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Behavioural Brain Research 109 (2000) 143165
Review article
Picture recognition in animals and humans
Dalila Bovet a, Jacques Vauclair a,b,*a Centre de recherche en
Neurosciences Cogniti6es, CNRS-CRNC, 31, chemin Joseph-Aiguier,
13402 Marseille Cedex 20, France
b Department of Psychology, Uni6ersite de Pro6ence, 29, a6.
Robert Schuman, 13621 Aux-en-Pro6ence Cedex 1, France
Received 4 June 1999; received in revised form 27 December 1999;
accepted 27 December 1999
Abstract
The question of objectpicture recognition has received
relatively little attention in both human and comparative
psychology;a paradoxical situation given the important use of image
technology (e.g. slides, digitised pictures) made by
neuroscientists in theirexperimental investigation of visual
cognition. The present review examines the relevant literature
pertaining to the question of thecorrespondence between and:or
equivalence of real objects and their pictorial representations in
animals and humans. Two classesof reactions towards pictures will
be considered in turn: acquired responses in picture recognition
experiments and spontaneousresponses to pictures of biologically
relevant objects (e.g. prey or conspecifics). Our survey will lead
to the conclusion that humansshow evidence of picture recognition
from an early age; this recognition is, however, facilitated by
prior exposure to pictures. Thissame exposure or training effect
appears also to be necessary in nonhuman primates as well as in
other mammals and in birds.Other factors are also identified as
playing a role in the acquired responses to pictures: familiarity
with and nature of the stimulusobjects, presence of motion in the
image, etc. Spontaneous and adapted reactions to pictures are a
wide phenomenon present indifferent phyla including invertebrates
but in most instances, this phenomenon is more likely to express
confusion between objectsand pictures than discrimination and
active correspondence between the two. Finally, given the nature of
a picture (e.g.bi-dimensionality, reduction of cues related to
depth), it is suggested that objectpicture recognition be
envisioned in variouslevels, with true equivalence being a limited
case, rarely observed in the behaviour of animals and even humans.
2000 ElsevierScience B.V. All rights reserved.
Keywords: Behaviour; Electrophysiology; Spontaneous responses;
Acquired responses; Invertebrates; Birds; Mammals; Humans
www.elsevier.com:locate:bbr
1. Introduction
Researchers in animal and in human cognition fre-quently use
photographs or slides in place of realobjects in their studies of
categorisation, face recogni-tion, etc., but paradoxically, there
are few experimentseither with animals or humans that have
explicitlyaddressed the question of the equivalence between
anobject and its picture. In other words, it is not obviousthat
animal and human subjects do really interpret the2-D stimuli as the
3-D objects they represent. Forexample, the success obtained in
training pigeons[40,63] or monkeys [13,80,112] to categorise
photo-graphic slides does not prove that the animals under-
stand what the pictures they categorise actuallyrepresent. In
fact, as we will see in this paper, somestudies have demonstrated
that this is not the case,while others have shown that the
establishment of someequivalence between the real object and its
pictorialrepresentation is dependent upon both the
stimulusdimensions and experimental and:or motivational
con-ditions. The present review tries to take stock of thisquestion
by examining the available literature for hu-mans (mostly infants)
and nonhuman subjects.
This review will first examine experiments concerninghumans and
will subsequently consider studies withnonhuman subjects, with the
latter being classified intothree categories. The first category
comprises of casesof convincing demonstrations in which animals are
ableto treat pictures like the stimuli they represent; we canassume
that a picture is recognised when animals react
* Corresponding author.E-mail address: [email protected]
(J. Vauclair)
0166-4328:00:$ - see front matter 2000 Elsevier Science B.V. All
rights reserved.PII: S01 6 6 -4328 (00 )00146 -7
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143165144
to a picture as they would react, spontaneously or aftersome
training, to the real object. Of course, such reac-tions may vary
according to the type of presentedpictures: social behaviour with
pictures of conspecifics,fear with threatening stimuli, predator
behaviours withpictures of prey, etc.; spontaneous responses and
trans-fer of various acquired responses (naming, categorisa-tion,
discrimination, cross-modal matching, etc.) inother cases. The
second category encompasses the ex-periments which could indicate
the existence of picturerecognition but are not really
demonstrations becausethe experimental design is questionable (for
examplewhen only one subject is involved) or the results (sub-jects
preferences or time viewing, discrimination ofindividuals or
species, or various spontaneous be-haviours) are not necessarily
specifically elicited by thepresented stimuli. The third category
includes thoseexperiments which show that animals may
havedifficulties with picture recognition.
In addition, two subclasses may be distinguishedamong the
studies that have used pictures of living orinanimate objects with
animal subjects; the first classreferring to the studies examining
learned reactions tostimuli (as it is often the case when the
subjects areprimates or birds), while the second class of
studiesmeasures spontaneous or natural reactions to the stim-uli
(this type of study is frequently seen in experimentsinvolving
lower vertebrates or invertebrates). In thislatter case, a very
salient feature of the releasing stimu-lus can suffice to induce
the reaction. For example, amale redbreast reacts to a lure (e.g. a
red tuft of redfeathers) as if it were a real conspecific, even if
the luredoes not look like a bird [61]; a colour photograph of
amale conspecific may induce a similar reaction but it isnot
certain whether, in such cases, the whole stimulushas to be
processed and recognised. However, when asubject is trained to
respond to real stimuli and thentransfers its response to pictures
of those stimuli, or isable to use video images to acquire some
informationabout the nature of a real object, this suggests that
themost significant features of the pictured stimuli areconsidered
and recognised. Therefore, as picture pro-cessing can differ as a
function of the kind of response(spontaneous or learned), these two
classes will beconsidered separately.
A third classificatory key concerns the issue of thestimuli
presented, that is, whether the image is static(photography, slide,
digitised picture) or a motion pic-ture, which of course implies
some movement and oftensound and may thus greatly facilitate the
subjectsreaction to the stimuli. For example, movement is wellknown
as releasing predatory behaviour [11], or mayplay an important role
in the courtship of many species(see for example Refs.
[35,94]).
2. Studies with humans: cross-cultural anddevelopmental
studies
Even in humans, recognition of photographs or pic-tures is not
as straightforward as it may first appear.Thus, Miller [70] showed
that there are interculturaldifferences in picture perception, with
humans whohave never seen pictures having difficulty
recognisingwhat is represented in black-and-white photographs;this
author gives an example originally reported byHerskovits ([41],
cited in Miller [70]) who imparts thata Bush Negro woman was
initially unable to recognisea photograph of her son until details
were pointed outto her. Similarly, Miller cites Kidd [56] who
reportedthat Bantus expressed difficulties in recognising objectsin
photographs until the details of those objects werehighlighted to
them and they then perceived them al-most instantly. Deregowski et
al. [29] encountered thesame difficulties with a remote Ethiopian
population,but these authors emphasised that members of
thispopulation were able to gradually recognise drawingsbut with
considerable effort and seemingly finding thetask stressful.
Kennedy [55] has suggested that the subjects studiedby
Deregowski and his colleagues could have initiallyrecognised
details and then progressively built up acomposite structure of the
picture. Miller [70] has alsoshown that even when people recognise
objects as rep-resented in pictures, they may experience
problemsperceiving the third dimension; thus, depth is often
notseen. For example, objects which were represented inthe
background appeared to some subjects as beingplaced upon objects
represented at the forefront of thepicture. Miller concluded that
the insight that pro-duces an overcoming of flatness cues may
require verylittle experience with pictures, but experience in
perceiv-ing objects in the three-dimensional world is not
suffi-cient to perceive those objects in pictorialrepresentations
and that direct experience with picturesmight also be necessary for
the perception of depth cuesin pictorial materials ([70], p. 148).
Deregowski [28]also relied on cross-cultural studies to understand
themechanisms of the perception and representation ofspace; coming
to the conclusion that even if peopleunfamiliar with 2-D
representations had moredifficulties with picture recognition, the
same kind ofdifficulties could occur in pictorial and
nonpictorialcultures.
An interesting experiment has presented results thatcontradict
the preceding conclusions. Hochberg andBrooks [44] studied the
behaviour of a child brought upuntil the age of 19 months without
being exposed topictures; when tested at the end of this period,
this childwas able to recognise and to name various objects fromhis
familiar environment represented in photographsand line drawings.
However, this childs environment
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143165 145
was not totally devoid of pictorial representations (it
isimpossible in the USA!), as he accidentally saw repre-sentations
of objects, for example, billboards on thehighway.
Even very early in life, infants are already able todiscriminate
between 2-D and 3-D stimuli. Thus, Pippand Haith [76] observed that
4-week-old infants differ-entiated 3-D from 2-D forms, with the 2-D
formsinvoking shorter fixation times. This experiment wasrepeated
with 8-week-old infants who also processed a3-D form in a different
manner than a 2-D form, not interms of overall fixation time but in
the visual scanpatterns elicited; there were more eye movements
inresponse to 3-D forms. An experiment carried out byAppel and
Campos [1] may offer some insight into howinfants differentiate 2-D
and 3-D stimuli; resultsshowed that 8-week-old infants could
discriminate be-tween stimuli differing only in terms of binocular
dis-parity, that is, when they were habituated tostereograms
without retinal disparity and then pre-sented with the same
stereogram with retinal disparity,heart rate increased indicating
that they dishabituated.
