Tactile sensitivity of normal and autistic children

Somatosensory and Motor Research, March–June 2007; 24(1–2): 21–33

Tactile sensitivity of normal and autistic children

BURAK GUCLU1, CANAN TANIDIR2, NAHIT MOTAVALLI MUKADDES2, &

FATIH UNAL3

1Biomedical Engineering Institute, Bogazici University, Istanbul, Turkey, 2Child and Adolescent Psychiatry

Department, Autism Clinic, Istanbul Medical School, Istanbul University, Istanbul, Turkey, and 3Child and Adolescent

Psychiatry Department, Hacettepe University, Ankara, Turkey

(Received 4 June 2006; revised 20 October 2006; accepted 11 December 2006)

AbstractMany children with autistic spectrum disorders have unusual reactions to certain sensory stimuli. These reactions vary alonga hyper- to hypo-responsivity continuum. For example, some children overreact to weak sensory input, but others do notrespond negatively to even strong stimuli. It is typically assumed that this deviant responsivity is linked to sensitivity,although the particular stage of sensory processing affected is not known. Psychophysical vibrotactile thresholds of six malechildren (age: 8–12) who were diagnosed to have autistic spectrum disorders and six normal male children (age: 7–11) weremeasured by using a two-alternative forced-choice task. The tactile stimuli were sinusoidal displacements and theywere applied on the terminal phalanx of the left middle finger of each subject. By using a forward-masking paradigm, 40- and250-Hz thresholds of the Pacinian tactile channel and 40-Hz threshold of the Non-Pacinian I tactile channel weredetermined. There was no significant difference between the thresholds of autistic and normal children, and the autisticchildren had the same detection and masking mechanisms as the normal children. The sensory responsivity of each subjectwas tested by clinical questionnaires, which showed again no difference between the two subject groups. Furthermore, nosignificant correlations could be found between the questionnaire data and the psychophysical thresholds. However, therewas a high correlation between the data from the tactile and emotional subsets of the questionnaires. These results supportthe hypothesis that the hyper- and hypo-responsivity to touch, which is sometimes observed in autistic spectrum disorders, isnot a perceptual sensory problem, but may probably be emotional in origin.

Keywords: Somatosensation, Pacinian channel, non-Pacinian I channel, hyper-sensitivity, hypo-sensitivity, pervasivedevelopmental disorder

Introduction

Autistic disorder (AD) is among the pervasive

developmental disorders (autistic spectrum disor-

ders), which include Rett’s disorder, childhood

disintegrative disorder, Asperger’s syndrome, and

pervasive developmental disorder not otherwise

specified (PDD-NOS). AD occurs in about 20 out

of 10 000 children, and an autistic spectrum disorder

in about 60 out of 10 000 (Fombonne 2003), but

the prevalence is increasing probably due to

new diagnostic criteria and heightened awareness.

According to the Diagnostic and Statistical Manual

of Mental Disorders (DSM-IV-TR, American

Psychiatric Association), the diagnostic criteria

for AD are (1) qualitative impairment in social

interaction, (2) qualitative impairment in commu-

nication, (3) restricted repetitive and stereotyped

patterns of behavior, interests, and activities,

(4) delays or abnormal functioning with onset prior

to age 3 years, and (5) the disturbance is not better

accounted for by Rett’s disorder or childhood

disintegrative disorder. AD is a lifelong disorder

Correspondence: B. Guclu, PhD, Assistant Professor, Biomedical Engineering Institute, Bogazici University, Bebek, Istanbul 34342, Turkey. Tel: +90 212

3596413. Fax: +90 212 2575030. E-mail: [email protected]

ISSN 0899–0220 print/ISSN 1369–1651 online � 2007 Informa UK Ltd.

DOI: 10.1080/08990220601179418

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and its symptoms appear mostly in the first 2 years

of life. Although people with AD have normal life

expectancy, the quality of their lives is hindered

by impaired social interaction, communication,

imagination, and limited daily activities and interests.