Bower [6] studied the reaction of neonates when areal ball or
its colour photograph was presented at sucha distance that the ball
could only be touched but nograsped. The real object was frequently
contacted butthe photograph not at all (it did not even elicit
handraising) although the infants attentively stared at it.
Incontrast, very young infants (under 23 days of age)were shown to
perform a similar amount of reaching inthe presence of either a
three-dimensional graspableobject (an orange textured sphere) or a
two-dimen-sional picture of it [32]. Slater et al. [91] showed
thatnew-borns (mean age: 2 days and 21 h) could discrimi-nate real
objects from their photographs; all partici-pants looked longer at
the real objects, even withmonocular viewing, leading the authors
to suggest thatmotion parallax was a salient cue for this
discrimina-tion. Interestingly, there was no evidence that the
new-borns were able to recognise stimulus similarity
acrossdimensions, thus, for these new-borns, differences be-tween
objects and their two-dimensional representa-tions seemed to be
more detectable or salient than theirsimilarities.
Other investigations indicate that human babies arenot only
able, with very little or no experience withphotographs, to
discriminate these from real objects,but also to recognise what
they represent. In an experi-ment carried out by Rose [81],
6-month-old infantspresented with various geometric stimuli were
not onlyable, in an habituation and visual preference test,
todiscriminate 2-D from 3-D stimuli, but also to
transferhabituation from 2-D to similar 3-D stimuli, or in-versely
from 3-D to similar 2-D objects. This apparentease of processing
objects and pictures in a similar wayis not, however, a
consistently reported result. For
example, some authors found that very young children(less than
30 months of age) did not interpret thepictures as representing
current reality; when the loca-tion of a hidden toy was
demonstrated using photo-graphic stimuli, 24-month-olds did not use
thisinformation to retrieve the toy, a task which 30-month-olds
readily performed [23].
Dirks and Gibson [30] have shown that 5-month-oldinfants,
without any experience of photographs, whohad previously been
habituated to an unfamiliar, liveface, showed no change in fixation
time when presentedwith a slide of the same person, but
dishabituated whenpresented with a slide of a novel person who
differed insex, hair colour, and hairstyle from the familiar
face.However, if the novel person was of the same sex, andhad the
same hair colour and hairstyle as the familiarface, no difference
in fixation time was observed, sug-gesting that these infants could
see the similarity be-tween a live person and their photograph
using rathergross physiognomic features. Similar findings are
re-ported in a study by Barrera and Maurer [2] who foundthat
3-month-old babies who had never seen photo-graphs looked longer at
their mothers photograph thanat a strangers one. Related evidence
of an early sensi-tivity of infants to pictures of conspecifics
comes fromthe study of the phenomenon of gaze following;
forexample, Hood et al. [46] showed that 3-month-oldscould detect
another individuals gaze shifts even whenpresented as digitised
pictures of adult faces.
Cross-modal experiments can also be valuable in thestudy of
transfer from objects to pictures or the reverse.Rose et al. [82]
showed that such a transfer was possiblewith 12-month-old infants,
but that it depended onfamiliarisation time: with a familiarisation
time of 30 s,infants were only able to perform a cross-modal
trans-fer from touch to vision (real objects), and to transferin a
visualvisual task from real objects to both theiroutline drawings
and coloured silhouettes. With anincreased familiarisation time of
45 s, subjects were ableto cross-modally transfer from touch to
real objects, totheir outline drawings and to their coloured
silhouettes.However, with a familiarisation time of 15 s,
infantswere no longer able to transfer in a visualvisual taskfrom
real objects to their outline drawings, or colouredsilhouettes.
Streri and Molina [92] conducted another experimenton
cross-modal transfer with infants of only 2 monthsof age. Somewhat
paradoxically, these authors foundthat pictures were more easily
recognised than realobjects in a transfer from vision to touch:
transferoccurred between felt objects and their 2-D visual
sil-houettes, but not between felt objects and their
visualcounterparts. The authors hypothesised that this rela-tive
ease could be explained by the fact that infants takemore
information from seeing stimuli than from touch-ing them. Thus, the
use of pictures could have sim-
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D. Bo6et, J. Vauclair : Beha6ioural Brain Research 109 (2000)
143165146
plified and reduced visual information, consequentlyrendering
the object more easily identifiable for theinfant who had only
tactile experience of the object.Subsequent experiments performed
by the same authorsusing the habituation procedure provided some
evi-dence that 2-month-old babies were able to perceiveboth the
commonalties and the differences of 3-D ob-jects and their 2-D
representations.
Experiments measuring event-related potentials havereached the
same conclusion as most of the experimentspresented above.
Recordings of ERPs in 7-month-oldbabies while they were watching
pictures of facesshowed that the active components differentiated
be-tween a happy face and a fearful face but not betweenan angry
face and a fearful face [71], suggesting that theinfants recognised
what was presented on the pictures.
Perhaps the most impressive demonstration of younginfants
abilities in interpreting pictorial representationscan be found in
a study on intermodal transfer betweenoral exploration of objects
and visual matching. Ineffect, in two experiments, Kaye and Bower
[52]showed that new-borns as young as 12 h old were ableto match
tactile shapes (pacifiers) with visual represen-tations of the
pacifiers shafts displayed as digitisedcoloured or black-and-white
images on a computerscreen.
In summary, cross-cultural studies have demon-strated that
adults who have never seen any two-dimen-sional representations may
experience difficultiesrecognising pictures; these participants
need some ex-planation and some experience with a photograph (or
adrawing) before being able to perceive what it repre-sents.
However, developmental studies reveal that theability to recognise
significant information in picturessuch as photographs is evident
even in very earlyinfancy (demonstrated at 3 months or even
younger).This apparent paradox between adult and infant
perfor-mance will be discussed in the conclusion of this
article.Human babies are also able to discriminate real objectsfrom
their pictorial representations; this precociousability does not,
however, preclude infants and eventoddlers from confusing an object
and its referents. Themethods, populations and main results of the
studiesdiscussed above are summarised in Table 1.
3. Studies with animals: convincing demonstrations
3.1. Spontaneous responses to pictures
In this section, studies that provide rather unambigu-ous
evidence of spontaneous responses to pictures as ifthey were real
objects are considered; studies usingstatic and motion pictures
will be presented in turn.
3.1.1. Responses to still picturesThe perception of still
pictures can elicit adapted
responses by monkeys. Thus, von Heusser [43] reportedthat a
tamed marmoset displayed grabbing responses infront of photographs
representing different prey (e.g.ants, butterflies) but displayed
fear reactions whenshown a picture of a cobra. In a similar vein,
Rosenfeldand van Hoesen [83] related that naive rhesus
monkeysreacted with hasty retreat, threat responses and
vocali-sations to the first presentation of slides of rhesusmonkeys
faces. These monkeys also displayed abortiveapproachretreat before
touching the stimuli (i.e. theslide projector), this behaviour did
not persist, however,because the subjects quickly realised that the
stimuliwere only pictures (i.e. unresponsive, immobile
stimuli).Comparable findings were obtained with cynomolgusmacaques
by Kyes et al. [60], namely, that dominantmonkeys produced
threatening gestures when shown,for example, pictures of gorillas
or humans, while sub-ordinates gave submissive responses to these
samestimuli.
Sackett [87,88] presented real-life-size coloured slidesof
various social stimuli to rhesus monkeys; subjectsexhibited
different responses to the various stimuli,many of these responses
being appropriate to the situa-tion. Moreover, the level of
responses to a given slidevaried according to age and rearing
conditions: subjectsreared in isolation showed more exploration of
non-monkey pictures and pictures without any social com-munication
content than with socially relevant pictures.Results of another
experiment [59] with hamadryasbaboons suggest that these monkeys
were able to recog-nise slides of conspecifics. Subjects were given
controlover slide selection and viewing time, with some
slidesdepicting individual troop members and others depict-ing
various facial areas of a troop member; the baboonswere highly
reliable in their choices, with dominancestatus seemingly a primary
factor in troop memberpreference and slides of full faces being
consistentlychosen and the eye region attracting the
greatestattention.
Overman and Doty [72] have investigated hemi-spheric
specialisation for face processing in pigtailmacaques. Prior to
testing, the authors examined iftheir subjects would react in a
similar manner to realmodels and pictures of humans and monkeys;
variousemotional reactions were measured (e.g.
vocalisations,lip-smacking, etc.) at the first presentation of
differentcategories of slides and results showed that pictures
ofhumans and monkeys were clearly differentiated fromother classes
of stimuli such as flowers, insects orlandscapes.
The available neurophysiological evidence supportsthe view that
nonhuman primates establish some corre-spondence between
photographs and the individualmonkeys they represent. For instance,
studies with
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143165 147
macaques have revealed that groups of neurones intheir left
inferotemporal cortex are responsive to facesof other monkeys and
are sensitive to the identity of themonkey [74,113]. It occurred
that similar groups of
neurones were firing in response to life facial stimuli,
topicture stimuli and to still video [75].
In addition to nonhuman primates, it seems thatsheep may also be
good candidates for studying picture
Table 1Studies with humans
Task Nature of pictures Age Results Reference
Increased heart rate in theStereogram or simple Eight weeks
Appel andDiscrimination of stimulipicture of a color drawingwith
binocular disparity Campos [1]dishabituation phase
from stimuli withoutbinocular disparity
Mothers photograph Barrera andPreference for mothersColor slides
Three monthspicture Maurer [2]recognition
Bower [6]NeonatesColour photographs Hand raising is elicited by
aBehavioural observationsreal ball but not by itsphotographERP
shows differenceSix months Nelson and deTwo-thirds-size
digitizedEvent-related potentials
Haan [71]colour photographsrecorded from babies between
observation of themothers face and awatching pictures of
theirstrangers face, but lookingmothers face or a
strangers face time does notFailure to find the
hiddenTwenty-four and 30 monthsPhotographsFinding a hidden object
when Deloache and
the location is Burns [23]object in 24-month-oldchildren but not
indemonstrated with
photographs 30-month-oldTrying to grasp the depicted Deloache et
al.Color photographsBehavioural observations Nine months
[25]objects, despitediscriminating betweenobjects and
picturesSame types of difficulty Deregowski [28]Adults and children
with orBlack-and-whiteCross-cultural studies ofoccur in pictorial
andhuman picture perception photographs and drawings without
experiencenonpictorial cultures
Line drawings Adults without experiencePresentation of pictures
Difficulty recognizing what Deregowski eta picture represents al.
[29]
Life-size colour slides Perception of similarityHabituation to a
live face, Dirks andFive monthsthen measurement of between a live
person and Gibson [30]
their photographfixation times for a slide ofthe same and a
novel face
Color photographs Similar amount of reachingTwenty-three days
Dodwell et al.Comparison of amount ofwith a real ball and with
itsreaching to a real ball and [32]
to its picture picturePresentation of pictures Adults without
experience Herskovits [41]Difficulty recognizing
whatBlack-and-white
a picture representsphotographsHochberg andNaming the
represented Photographs and line Nineteen months without Correct
naming
experience Brooks [44]drawingsobjectsThree months Correct
detection Hood et al. [46]Colour digitized picturesDetection of an
adults
change of gaze directionIntermodal transfer occursTwelve hours
Kaye andDigitized colored orIntermodal transfer between
oral and visual exploration black-and-white images Bower [52]of
objects
Kidd [56]Presentation of pictures Black-and-white Difficulty
recognizing whatAdults without experiencephotographs a picture
represents
Adults and children withBlack-and-whiteReview of cross-cultural
Difficulty in perception and Miller [70]recognition of various
typesresearch more or less experiencephotographs and drawingsof
pictures
Recording of event-related Nelson and deEvent-related potentials
varyBlack-and-white slides Seven monthsHaan [71]potentials with the
expression of the
faces presentedSimple black-and-white Four weeks and 8
weeksDiscrimination between 2-D Pipp and HaithVisual behavior
[76]differentiates between 2-Dsilhouettesand 3-D stimuliand 3-D
stimuli
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Table 1 (Continued)
Age ResultsNature of pictures ReferenceTask
Six months Rose [81]Babies are able to discrim-Discrimination
between 3-D Black-and-whitephotographs inate between 3-D andand 2-D
stimuli, and transfer
2-D stimuli, but also toof habituation from 2-D tosimilar 3-D
stimuli, or perceive similarity across
dimensionsinversely from 3-D to similar2-D objects
Cross-modal transfer isCross-modal transfer between Colored
outline drawings Rose et al. [82]Twelve monthsand
silhouettestouched objects and their possible but depends on
familiarization timepicturesTwo days and 21 hours Neonates
discriminateDiscrimination between 3-D Slater et al.
[91]Black-and-white
between 3-D and 2-D stimuli,photographsand 2-D stimuli, and
transferbut do not perceive similarityof habituation from 2-D
to
similar 3-D stimuli, or across dimensionsinversely from 3-D to
similar2-D objects
SilhouettesCross-modal transfer from Two months Pictures are
more easily Streri and Molinavision to touch recognized than real
objects [92]
in this kind of cross-modaltransfer
recognition in animals. Thus, a study by Vandenheedeand Bouissou
[101] indicates that sheep recognised a 2-Dstimulus at its first
presentation. In this study, the fearreactions of ewes were tested
when the subjects wereseparately presented with a full-size slide
of a human, asheep, or a control stimulus (a traffic cone); the
ewesshowed reduced fear reactions in the presence of a
sheepsphotograph, as with real conspecifics and, moreover,sniffing
was primarily directed towards the anogenitalregion and the head,
which corresponds to behavioursdirected towards real conspecifics.
However, the humanslide failed to induce fear reactions, as they
occurred witha real human or even a human-like model [101],
thussuggesting the possibility that recognition of 2-D stimulicould
be easier when stimuli are conspecifics. A subse-quent experiment
[4] showed that, in ewes, a slide of anunknown individual of its
own breed significantly re-duced fear reactions compared to a slide
of an unknownindividual of a different breed, this latter result
suggest-ing that the ewes can recognise the characteristics of
theirbreed on the slide.
Finally, Clun Forest and Dalesbred sheep showed anability to
discriminate black-and-white photographs de-picting faces of sheep
versus human faces when they weretested with a procedure of
spontaneous choice in aY-maze [54]. In addition, the same study
demonstratedthat Clun ewes could also distinguish between the
facesof male and female breed members and between a troughwith and
without food. Moreover, some subjects wereobserved displaying overt
social reactions toward stimuli,such as licking the pictures (C.
Fabre-Nys, pers. com-mun., May 1998).
3.1.2. Reactions to motion picturesPlimpton et al. [77] showed
social stimuli (for example
a threatening male stimulus) to juvenile bonnet macaques
via colour videotape recordings and observed thesesubjects in
the presence of their mother; they exhibitedappropriate responses
depending on the nature of thesocial display, that is, they behaved
submissively towardthe threatening male and searched for contact
with theirmother while they approached a passive female. Herzogand
Hopf [42] showed different colour films to wild-bornand
laboratory-born squirrel monkeys. While the presen-tation of
predators (cats, snakes or avian predators)caused specific alarm
and flight reactions, the subjects didnot emit any alarm response
when nonpredator mam-mals were shown. Further, they reacted in the
same wayas they did in real situations upon seeing preparation
offood or insects walking and also reacted to films ofhuman beings
as to real people. These subjects demon-strated face recognition;
upon seeing in the film acaretaker who had recently removed a dead
neonate, thesquirrel monkeys behaved as if they were facing a
realterrestrial predator. No difference was shown betweenwild and
laboratory born-subjects.
In studies with birds, the use of predators picturesalso yielded
positive indications of picture recognition.For example, in a study
by Evans and Marler [34]which used video images as stimuli,
domestic cockerelswere shown to respond with similar alarm calls
inresponse to an aerial predator model when either videosof hens or
real caged hens were present. The use ofsocial stimuli, notably in
tasks requiring the recognitionof conspecifics, can provide
important insight on theability of birds to match objects with
their pictures.Shimizu [90] observed that when video images of
fe-males were presented to male pigeons, the duration ofthe males
social display was not significantly differentfrom that which they
performed in front of the livebird. When video images of another
bird (a cockatoo)
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143165 149
or of an empty room were presented to the subjects,they showed
much shorter, or no, display. Finally, theduration of the display
was longer when video imageswere in motion rather than still and
when only the headregion was visible rather then when only the
bodyregion was visible.
An experiment carried out by McQuoid and Galef[67] with juvenile
Burmese fowls provides evidence thatobserving feeding among
conspecifics via video tapeshas similar effects as observing live
conspecifics onsubsequent feeding behaviour (i.e. preferences for
fooddishes); thus, the sight of a video of a conspecificfeeding
before testing, even without sound, reduced thelatency of the first
peck. Further, a videotape of a fowlactually feeding on a food dish
was more effective inreducing the latency of the first peck and in
enhancingpreference for that type of food dish than a
videotapesimply representing a fowl (either active or immobile)near
a food dish.
Jenssen [49] studied the recognition of motion pic-tures by
lizards. In this study, female lizards couldchoose between two
films, one with males displayingthe normal courtship behaviour and
the other withmales presenting altered displays; in most cases,
thefemales chose the normal displays. This experimentsuggests that
female lizards could recognise male lizardsin the film and that
they could be sensitive to theirbehaviour. When video sequences
displaying aggressivedisplays of their own species were shown to
malelizards, these animals responded with the appropriateresponses
(e.g. head-bobbing, crest erection) which theywould exhibit in
front of live opponents. Moreover,such behaviours were inhibited
when video-recordedsequences of heterospecific lizards were
presented [65].
Some experiments have reported evidence of imagerecognition in
fishes, at least when video images (imply-ing motion) were used.
Thus, Rowland [85] showed thatmale and female sticklebacks reacted
to video images ofa zigzag dancing male played at a normal or
slightlyfaster tempo in a similar way as they would react to alive
male; when the tempo was slower or much fasterthan the normal
tempo, the animals were less attracted.In guppies, consistency of
mate preference was studiedby presenting females with males under
three experi-mental procedures: live males behind clear glass,
livemales behind one-way glass and images of males onvideotapes.
Females spent significantly more time inproximity to males behind
clear glass than in video andone-way glass presentations, but they
spent equal timewith males behind one-way glass and videotaped
males[58]. In all three experiments, females responded tostimuli by
displaying sexually oriented behaviours andthe results show that
when interaction was not possible,a videotaped male was as
attractive as a real male.
Some studies suggest that even invertebrates canrecognise video
images. For example, Clark and Uetz
[11] carried out an experiment with jumping spiders andfound
that in a V-maze choice, spiders preferentiallychose a videotape
with moving prey to a videotapewithout prey. In addition, the
spiders did not discrimi-nate between a live prey and its
simultaneously pre-sented video image and they behaved in a
mannercomparable to their reactions with life stimuli whenthey were
presented with televised images of prey in-sects (attack),
conspecifics (courtship) and heterospe-cific spider species
(retreat).