Many children with autistic spectrum disorders

have also strong reactions, positive or negative,

to sensations such as smell, touch, sight, or sound

(Ornitz 1983; Baranek 2002; Harrison and Hare

2004; Rogers and Ozonoff 2005). Since these

reactions are not yet included in the diagnostic

criteria, there is limited experimental data on this

subject and the basic sensory processing in autistic

children is still not clear. Specifically, some children

may have tactile defensiveness and avoid tactile

experiences, or they may react with pleasure to

rough-and-tumble games. Unusual sensory features

appear early in the development of autistic children

and in the absence of known peripheral dysfunction

(Baranek 2002). This deviant behavior, which may

vary in a hyper- and hypo-responsivity continuum,

was attributed to sensory registration and modula-

tion dysfunctions in various developmental disabil-

ities, including AD (Ayres 1979). It was shown that

autistic children who registered tactile input but

failed to modulate it responded better to therapy than

those who were hypo-responsive (Ayres and Tickle

1980). However, the tests used in such clinical

settings typically involve qualitative questions and

often do not allow for the physical measurement of

sensitivity. This makes the interpretation of the

clinical data difficult with respect to the neuropsy-

chology of touch. In this study, the main goal is to

measure the psychophysical thresholds of autistic

children in accordance with the current theories of

the tactile sense. This is not only important from a

basic scientific point of view, but may be significant

regarding the etiology of the disorder.

It may be assumed that a hypo-responsive child

does not react to a normal-level sensory input

adequately because of hypo-sensitivity. Similarly,

a hyper-responsive child may overreact to a normal-

level stimulus because of hyper-sensitivity. If the

absolute psychophysical thresholds of the autistic

children correlate with hypo- and hyper-responsivity,

this supports the hypothesis that the clinical sensory

problems of autistic children are due to perceptual

deficits in processing tactile inputs. Inversely, if there

is no correlation, sensory problems in autism may be

attributed to higher cognitive factors (e.g., emo-

tions). Tactile thresholds of autistic children can also

be compared to the thresholds of normal children to

find out if there are perceptual deficits unrelated to

clinical data. This is an important test, because even

if the clinical sensory problems are not perceptual, or

even if there are no clinical sensory problems, there

may be perceptual deficits clinically unreported.

Since it is difficult to elicit the required duration/

level of performance necessary for task completion in

the autistic population, data from only six higher-

functioning autistic children could be obtained.

Therefore, although the psychophysical results are

reliable, the neuropsychological conclusions given

in this paper are tentative, but may nevertheless

be helpful for guiding new experiments and

assessments.

According to the four-channel theory of tactile

perception (Bolanowski et al. 1988; Gescheider et al.

2001), the tactile psychophysical channels are media-

ted by four receptor systems which have specific

frequency characteristics. Humans are most sensitive

to vibrations at frequencies between 200 and 300Hz.

This is attributed to the Pacinian (P) channel which

has the lowest threshold in that frequency range.

In this study, detection by the P tactile channel was

tested at two frequencies (40 and 250Hz).

Furthermore, a masking procedure was used to

elevate the sensitivity of the P channel and to test

the detection of a 40-Hz stimulus by the

Non-Pacinian I (NP I) channel, which is mediated

by Meissner corpuscles in the skin. Therefore, the

results yield information about the basic operation

(i.e., detection and masking) of the two prominent

psychophysical channels. These experiments were

previously performed on normal adults (Guclu and

Bolanowski 2005), but not on normal or autistic

children. The experimental data may also be used in

modeling studies (see, e.g., Guclu and Bolanowski

2002, 2004). Minor parts of this paper were

presented as abstracts (Tan|d|r et al. 2005; Guclu

et al. 2006).

Materials and methods

Subjects

The experiments do not pose any harm and they

adhere to the US NIH ethical guidelines for testing

human subjects. The parents of the subjects gave

written consent and approvals were obtained from

the Ethics Committee for Human Subjects of

Bogazici University and the Istanbul Medical

School Ethics Committee of Istanbul University.

None of the subjects had any conditions that could

interfere with the psychophysical experiments, such

as being on psychotropic medication or having

serious medical or neurological diseases.