3.2. Acquired responses in picture recognition
We will now turn to a consideration of studies thathave
attempted to test the abilities of different animalspecies using
pictures (still and in motion) as stimuli.Although most of the
experiments summarised belowwere aimed at investigating picture
recognition, wehave also considered experiments for which the
explicitgoal was not picture recognition as these studies
alsopresent findings relevant to this topic.
3.2.1. Reactions to still picturesHayes and Hayes [39] reared a
female chimpanzee,
Vicki, in their home, almost like a human child. Whilethey did
not specifically train her in picture perception,they did test her
ability to recognise pictures depicted inbooks and other materials
and to imitate actions illus-trated in films, photographs, and line
drawings. Vickiwas able to recognise most of the pictures she saw,
evenwhen the pictures were presented as black-and-whitedrawings.
Nevertheless, those authors reported thatVicki did not confuse
photographs with real objects;Vicki did not try to grasp 2-D
objects and when shepointed for example to pictures of beverages,
she saidcup, and let the person who was with her go to thekitchen
and get a drink. Gardner and Gardner [38]showed that four
chimpanzees that had been famil-iarised with pictures could also
recognise and name inAmerican Sign Language various objects
representedon new slides.
Sarah, an adult chimpanzee experienced with filmsand photographs
was shown videotaped scenes of ahuman actor struggling with one of
eight problems andthen presented with two photographs that could
consti-tute a solution to the problem [78]. Sarah chose thecorrect
photograph on seven of the eight problemssuggesting that she
recognised both what happened inthe films and the objects as
depicted in photographsand as films. Furthermore, Savage-Rumbaugh
et al.[89] trained chimpanzees to categorise various objectsinto
the two categories tools or foods; when theobjects were replaced by
photographs (the subjects werealready familiarised with pictures),
the two chimpanzeestested were still able to categorise (and thus
to recog-nise) them.
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An interesting study on the recognition of pictures ofindividual
faces by chimpanzees was carried out byBauer and Philip [3]. The
authors found that threechimpanzees initially trained to match
photographs offaces of the same individuals were able to later
matchdifferent vocalisations to facial portraits of the
familiarindividuals.
Experiments on cross-modal perception can alsoprovide us with
useful indications concerning the ques-tion of picture recognition
in nonhuman primates. Forexample, Davenport and Rogers [16] trained
three apes,one orang-utan and two chimpanzees, who were unfa-miliar
with photographs, to match a visual sample (thereal object) to an
haptically presented object. When thevisual sample was a
photograph, subjects responseswere clearly above chance (80% or
more) from thebeginning. Further, there was little difference in
accu-racy between colour and black-and-white photographs,and no
difference between real objects and their colourphotographs. Thus,
it seems that apes are able, evenwithout any familiarity with
photographs, to recognisethem and treat them like real objects. In
another exper-iment [17], five chimpanzees trained only in a haptic
tovisual cross-modal experiment with real objects weresubmitted to
the same sort of problems with photo-graphs or drawings; four of
the subjects performedsignificantly above chance with full-size
colour photo-graphs, with full-size or half-size black-and-white
pho-tographs and with full-size line drawings. However,cross
modal-perception was better with photographsthan with line
drawings.
An experiment of cross-modal matching of objectswith their
photographs was conducted by Malone et al.[66] with two adolescent
male rhesus monkeys. One ofthe monkeys was trained on visual to
haptic matchingto sample, and the other on haptic to visual
matchingto sample (the visual stimuli were full-sized
colourphotographs of the haptic objects). While both mon-keys
succeeded on the task, they needed prior trainingto do so and the
authors highlighted that it was unclearwhether training was
necessary because subjects haddifficulty mastering the
matching-to-sample procedure,or if they had to learn first the
equivalence betweenphotographs and objects. In a subsequent
experiment[97], the same subjects were required to perform thesame
task but with black-and-white photographs, sil-houette photographs,
and outlines drawings of the ob-jects, that is, forms of stimuli
with which they had hadno prior training; the monkeys were still
able to per-form the task when visual stimuli were
black-and-whitephotographs and silhouette photographs, but not
withoutline drawings.
Zimmermann and Hochberg [114] trained infant rhe-sus monkeys
(5150 days of age) to discriminate be-tween flat and solid objects
(e.g. squares and cubes) andthen tested the transfer to photographs
and outlines
figures of these objects. The results showed that thesesubjects
were able to make consistent responses topictorial representations
of the stimuli (photographs ordrawings) and that while the presence
of shadow facili-tated transfer, this feature was not
necessary.
Dasser [14] showed that two Java monkeys were ableafter a few
trials, to identify novel views (full face orfull animal) of a
familiar conspecific presented onslides, and that another subject
could match differentbody parts of the same familiar group members.
In asubsequent experiment, Dasser [15] showed that twoadult female
Java monkeys, first familiarised with slidesof their conspecifics,
were able to identify motheroff-spring pairs or to match views of
offspring to theirmother, a task which requires the recognition of
groupmembers on the slides.
In a recent study by our group [5], olive baboonswere trained on
the natural category of food versusnon-food with real objects.
After categorical transferwith novel items, subjects were trained
again with onepair of cut-out pictures each of which belonged to
thetwo previously learned categories; after this limitedtraining,
categorical transfer was high in both baboonsfor cut-out photos of
the food and non-food objects.Results of the experiment and of
additional controlsituations involving various modes of picture
presenta-tions further demonstrated the abilities of the baboonsto
relate real objects to their pictorial representations.
A consideration of studies using nonprimates revealsthat most of
these experiments have been conductedwith birds, especially
pigeons, but studies with otherzoological groups will be considered
subsequently. Oneof the first studies concerned with the question
ofpicture recognition in animals addressed the ability ofpigeons to
recognise a conspecific shown in a picture[62]. The authors used
three pigeons with experimentalhistories of attacking a mirror
target, but not pictures,and submitted them to an intermittent
schedule ofreinforcement for key pecking; they found
comparableresults for both temporal pattern and locus of
attacks(the head region) to that reported in studies with
live,taxidermally stuffed pigeons or mirror targets. Subse-quent
experiments demonstrated that an upright silhou-ette,
white-on-black silhouette of a pigeon, with orwithout eye, was more
effective in controlling attackthan an inverted silhouette, an
outline of a pigeon, or apiece of coloured paper.
A study of transfer of discrimination from solidobjects to
pictures by pigeons was carried out by Cabe[10]. The pigeons, which
were naive with respect topictorial stimuli, were first trained to
discriminate twoobjects, and then four groups of four pigeons
weretested in reinforcement reversal with objects, black-and-white
photographs, silhouettes or line drawings of thoseobjects; negative
transfer was expected in all cases if thepigeons recognised the
pictures. In fact, negative trans-
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143165 151
fer occurred with objects (for all four pigeons),
withphotographs (for three of the four pigeons) and withsilhouettes
(for all four pigeons), but not with linedrawings (none of the four
pigeons). Four birds testedonly with objects were then re-tested
without reversalusing black-and-white photographs and showed a
posi-tive transfer and, in addition, subsequent training
todiscriminate objects from their photographs showedthat this was
fairly easily obtained. These results sug-gest that the birds did
not confound objects and theirpictures and, altogether, these
experiments also attestthat prior experience is not necessary for
picturerecognition.
Delius [26] trained eight pigeons to discriminatespherical
objects from nonspherical objects and follow-ing this training,
seven birds out of eight were able totransfer to black-and-white
photographs, colour photo-graphs, or drawings of these objects, as
well as tophotographs of novel objects; the overall transfer
toblack-and-white photographs was best, that to draw-ings
intermediate, and that to colour photographsworst. The author
suggested that the pigeons did worstwith colour photographs because
their colour vision isat least pentachromatic and the colour
photographywas matched to the trichromatic colour vision of
hu-mans. Watanabe [105] trained 12 experimentally naivepigeons to
discriminate, for one half, between realobjects (edible and
nonedible objects) and their photo-graphs, and for the remaining
half between food andnon-food objects in the same set of stimuli
(real objectsand their photographs). The pigeons performed high
oneither task and showed generalisation to novel stimulifor the two
tasks, hence demonstrating that pigeonswere able to treat
photographs like real objects, and todiscriminate between them,
according to their trainingand categorical judgement.
Jitsumori and Ohkubo [51] found that four experi-mentally naive
pigeons trained to discriminate right-side-up and upside-down
orientations of colour slidesof natural scenes depicting humans,
transferred thisdiscrimination to new slides of the same kind. Both
theorientations of the human figures and of the back-ground scenes
controlled this discrimination, but whenthese slides were oriented
in the opposite direction, thebackground orientation cue was the
dominant feature.The birds were also able to categorise by
orientationnatural objects (humans, apes, monkeys and birds) on
awhite background, indicating that the subjects recog-nised the
objects presented.
3.2.2. Reactions to motion picturesInfant chimpanzees [68] and
even baboons [102] were
able, after limited experience, to match what they ob-served on
a television screen to events occurring else-where in order to
determine the location of a hiddengoal object in a familiar outdoor
field. Along similar
lines of research, Menzel et al. [69] showed that twochimpanzees
could use mirrors or live video images tomove their hands in the
appropriate direction and makecontact with target (food)
objects.