Normal group. Six normal male children (N1–6;

age range: 7–11, mean age: 9.2) were recruited

locally from a primary school. Their psychiatric

interviews did not show any psychopathology.

They did not have behavioral problems according

22 B. Guclu et al.

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to the Child Behavior Checklist (T-scores560; see

below).

Autistic group. Six male children (A1–6; age range:

8–12, mean age: 10.0) diagnosed with autistic

spectrum disorders were recruited from the Autism

Clinic of the Istanbul Medical School. Five of those

children had AD diagnosis, and one had PDD-NOS

diagnosis according to the DSM-IV-TR. Their

diagnostic interviews were done by two child

psychiatrists, of whom one is the founder of the

autism clinic and has 14 years of experience with this

subject group. All autistic subjects were higher

functioning (IQ470) and their additional diagnostic

data are given in Table I. There was no significant

age difference between the normal and autistic

subject groups (t-test; p¼ 0.415).

It is important to note that initially 12 higher-

functioning autistic subjects were invited to the

study. However, only six subjects, presented here,

could complete the experiments. One subject quit

because of family problems; the remaining five

subjects could not perform the psychophysical task

described below because of attention deficits.

Therefore, well-controlled psychophysical experi-

ments may be difficult to accomplish even for the

higher-functioning autistic subjects.

Clinical assessments

Routine psychiatric assessments of the children were

performed according to the DSM-IV-TR.

Specifically, language development, socialization

capability, stereotypical behaviors, and sensory

responses displayed in daily life were investigated.

Behavioral problems of normal children were

measured by the Child Behavior Checklist; and the

severity of autistic symptoms in autistic children

were measured by the Childhood Autism Rating

Scale. The sensory responsivities of normal and

autistic children were tested by two questionnaires

(the Sensory Profile and the Touch Inventory),

which are typically used in the field of occupational

therapy.

Childhood Behavior Checklist (CBCL; Achenbach

1991). This test has 118 items which describe

specific behavioral and emotional problems, plus two

open-ended items for reporting additional problems.

By using a three-point scale, normal subjects’ parents

rated their children based on their activities, social

relations, and school performance. Raw scores were

converted to normalized T-scores. T-scores above 60

represent deviant behavior. All normal subjects had

T-scores below 60.

Childhood Autism Rating Scale (CARS; Schopler et al.

1980). This is a 15-item test which combines

parent reports and direct observations by profes-

sionals. The items are relationship with people,

imitation, affect, use of body, relation to non-human

objects, adaptation to environmental change, visual

responsiveness, auditory responsiveness, near recep-

tor responsiveness, anxiety reaction, verbal commu-

nication, nonverbal communication, activity level,

intellectual functioning, and the clinician’s general

impression. The total scores range from 15 to 60.

Scores between 30 and 36 indicate mild autism, and

Table I. Clinical data of normal (N1–6) and autistic (A1–6) subjects.

Subject Age Diagnosis Comorbidity Sensory problems IQ CARS TI SP

SP

(touch)

SP

(emotional/social)

N1 11 Normal – Normal – – 54 (95%) 87 18 17

N2 8 Normal – Normal – – 59 (99%) 106 22 22

N3 7 Normal – Normal – – 44 (65%) 80 17 19

N4 8 Normal – Normal – – 30 (8%) 30 3 5

N5 10 Normal – Normal – – 44 (65%) 60 12 14

N6 11 Normal – Normal – – 42 (55%) 44 6 15

A1 8 HFA None Hypo-responsivity 90 30 36 (26%) 60 8 17

A2 12 HFA ADHD Normal 70 30 42 (55%) 68 13 12

A3 11 HFA ADHD Hyper-responsivity 108 32 58 (99%) 94 21 20

A4 8 HFA ADHD Hypo-responsivity 73 33 49 (85%) 58 10 18

A5 10 HFA ADHD Normal 72 31 53 (94%) 44 7 18

A6 11 PDD-NOS ADHD Hypo-responsivity 80 26 37 (30%) 42 5 11

The results of the psychiatric assessments are given in columns 3–5. The questionnaire data are given in columns 6–9. For the SensoryProfile questionnaire, the responses from the entire question set, the touch subset, and the emotional/social subset are given separately.HFA¼ high-functioning autism; PDD-NOS¼ pervasive developmental disorder not otherwise specified; ADHD¼ attention-deficit/hyperactivity disorder; IQ¼ Intelligence Quotient; CARS¼Childhood Autism Rating Scale; TI¼Touch Inventory for Elementary-School-Aged Children (raw scores and percentiles); SP¼Sensory Profile questionnaire (positive scores, see text). Empty fields were not measured.

Tactile sensitivity of normal and autistic children 23

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scores above 36 indicate severe autism. CARS scores

of autistic subjects are given in Table I.

Sensory Profile (SP; Dunn and Westman 1997). This

is a 125-item test, with questions grouped under the

following categories: Auditory (10 questions), Visual

(18 questions), Activity Level (6 questions), Taste/

Smell (10 questions), Body Position (11 questions),

Movement (22 questions), Touch (24 questions),

Emotional/Social (24 questions). The subjects’

parents responded to these questions by using a

five-point Likert scale (the child displays the

described condition 100, 75, 50, 25, or 0% of the

time). The answers were scored according to

the percentages of typical children who seldom or

never displayed the described behaviors ( p%) given

by Dunn and Westman (1997). Specifically, if the

Likert-scale response to a particular question was

greater than the percentage of typical children who

displayed the given behavior, that is, (100� p)%, the

question was scored as positive, otherwise negative.

Here, the number of positive scores is presented. A

high positive score implies more behavioral problems

related to the given subset of the questionnaire.

Touch Inventory for Elementary-School-Aged Children

(TI; Royeen and Fortune 1990). This test has

26 items exclusively for tactile defensiveness.

The questions were answered by the subjects as

‘‘no’’, ‘‘a little’’, or ‘‘a lot’’. The number of ‘‘no’’

responses was weighted by 1, the number of ‘‘a little’’

responses by 2, and the number of ‘‘a lot’’ responses

by 3. The weighted sum of responses was calculated

as the raw score, which was then converted to a

percentile (q%) according to the normative data

given by Royeen and Fortune (1990). In this paper,

both the raw scores and percentiles are presented.

(100� q)% signifies the percentage of normative

subjects who selected more items associated with

tactually defensive behaviors. A high score on this

test implies hyper-responsivity, and a low score

implies hypo-responsivity.

Apparatus

The apparatus was similar to the one used by Guclu

and Bolanowski (2005). The stimuli were generated

by a digital-to-analog converter card (DaqBoard/

2000; IOtech, Inc., Cleveland, OH, USA) in a

personal computer and amplified to drive a mechan-

ical shaker (V201; Ling Dynamic Systems Ltd,

Royston, Herts., UK). The displacements generated

by the shaker were measured by a LVDT

(HR050; Measurement Specialties, Inc., Hampton,

VA, USA). The stimuli were applied with a circular

contactor (radius: 2mm, area: 0.126 cm2) without a

surround. The subject’s left middle finger was

molded in modeling clay to prevent movement.

The fingertip was continuously monitored by a

CCD camera and the experiment was stopped if

the subject moved the stimulated finger. The

response of the subject was obtained by buttons on

a response box connected to the computer. The time

intervals (see below) were signaled to the subject

by LED lights of the response box. Additionally,

the subject knew if his/her response was correct or

incorrect by the signaling of another LED.

Tactile stimuli

The stimuli were bursts of sine waves superimposed

on a 0.5-mm static indentation. The burst stimuli

started and ended as cosine-squared ramps with

50-ms rise and fall times. The stimulus timing

diagrams are given in Figure 1. The duration of the

test stimulus was 0.5 s as measured between half-

power points of the bursts. Depending on the

experiment, the frequency of the test stimulus was

either 40 or 250Hz (Figure 1A). In forward-masking

experiments, a high-level 250-Hz masking stimulus

was applied in the beginning of each time interval

(Figure 1B). The duration of the masking stimulus

was 1 s and there was a 150-ms gap between the

masking and test stimuli when the test stimulus was

presented. The test stimulus occurred either in the

first or the second time interval. Each interval was

cued by a LED light and lasted 2 s.