It has been demonstrated that fear reactions can alsobe learned
by use of videotaped demonstrators. Forexample, naive rhesus
monkeys acquired a fear ofsnakes through watching videotapes of
conspecifics re-acting fearfully to snakes; note however, that the
mon-keys did not acquire a fear of flowers through
watchingvideotapes of monkeys reacting fearfully to flowers
[12].
To summarise, it appears that the experiments pre-sented in this
section reveal that picture recognition ispossible by animals, even
without previous experience.Thus, spontaneous adapted responses
were displayed tosignificant stimuli photographs (prey, predators,
orconspecifics) by monkeys. Other mammals (sheep)showed adapted
responses to slides of conspecifics andsimilar responses to
pictures of conspecifics were alsoevident in other species (birds,
lizards, fishes and evenin some invertebrates) when the pictures
were presentedin motion; such spontaneous responses to these
stimulidid not require any previous training with pictures.
Apes, monkeys and pigeons were also able to transferacquired
responses from objects to pictures in varioustasks, however, it is
difficult to know in such caseswhether familiarisation with the
pictures is necessaryfor their recognition. The methods, species
and mainfindings of the papers that have reported some evidenceof
picture recognition in animals are outlined in Table2.
4. Studies with animals: experiments that could indicatepicture
recognition
This section reviews those experiments which providesome cues
indicating the presence of picture recognitionbut that may not
constitute real proof of such anability.
4.1. Spontaneous responses to pictures
As in the previous section, studies that used still andmotion
pictures will be considered in turn.
4.1.1. Reactions to still picturesHumphrey [47] used visual
stimuli, either plain fields
of light colour, or photographs or films, and a simplechoice
procedure with two adolescent rhesus monkeys;subjects could push
two buttons to choose one of thetwo stimuli presented on a screen.
Humphrey inter-preted rhesus monkeys preferences in terms of
interest(determined by the information content in the stimuli)and
pleasure (determined by features such as colourand brightness),
with these two factors determining the
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Table 2Convincing demonstrations of picture recognition in
animals
Task ResultsNature of pictures ReferenceSpecies
Black-and-white photographs Chimpanzee Correct matching Bauer
and PhilipMatching vocalizations to facialportraits of familiar
conspe- [3]cifics
Behavioural observations Sheep Appropriate spontaneous
re-Life-size colour slides Bouissou et al. [4]sponses to images of
conspe-cifics
Colour photographsCategorization of objects and Olive baboons
Correct transfer occurs from Bovet and Vau-objects to pictures
clair [5]their pictures into food and
non-food categoriesDiscrimination transfer from Black-and-white
photographs or Pigeon Correct transfer occurs from Cabe [10]
objects to picturesdrawingstwo objects to their
picturesBehavioural observations Appropriate spontaneous
re-Black-and-white video images Jumping spider Clark and Uetz
[11]sponses to images of conspe-cifics, prey and
heterospecificspiders
Colour video images Rhesus monkey Fear of snakes acquired but
notAcquisition of fear of snakes or Cook and Minekaof flowers by
observation of fear of flowers [12]conspecifics
Long-tailedMatching various novel views of Colour slides Correct
matching Dasser [14]macaquea conspecific
Colour slides Long-tailed Dasser [15]Categorization of
motheroff- Correct categorization ormatchingspring pairs or
matching macaque
mother to offspringMatching a touched but unseen Correct
cross-modal matching Davenport andChimpanzeeBlack-and-white and
colour pho-
tographsobject to its photograph Rogers [16]Black-and-white and
colour pho-Matching a touched but unseen Chimpanzee Davenport et
al.Correct cross-modal matching
object to various 2-D repre- tographs (life-size and half-size)
[17]and line drawingssentationsBlack-and-white and colour pho-
Delius [26]Categorization of objects and Pigeon Correct transfer
occurs from
objects to picturestographs or drawingspictures as spherical and
non-spherical objects
Colour video imagesBehavioural observations Appropriate
spontaneous re-Domestic Evans and Marler[34]sponses to
predatorscockerel
ChimpanzeeNaming the represented objects Correct namingColour
slides Gardner andin American Sign Language Gardner [38]
Black-and-white films, photo- ChimpanzeeMatching pictures and
imitating Correct matching and imitation Hayes and Hayes[39]actions
illustrated in pictures graphs, and line drawings
Colour films Squirrel monkey Appropriate spontaneous
re-Behavioural observations Herzog and Hopf[42]sponses to
predators, food, and
humansLizard Female choice of normal maleKodachrome II indoor
films (BW Jenssen [49]Behavioural observations
displaysor NB unspecified)Colour slidesDiscrimination of
right-side-up Pigeon Correct discrimination transfer Jitsumori
and
and upside-down orientations Ohkubo [51]to novel viewsof
scenes
Life-size black-and-white photo-Spontaneous choice in a Y- Sheep
Discrimination between human Kendrick et al.maze between pictures
and sheep faces, between malegraphs [54]
and females conspecifics andbetween a trough with andwithout
foodAppropriate spontaneous re-Guppy Kodric-BrownLife-size colour
video imagesBehavioural observationssponses to images of conspe-
and Nicoletto [58]cifics
HamadryasColour slidesSpontaneous choice between Choices
consistent with social Kyes and Cand-context land [59]slides of
conspecifics baboon
Long-tailedColour slidesBehavioural observations Appropriate
spontaneous re- Kyes et al. [60]macaque sponses to pictures of
gorillas
and humansColour photographs and draw-Reinforced attack of a
pigeon Looney andAttack of the target picturePigeon
target Cohen [62]comparable to attack of a liveingstarget
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Table 2 (Continued)
Task SpeciesNature of pictures Results Reference
LizardBehavioural observations Appropriate spontaneous
re-Life-size colour video images Macedonia et al.[65]sponses to
images of conspecifics
Cross-modal matching of objects Full-size colour photographs
Rhesus monkey Correct cross-modal matching Malone et al. [66]and
their photographs
Burmese fowlBehavioural observations Acquisition of preferences
for McQuoid andLife-size colour video imagesfood dishes by
observation of Galef [67]conspecifics
Finding a hidden object with Menzel et al. [68]Black-and-white
video images Chimpanzee Correct transfer from video im-ages to real
situationvideo images used to demon-
strate locationMenzel et al. [69]Moving their hands to make
Colour live video images Chimpanzee Correct hand movements
contact with target objectsshown on video
Pigtail macaqueBehavioural observations Spontaneous choice and
appro- Overman and DotyColour slidespriate spontaneous responses to
[72]slides of humans and conspecifics
Patterson-Kane etColour video imagesDiscrimination transfer
between Domestic hen Transfer occurs only when thetwo stimuli to
discriminate havevarious hens and objects to al. [73]different
colourstheir pictures
Drawings and still video images Rhesus monkey Similar neuronal
responses toElectrophysiological recording Perrett et al.
[75]images as to life facial stimuli
Bonnet macaque Appropriate spontaneous re-Colour video
imagesBehavioural observations Plimpton et al. [77]sponses to
pictures of conspecifics
Premack andPhotographs and film ChimpanzeeChoice of a photograph
as the Correct choicesolution to a problem presented Woodruff
[78]in a film
Discriminations of conspecifics Slides Rosenfeld and vanRhesus
monkey Correct discrimination and ap-propriate spontaneous
responsesfaces and behavioural observa- Hoesen [83]to conspecifics
facestions
Stickleback Appropriate spontaneous re-Colour video images
Rowland [85]Behavioural observationssponses to images of
conspecifics
Colour slidesMeasurement of visual and Rhesus monkey Sackett
[87]Visual and tactile responses varyconsistently with the nature
oftactile responsesstimuli, and with the subjectsage and
experience
Rhesus monkeyBehavioural observations Appropriate spontaneous
re-Colour slides Sackett [88]sponses to pictures of
conspecifics
Savage-RumbaughCategorization of objects and Photographs
Chimpanzee Correct transfer occurs fromet al. [89]their pictures
into food and objects to pictures
non-food categoriesPigeon Courtship display to images ofMotion
and still colour video im-Behavioural observations Shimizu [90]
ages conspecificsMatching an object touched but Black-and-white
and colour full- Correct cross-modal matching Tolan et al.
[97]Rhesus monkey
size photographs, silhouettes andunseen to various 2-D for
photographs and silhouettes,but not for outline drawingsoutline
drawingsrepresentations
Behavioural observations Colour slides Sheep Reduction of fear
and appropri- Vandenheede andBouissou [100]ate spontaneous
responses to im-
ages of conspecificsVauclair [102]Colour video images Guinea
baboon Correct transfer from video im-Finding a hidden object
when
ages to real situationthe location is demonstratedusing
video
MarmosetBehavioural observations Appropriate spontaneous
re-Photographs von Heusser [43]sponses to prey and predatorsCorrect
transfer from objects to Watanabe [105]Colour slidesCategorization
of objects and Pigeon
their pictures into food and pictures, and correct
discrimina-non-food categories, and dis- tion between objects and
picturescrimination between objectsand pictures
Black-and-white photographs or Zimmermann andRhesus
monkeyDiscrimination transfer between Discrimination transfer
occurssquares and cubes to their drawings Hochberg
[114]pictures
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strength and direction of preferences; when the twofactors are
set against each other, interest overrodepleasure in determining
the preference. When naivemonkeys were tested for preferences for
coloured pho-tographs paired with plain fields of light, they
firstshowed negative preferences and signs of fear. How-ever, as
they became more experienced, the signs of feardropped away and
they showed positive preferences forcolour photographs (the same
pattern of responsechange was observed when films were introduced
in-stead of photographs). The photographs were dividedinto six
classes, and the rank order of preferencesexhibited by the monkeys
was other animals, mon-keys, men, flowers, abstract paintings and
foods.In a further experiment, Humphrey [48] used noveltypreference
for slides to investigate how rhesus monkeyscould differentiate
between individual animals of thesame species; the finding was that
monkeys to whichdomestic animals were unfamiliar treated
individualdomestic animals of the same species as being
closelysimilar, but treated individual monkeys as being differ-ent
from each other. However, monkeys who had beenexposed for 6 months
to many pictures of animals,treated all individuals as different
from each other.Demaria and Thierry [27] conducted a rather
similarexperiment with female stumptailed macaques. Thesefemales
were submitted to slides displaying individualprimates or
non-primates and the results showed thatsubjects looked longer at
slides of individuals of theirown species than at slides depicting
other macaquesspecies; moreover, they looked more at adult
femalescarrying infants than at adult females alone. Withpictures
of non-primates animals, subjects looked mostat slides of felids.
However, spontaneous social re-sponses, like facial expressions,
were very rare.
Fox [36] observed the responses of young and adultdogs (the
breed was not given) presented for the firsttime with a life-size
dog painting. The young dogssniffed more at hind leg and inguinal
regions, whileadults sniffed more at ear and anal areas; as all
theseareas are normally investigated by conspecifics, we caninfer
that the subjects had perceived the correspondencebetween the
painting and a real dog.
In sheep, Kendrick and Baldwin [53] recorded re-sponses of cells
in the temporal cortex of awake sub-jects and demonstrated that
some of these specificallyresponded to slides (photographs and
drawings) offaces (but not to upside-down faces or profiles).
More-over, different groups of these cells were influenced
byrelevant social factors, such as dominance, breed, famil-iarity,
and facial expression. Franklin and Hutson [37]investigated the
reactions of sheep to full-size colourphotographs of one sheep and
to colour films of mov-ing sheep and found that the sheep reacted
to 2-Dimages as if they were real animals: subjects were slowto
approach a sheep facing them, but they approached
without hesitation or followed a sheep displayed inprofile;
reactions which were heightened when the im-age was moving. The
most attractive stimulus was thefilm of sheep moving across the
screen towards the exit.
Several studies are available concerning the ability ofbirds to
spontaneously display adapted responses topictures of biologically
relevant situations. For exam-ple, dark-eyed juncos which could
choose betweenslides of their winter and summer habitats spent
moretime in front of the pictures that were consistent withtheir
season of capture and laboratory photoperiodconditions compared to
the inconsistent habitat [79].Klopfer [57] performed imprinting
tests with Pekingducklings, using various decoys or images and
demon-strated that the ducklings followed both decoys andimages.
However, the images elicited responses thatwere not strictly
equivalent to the three-dimensionaldecoys; in effect, the ducklings
reacted differently todifferent decoys (according to their colours,
and ac-cording to those they were accustomed to follow) butnot to
different two-dimensional representations ofducks. One aspect of
the task is particularly interestingin this experiment: the
comparison between decoys andimages, because the only difference
between those tworepresentations is the presence or absence of the
thirddimension, that is, the lack of three-dimensionality
wassufficient to cause a decrease in attention or in re-sponses to
feature differences.
4.1.2. Reactions to motion picturesRosenthal et al. [84]
presented green swordtail fe-
males with video-recorded sequences of the same malewhich was
either engaged in an active courtship display,or which performed
similar levels of feeding activity, orwhich remained inactive
(control sequences showedfood particles in movement or an empty
aquarium).Female behaviour patterns differentiated between
thepre-stimulus, stimulus and post-stimulus periods for thethree
stimuli showing a male, but not for the twocontrols; courtship
displays elicited more activity thanany other stimulus, and there
were no significant differ-ences between the responses to the
feeding and inactivesequences.
4.2. Acquired responses in picture recognition
We will now examine studies of acquired reactionswhich fall into
the category of responses that couldpossibly be evidence for the
ability of picture recogni-tion in animals.
In an experiment conducted by Tomonaga et al. [98],a sample of
students and a language trained femalechimpanzee (called Ai) were
trained to recognise videostill pictures of individual faces of
humans or chim-panzees, presented at various orientations. The
experi-ment yielded two main findings indicative of picture
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recognition in the chimpanzee; firstly, it was moredifficult for
Ai to recognise human faces than chim-panzees faces, while the
opposite was shown for hu-man subjects (they had more difficulty
recognisingchimpanzees faces than human faces) and, secondly, itwas
more difficult for her to recognise inverted faces orhorizontal
faces than to recognise upright faces (asimilar, but more
pronounced effect being obtainedwith human participants). It must
be noted that experi-ments conducted with macaque monkeys [8,83]
failed toshow this inversion effect, but this could be due
todifferences in processing rotated complex visual stimuli.For
example, monkeys and some apes could be moresuited to doing this
type of rotation because they live inan environment in which they
often hang upside-down[103], the above findings do not, therefore,
necessarilyimply that subjects were unable to recognise the
pic-tures presented. Nevertheless, Swartz [93] showed, witha visual
fixation habituationdishabituation paradigm,that infant pigtail
macaques (3 months old) could dis-criminate between colour
photographs of faces of threemacaque species (pigtail, cynomolgus,
and stumptailed)when they were presented upright, but not when
thefaces were displayed upside-down.
Other experiments using schematic drawings of mon-keys bodies
indicate that longtailed macaques wereable to discriminate one
monkey from other monkeys,basing their recognition on the limited
informationprovided by the black-and-white shape and texture
oftheir body characteristics [31]. Such findings, however,do not
present any obvious interpretation regardingpicture recognition.
Firstly, as stated by the author ofthe above study it remains
questionable whether themonkey has a knowledge of the
representational natureof the image (Dittrich [31], p. 150).
Secondly, we knowfrom other studies that monkeys can correctly
cate-gorise images of different classes of stimuli by usingsome
absolute cues which are not constitutive of the tobe categorised
stimuli. For example, DAmato and vanSant [13] have shown that their
monkeys used anirrelevant red patch to form the person versus
non-per-son category. Thus, caution is in order before conclud-ing
that seemingly appropriate classification skills meanthat the
animal realises the relationship between apicture and the real
object.
Watanabe and Ito [106] trained pigeons to discrimi-nate between
colour slides of two pigeons faces. Whilediscrimination was
apparently effortless for two stimulieasily discriminated by human
observers, it was difficultwith the stimuli which humans also
struggled to differ-entiate. When the S stimulus was replaced by
itsscrambled parts, subjects did not respond; such a reac-tion
seems to indicate that the birds recognised that thestimuli
depicted on the slides represented conspecifics(which could be
recognised only when the faces werenot scrambled).
Lumsden [64] conducted an experiment with onepigeon; the bird
was trained to discriminate one geo-metric object from two others,
after which transfer wasexamined when the object, its cut-out
photograph, orits line drawing was shown at various
orientations.Response curves were the same for photographs andfor
three-dimensional objects: generalisation was goodat 0, 45, and
135, poor at 180, and was absent for 90.Although the line drawings
were responded to at thelowest rate, the pattern of responding was
similar. In asubsequent experiment, the pigeon was trained to
dis-criminate between the object and its photograph dis-played at
45: the bird then generalised thatdiscrimination to photos
presented at other orienta-tions. We should note however that there
was only onesubject involved in this experiment.
Wilkie et al. [108] trained four pigeons to discrimi-nate
between pictures taken in the vicinity of the loft towhich they had
been raised and other areas they hadnot visited, with four other
pigeons that were nottrained to home being tested as a control. The
resultsshowed that after training with only eight slides,
bothgroups were able to transfer and then to discriminatethe two
categories of slides, but homing pigeons werebetter than nonhoming
pigeons. In the same paper, theauthors mention the experiment of
Honig and Ouellette[45], in which eight pigeons were taught to
discriminatecolour pictures of various views of two ends of a
longroom. Following this task, the pigeons had to discrimi-nate
between the two ends of the real room; a feederwas placed at each
end of the of the test room but onlyone feeder contained food: for
the congruent group, itwas in the same location that had been
positive duringthe previous slide discrimination procedure, while
forthe incongruent group, it was the opposite end of theroom. The
subjects in the congruent condition tookconsistently less time to
find the correct feeder than theincongruent subjects. In a similar
study, Wilkie et al.cited an unpublished study by Willson et al.
[109] inwhich eight pigeons were placed outside the laboratoryfor
20 min prior to each training session in a picturediscrimination
task; for four of the pigeons the placethey had seen outside was
presented as the positivestimuli (relevant place), whereas for the
other fourpigeons, the visited place was not pictured at all
(irrel-evant place). The relevant place birds acquired
thediscrimination more quickly than did the irrelevantplace
subjects. Such experiments suggest that pigeonscan perceive the
correspondence between pictorial stim-uli and the place they
represent.