Most clinical descriptions of sensory problems

refer to more complex stimuli, for example, textures.

However, any tactile stimulus, for example, texture,

tap, or pressure can be considered to be a weighted

sum of sinusoidal vibrations with different frequen-

cies according to Fourier decomposition. The

response of a system, that is, the tactile sensory

system, is typically first characterized with respect to

a single-frequency vibration, before the effects of

more complex stimuli like textures, which have both

spatially and temporally varying parameters, can be

understood.

Psychophysical procedures

The terminal phalanx of the left middle finger was

stimulated on the midpoint of the volar surface.

All subjects except N2 declared that they were right-

handed. The average surface temperature of the

second finger in the left hands of both normal and

autistic subject groups was 33–35�C during the

experiments. Each experimental session took about

1–2 h with short breaks. Four to five sessions per

subject were needed to complete the experiments.

Threshold measurements from autistic subjects

24 B. Guclu et al.

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required more trials, because their attention some-

times drifted away from the tactile stimulus during

the experiments.

The experiments used the two-interval forced-

choice task, which generates results independent of

the subject’s criterion. The subject’s task was to

decide whether the test stimulus was in the first or

the second interval. The stimulus levels were

changed by using an up–down rule that tracked

thresholds at 0.75 correct probability of detection

(Zwislocki and Relkin 2001). A random selection of

intervals yields 0.50 correct probability, and any bias

for choosing the intervals results in no tracking.

Therefore, if the subject gave inconsistent responses

deliberately, no measurement could be obtained.

This procedure avoids inaccurate threshold measure-

ments. The up–down rule increased the stimulus

level by 1 dB for each incorrect response, and

decreased the stimulus level by 1 dB for three,

not necessarily consecutive, correct responses. The

experiment automatically stopped when the stimulus

level was within �1dB range for the last 20 trials.

The mean intensity level in the final �1dB range was

recorded as the threshold.

First, absolute thresholds of the subjects were

measured separately at 40 and 250Hz without using

a masking stimulus (Figure 1A). Then, the sensitivity

of the P channel was elevated by using a 250-Hz

masking stimulus. During this experiment, the

250-Hz masking stimulus preceded a 250-Hz test

stimulus (Figure 1B). The masking level was

adjusted to yield a shift (5–15 dB) in the threshold

of the P channel for each subject. Finally, the

threshold of the NP I channel was supposedly

measured at 40Hz by using the adjusted masking

level. For this experiment, the 250-Hz masking

stimulus preceded a 40-Hz test stimulus

(Figure 1B). This forward-masking procedure is

required to measure the threshold of the NP I

channel at 40Hz, because at 40Hz, the P channel is

usually more sensitive than the NP I channel (Guclu

and Bolanowski 2005).

Analyses

The psychophysical thresholds are presented in dB

units referenced to 1 mm peak displacement ampli-

tude. The statistical analyses were performed in

MATLAB (The MathWorks, Inc., Natick, MA,

USA). During the psychophysical experiments,

each measurement was repeated 4 times for each

subject. The graphs plot the averages of those

measurements. The error bars are the standard

errors of the mean. Hypothesis testing (two-sample

t-tests) was performed by using the threshold-

displacement values. The correlation coefficients

Figure 1. Stimulus timing diagrams for experiments without masking (A) and with masking (B). The 40- or 250-Hz teststimulus (small burst) occurs either in the first interval (cued by red light) or the second interval (cued by green light).The subject responds while the yellow light is on. In masking experiments, a high-level 250-Hz masking stimulus(large burst) is present in the beginning of each interval.

Tactile sensitivity of normal and autistic children 25

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(Pearson’s) were found between the questionnaire

results and the threshold-displacement values.

Results

Questionnaire data

The results obtained from the Sensory Profile (SP)

and the Touch Inventory (TI) questionnaires are

given in Table I. Both normal and autistic subject

groups have members who scored high or low on

these tests. According to the total questions available

in a given test or the percentile result, the results

were grouped into three equal ranges. The results are

considered to be low if they are in the lowest range,

high if they are in the highest range, and normal if

they are in the middle range.