The above findings provide further information re-garding the
issue of picture recognition. In some exper-iments, animals showed
differential preferences forpictures and appeared to be able to
discriminate be-tween them. However, they did not treat them as
theconspecifics they represented: for example macaques
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did not display social behaviour toward pictures ofconspecifics
[27,47] and ducklings did not react to thepictures in exactly the
same way as they did with decoys[57]. In other experiments,
behaviours were indicativeof picture discrimination [79,84] but
were not specifi-cally directed toward the stimuli. While visual
experi-ence with real locations can facilitate the discriminationof
photographs by pigeons, and vice versa, it is not yetclear how this
facilitation occurs.
Table 3 summarises the methods, species and mainresults of the
studies that could be indicative of picturerecognition in animal
species.
5. Difficulty with picture recognition
This section reviews those studies which show adifficulty or
failure of the subject to react to 2-D stimulias if they were
meaningful or 3-D stimuli.
5.1. Spontaneous responses
It is important to note that socially salient stimulipresented
as pictures do not always elicit overt re-sponses in birds or even
in monkeys. Butler andWoolpy [9] studied visual attention in rhesus
monkeyssubmitted to various slides or motion pictures of
otherrhesus monkeys but their results are not easy to inter-pret
because they appear to be quite contradictory; theamount of visual
attention given to slides of conspe-cifics was not different from
attention devoted to anhomogeneous illuminated screen. Such a
result seems toimply that the monkeys did not recognise the slide
asrepresenting one of their conspecifics although viewingbehaviour
(and thus attention) was more important (i.e.monkeys looked longer)
when motion pictures wereprojected in the normal orientation than
when theywere projected upside-down.
Table 3Experiments which could indicate picture recognition in
animals
Task Nature of pictures Species Results Reference
Dog Fox [36]PaintingsBehavioural observations Sniffing the areas
normally investi-gated on conspecifics
Life-size colour slides and Franklin andBehavioural observations
Sheep Following images of conspecificsfilms Hutson [37]
Transfer occursColour slides Honig andPigeonTransfer of
discrimination of twoOuellette [45]ends of a room from pictures
to
real placesConsistent choice Humphrey [47]Choice between two
visual stimuli Rhesus monkeyColour slides and films
(plain field or various stimuli)Humphrey [48]Discrimination of
individuals de-Rhesus monkeyNovelty preference Slides
pends on subjects experienceSpecifics cells in the temporal
cor-Electrophysiological recording Kendrick andSheepColour slides
and black-and-
white drawings tex respond to faces Baldwin [53]Colour
filmBehavioural observations Pekin duckling Spontaneous following
of moving Klopfer [57]
ducksTransfer of discrimination between PigeonCut-out photograph
and line Same type of curve depending on Lumsden [64]
drawing object orientation, but overall levelobjects at various
orientationsof responding considerably lessto pictures
Measurement of time spent in Roberts andDark-eyed junco
Spontaneous choice of appropriateColour slideshabitats according to
the seasonfront of slides of habitats Weigl [79]
Measurement of female interest Colour video images Green
swordtail Frequency of behavior patterns in Rosenthal et al.females
depends on the male [84]courtship
Swartz [93]Colour photographsDiscrimination between faces of
Pigtail macaque Difficulty recognizing invertedthree macaque
species faces
Discrimination between faces of Tomonaga etColour video still
pictures Difficulty recognizing invertedChimpanzeeal.
[98]faceschimpanzees and humans indi-
vidualsWilkie et al.Colour slidesDiscrimination between pictures
of Pigeon Discrimination between pictures
facilitated by prior experience withlocations [108]the
locationDiscrimination between picturesPigeonColour
slidesDiscrimination between pictures of Willson et al.
[109]facilitated by prior experience withlocationsthe
location
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Social recognition experiments which used pictorialstimuli with
hens, failed to indicate any transfer ofdiscrimination from live
birds to photographs [19]: henspreferred flock-mates rather than
unfamiliar conspe-cifics (even when they saw only their heads and
necks)when presented with live stimuli, but they failed toshow any
preference with photographic stimuli. Similarfindings were observed
in a study of hens shown videosequences [21]: hens neither took
longer to eat nearunfamiliar conspecifics than near flockmates, nor
nearhigh-ranking flockmates than near low-ranking flock-mates, as
they usually did when they saw live stimulibehind a clear screen.
Pigeons also failed to exhibit anynatural social response when they
were presented withlife-size moving video images of conspecifics
[86].
5.2. Acquired responses
5.2.1. Reactions to still picturesWinner and Ettlinger [110]
trained two chimpanzees
with no prior experience with photographs in a
match-ing-to-sample task. First, the subjects had to match
realobjects to real objects and subsequently had to matchobjects
with their photographs. Initially, they were un-able to perform the
task successfully with performanceremaining at chance levels for
the first 4 days and thenrising moderately, but not consistently,
above chance.The second experiment of Winner and Ettlinger
at-tempted to replicate the results obtained by Davenportet al.
[17] (see above); in their experiment, two chim-panzees with no
prior experience with photographswere required to transfer a
discrimination between pairsof objects that were felt but not seen
to their photo-graphic representations or vice versa. The new
subjectsresponded significantly above chance when tested onlywith
objects and at chance level when required totransfer a learned
response from a felt object to aphotograph or from a photograph to
a felt object. Theauthors suggest that in Davenport et al.s
experimentthe objects were not paired by size: consequently,
sub-jects might have succeeded by choosing the bigger one.
Jitsumori [50] has also demonstrated that picturerecognition is
difficult for untrained animals. The taskconsisted of training four
monkeys (two of them hadprior experience with discrimination
problems betweenpictures containing or not containing monkeys)
andfour experimentally naive pigeons to discriminate be-tween
normally oriented displays and topbottom re-versals. If the monkeys
saw meaningful objects in theseslides, then transfer was supposed
to occur with variousnovel slides; subjects were trained with a
go:no-godiscrimination task with colour pictures of full humans,and
then tested with other pictures of humans, mon-keys, birds, mammals
and man-made objects. Bothmonkeys and pigeons showed good transfer
to novel
human pictures but when tested with other pictures,levels of
performance revealed considerable interindi-vidual variation,
namely, in pigeons and in nonexperi-enced monkeys transfer was
relatively good for somepictures but not for others. Thus, the
overall perfor-mance was inconsistent and successful transfer might
beexplained by perceptual similarities among the slidespresented in
a fixed orientation. In this study, only oneof the experienced
monkeys produced results suggestiveof the perception of meaningful
objects in pictures.
Another experiment [19] attempted to establishwhether or not
transfer between geographical locationsand photographs of those
locations occurred in homingpigeons. Eight pigeons were trained to
discriminatephotographs of two geographical locations, havingbeen
given visual experience of a real geographicallocation beforehand.
Half of the birds were transportedto one of the two locations that
appeared in the photo-graphs, while the remaining subjects were
transportedto a third, irrelevant location. Although there was
nosignificant difference in acquisition or transfer to novelstimuli
between the two groups, the authors suggestthat this might be due
to their methods (inadequateamount of experience outside or lack of
immediatereward for learning about the environment), but also
todifferences between human and bird vision (see above).Moreover,
it is possible that the pigeons were process-ing the far-distance
views and the near-distance views indifferent manners.
A study with laying hens by Bradshaw and Dawkins[7] attempted to
replicate the experiment performed byDasser [14] (see above) with
macaques. Hens weretrained to discriminate between slides of either
familiaror unfamiliar conspecifics and were then presented
withnovel views of these birds; during training, the right-hand
side of a hens head was presented, whereas thenovel stimulus set
was composed of pictures of left-hand side of the corresponding
hens head, a frontalview, or a view of the tail or feet. The birds
failed togeneralise discrimination from training slides (both
fa-miliar and unfamiliar) to novel view categories and theauthors
concluded that their study provided no evi-dence that the hens
perceived the slides presented asrepresentations of their group
members. Ryan and Lea[86] obtained somewhat comparable results in a
studyin which pigeons and chickens were trained to discrimi-nate
between slides of two individuals (two pigeons ortwo chickens). For
both species, the chicken slides werelearned faster and better than
the pigeon slides, withthe pigeons performances being much worse
thanchickens on both chicken and pigeon stimuli. More-over, only
one pigeon out of six was able to discrimi-nate slides of pigeons,
and none learned to discriminatebetween two different stuffed
pigeons, even though asubsequent experiment proved that they
readily dis-criminated individual live pigeons.
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Table 4Experiments showing difficulties recognizing pictures in
animals
Nature of picturesTask Species Results Reference
Color slides Bradshaw andDiscrimination not facilitated byLaying
henDiscrimination between picturesfamiliarityof familiar or
unfamiliar con- Dawkins [7]
specificsBlack-and-white and colour Butler andBehavioural
observations Rhesus monkey No spontaneous responses; noslides and
motion pictures more attention to slides of con- Woolpy [9]
specifics than to a homoge-neously illuminated screen
Spontaneous discrimination be- Dawkins [18]Discrimination occurs
for liveDomestic hensLife-size colour photographstween familiar and
unfamiliar hens but not for photographs of
hensconspecifics, either live or pre-sented on photographs
Dawkins et al.Discrimination between pictures Colour slides
Discrimination between picturesPigeonof locations [19]of locations
is not facilitated by
experienceDEath andLife-size colour video sequences Domestic
hensSpontaneous discrimination be- Discrimination of live hens
but
tween familiar and unfamiliar Dawkins [21]not for those
presented on videoconspecifics, either live or pre-sented on
video
Jitsumori [50]Colour slides Difficulty transferring
discrimi-Monkey and pi-Discrimination of right-side-upand
upside-down orientations geon nation to various classes ofof scenes
slides
Transfer occurs only when theDiscrimination transfer between
Patterson-Kane etColour video images Domestic hentwo stimuli to
discriminate havevarious hens and objects to al. [73]different
colourstheir pictures
Colour slides and moving videoDiscrimination of individual pi-
Ryan and LeaPigeon and Great difficulty in
identifying[86]chickengeons and chickens and be- novel views of an
individual, noimages
spontaneous responseshavioural observationsCategorization of
objects and Watanabe [104]Correct transfer from objects
toPigeonColour slides
their pictures into food and pictures occurs only for a
natu-non-food categories or into ral category (food)arbitrary
categories
Matching an object touched but Chimpanzee Failure to match
objects withBlack-and-white and colour full- Winner andEttlinger
[110]unseen to its photograph their photographssize photographs
A study by Watanabe [105] is particularly interestingin the
discussion of picture recognition because it sug-gests that
objectpicture equivalence can be performedrelatively easily when
there is some functional basis.Twenty-four pigeons were divided
into four experimen-tal groups: two object-to-picture groups and
two pic-ture-to-object groups; one of the object-to-picturegroups
and one of the picture-to-object groups weretrained on a natural
concept (food objects were S forhalf, and non-food objects were S ,
and it was theopposite for the remaining half) while the other
twogroups were trained on a pseudoconcept (an arbitrarygrouping of
edible and nonedible objects as positive andnegative stimuli). When
tested with the natural concept,the subjects showed a good transfer
of discrimination inboth object-to-picture and picture-to-object
conditions,but no transfer was observed with the pseudoconcept.Such
a result indicates that picture recognition candepend on the
consistency of the task.
5.2.2. Reactions to motion pictureAttempts to train domestic
hens to transfer from real
stimuli to video images generally produced negativeresults,
although, depending on experimental condi-tions, the birds could
use some features of the patterns(e.g. the colour) in their
discrimination [73]. It wasconcluded from this study that complex
video images,such as those required to recognise social stimuli,
arenot equivalent to the real stimuli. In addition, somepigeons did
not transfer a learned discrimination fromlive conspecifics to
their photographs and had greatdifficulties in discriminating
between slides of individu-als (although they easily discriminated
live conspecifics).
The results of the studies reported in this section aresomewhat
contradictory to the findings summarised inSections 3 and 4. In
effect, they demonstrate that picturerecognition in animals is not
obvious and is dependenton experimental factors. In several
experiments, mon-keys and birds (such as pigeons and chicken)
failed todisplay an interest in photographs of
conspecifics.Moreover, different tasks involving picture
recognitionhave reported a failure to demonstrate such an
ability;thus chimpanzees failed to realise a cross modal match-ing
and only one monkey (out of four), which was
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already familiarised with photographs, was able to dis-criminate
the orientation of photographs. In addition,it was shown that
experience with a particular place didnot facilitate the
discrimination between that locationand another. Finally, it was
shown that when testedwith a natural concept (food), pigeons
transferred adiscrimination from object to picture and vice
versa,but they did not demonstrate such transfer when theywere
tested with an arbitrary pseudoconcept. In brief,the experiments
summarised in this section highlight theimportance of biological
relevance in picture recogni-tion tasks.
Table 4 presents an outline of the investigations
thatdemonstrated a failure to recognise pictures in animals.
6. Conclusion
One of the main conclusions of this survey is thatvisual stimuli
presented as pictures (either as black-and-white photos, colour
photos, slides, or videos) are notnecessarily immediately
recognised by non-human andeven human subjects. In this final
section, we willsummarise the principal results concerning
picturerecognition in the much-studied species and attempt
toidentify some of the factors which may be responsiblefor the
apparent difficulties in recognising pictorialstimuli. Finally, we
will suggest some possible stepswhich could be useful in describing
picture processing;ranging from feature discrimination, to
correspondence,and ultimately, to strict equivalence between a
3-Dobject and its pictorial representation and a consider-ation of
the issue of confusion between pictures and theobjects they
represent.
6.1. Summary of main findings
Very young humans appear to be able to recognisephotographs from
2 or 3 months of age and at an evenearlier age they are likely to
discriminate real objectsfrom their pictorial representations.
Paradoxically, pic-ture recognition seems to present greater
difficulties foradults who are unaccustomed to seeing
photographsand drawings. Thus, it could be hypothesised that suchan
ability is innate but that this ability diminishes if theperson has
grown up without opportunities to see 2-Drepresentations. In this
case (which is less and less likelyto happen in contemporary
societies), such individualsare accustomed to seeing the world of
objects aroundwhich are inevitably characterised by the presence
offeatures like colours, depth, and motion parallax; whenpresented
with pictures, these people, who have lived inan exclusively 3-D
environment, would experiencedifficulties and some familiarity
and:or training withpictures would then be required in order to
recognisethe stimuli which lack those features.
The available literature is quite convincing concern-ing the
abilities of several animal species familiar withpictures to
recognise such 2-D stimuli. In mammals ingeneral (but with most
evidence coming from experi-ments conducted with monkeys and apes),
it also seemsthat picture recognition is possible for both adults
andyoung even if the animals have never been exposed toany picture
prior to the experiment. In this latter case,recognition seems to
be more difficult and appears todepend on the nature of stimuli and
on the experimen-tal conditions (see below). Some experiments, in
whichsubjects have to transfer what they have learned withreal
objects to the pictures of these objects (e.g. incross-modal
transfer between touch and vision), haveshown that these tasks
present serious difficulties forthe subjects. Furthermore, because
some training isoften necessary in order to perform picture
recognitiontasks, the training phase probably allows subjects to
getfamiliarised with pictures prior to testing; this require-ment
implies that subjects are rarely naive with respectto viewing
pictures or their discrimination.
The studies reporting spontaneous responses to pic-tures are
interesting because they can provide usefulindications on the
perceptual and cognitive processesinvolved in picture recognition
performed by trulynaive subjects. These studies have shown that
monkeysand other mammals (sheep and dogs) can, at first
sight,adaptively respond to various animals or foods (al-though
pictures of conspecifics seem to be respondedmore easily to than
pictures of other categories ofstimuli) presented on slides. We can
speculate that thisease presumably expresses the fact that these
animalsconfuse the real objects and their pictures;
nevertheless,this recognition can be quite precise, if we consider
thatsome animals are able (e.g. [4]) to differentiate individu-als
from their own breed from individuals belonging toother breeds. It
is also worth noting that transfer isgenerally better for pictorial
stimuli which better match(at least for a human viewpoint) real
objects, that is,motion films are more easily recognised than still
pic-tures, slides are better recognised that colour photo-graphs,
the latter leading to better performancecompared to black-and-white
photographs and linedrawings.
It may be an interesting observation that in the firstsection of
our review (convincing demonstrations),studies concerning mammals
are more numerous thanstudies concerning birds, and that the
opposite is trueof the third section (difficulties with picture
recogni-tion). Actually, the pattern of results obtained withbirds
is quite different compared to other zoologicalgroups; a divergence
which may be explained in part bythe fact that bird vision is
different from mammalvision. With regard to pictures, photographs
used inexperiments with these animals are usually matched forhuman
vision and lack some critical features of birds
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143165160
vision, such as UV light [111], and offer false
colourrepresentations for these subjects because the stimuliare
based on the trichromatic colour vision of humansand not on the
pentachromatic vision of birds (e.g.[33]). Motion pictures are also
matched for humanvision. In motion pictures, still images are shown
insuccession to produce the impression of a continuousmoving image.
But the frequency at which a flickeringstimulus starts to appear
continuous is higher in birds(notably in pigeons and in chicken)
than in humans[20,107]. Further, chickens are myopic (they can
recog-nise a conspecific only at a very short distance [18]) andit
is possible that they process far-distance views in amanner
distinct from that used for near-distance views.The above
considerations may thus explain some of theproblems encountered by
birds in interpreting picturesand why, for example, transfer is
sometimes better forblack-and-white photographs than for colour
photo-graphs. Nevertheless, some of the experiments reportedearlier
showed a good transfer from objects to picturesor confusion between
the slides or films and the animalsthey represented.
Finally, a handful of studies have convincinglydemonstrated that
responses to pictures are not limitedto birds and mammals;
reptiles, fishes and even inverte-brates reacted strongly to video
images depicting bio-logically significant stimuli (conspecifics,
prey orpredators, for examples). It is, however, not surprisingthat
animals of different phyla respond to salient visualcues in similar
ways, i.e. as they would respond to realobjects; for many years,
experimental ethologys tech-niques have used visual lures for
identifying the stimu-lus characteristics of social and aggressive
behaviour inanimals (e.g. the pioneering and now classic
studiesemploying cardboard models of a Herring gulls headby
Tinbergen and Perdeck, [95]).
6.2. Factors influencing picture recognition
The experiments reported in the section on acquiredresponses to
pictures fail to provide a clear and defini-tive answer to the
question of capacities of differentanimal species for processing
pictorial representation ofobjects. Given that the most advanced
and detailedstudies have been conducted with birds, it may beuseful
to list some of the factors that authors havehighlighted as playing
a determining role in the extentand limits of picture recognition.
Some of these factorshave been summarised in the discussion of
dEath andDawkins [21] article reporting a failure of domestichens
to discriminate between familiar and unfamiliarconspecifics on
videos and are thoroughly described inthe review by dEath [20]. A
first and obvious factor hasto be mentioned: pictures, being still
or in motion, area