N1, N2, A3, A4, and A5 have tactually

defensive (hyper-responsive) behaviors according

to the TI (467%). On the other hand, N4, A1,

and A6 show hypo-responsivity according to the

TI (533%). The other subjects have scores

between the scores of the subjects who show

hypo- and hyper-responsivity. It is interesting to

compare the TI data of autistic children with

clinically reported somatosensory problems, which

are listed in the fourth column of Table I. The

psychiatric evaluation revealed that A1, A4, and

A6 did not react normally to pain. A3 was

reported to overreact to various sensory inputs

which include tactile, auditory, and visual stimuli.

The parents of A2 and A5 reported that their

autistic children did not display excessive

responses to somatosensory inputs. The TI data

of A1, A2, A3, and A6 are consistent with the

psychiatric evaluations. However, although A4 was

reported to be hypo-responsive, his TI score is

higher than 85% of the normal population.

Similarly, the parental report of A5 did not suggest

any excessive sensory problems, but A5 scored

very high in the TI test (i.e., tactually defensive

behavior). The normal subjects do not all have

TI scores which lie close to the average of the

normative population. N1 and N2 have very high

scores; N4 has a very low score. These results

show that the tactile defensiveness as measured by

the TI test may vary considerably across the

normal population as well as the autistic popula-

tion. However, there is no statistical difference

between the TI scores of normal and autistic

children (t-test; p¼ 0.986).

The SP test results are not very different from the

TI test results. N1, N2, and A3 have high scores

(483). If only the tactile subset of the SP test is

considered, N1, N2, N3, and A3 have high scores

(416). If only the emotional/social subset of the test

is considered, N1, N2, N3, A1, A3, A4, and A5 have

high scores (416). The results from the tactile subset

of the SP test are generally consistent with the

psychiatric evaluations. Specifically, the subject who

is hyper-sensitive according to the psychiatric evalua-

tion, A3, has the highest score (21). Again, both

normal and autistic subject groups have members

who have high, medium, or low scores; therefore,

there is a large variation among subjects. However,

there is no statistical difference between the SP

scores of normal and autistic children (t-test;

entire SP set: p¼ 0.635, touch subset: p¼ 0.555,

emotional/social subset: p¼ 0.817). It is interesting

to note that some subjects scored high on both

emotional/social and tactile subsets. This suggests

that there may be a correlation between hypo- and

hyper-responsivity and emotional reactions.

Correlation analyses were performed on data

which include all normal and autistic subjects. A

high (r¼ 0.715) and significant ( p¼ 0.009) correla-

tion was found between the results from the tactile

and emotional/social subsets of the SP test

(Figure 2A; n¼ 12). Similarly, a high (r¼ 0.822)

and significant ( p¼ 0.001) correlation was found

between the results from the TI test and the

emotional/social subset of the SP test (Figure 2B;

n¼ 12). This latter correlation is important because,

the TI test was answered by the subjects, but the SP

test was answered by the parents. As expected, there

is also a high (r¼ 0.780) and significant ( p¼ 0.003)

correlation between the results from the TI test and

the tactile subset of the SP test (Figure 2C; n¼ 12).

Finally, a high (r¼ 0.753) and significant ( p¼ 0.005)

correlation was found between the results of the TI

test and the entire SP test (Figure 2D; n¼ 12). These

correlation data show that although the questionnaire

results vary considerably among normal and autistic

subjects, there is a consistent pattern. The subjects

who have more emotional problems according to the

SP test also have more tactile problems according to

the SP test, and display more tactually defensive

behaviors according to the TI test. In addition,

the subjects who have more sensory problems in

general according to the SP test are more tactually

defensive according to the TI test, which also

correlates strongly with the results from the tactile

subset of the SP. Furthermore, the inverse state-

ments are implied because of the given correlations.

For example, the subjects who have very few

emotional problems display hypo-responsive beha-

viors. Similar correlation analyses were performed

also by using the CARS scores for the autistic

subjects. There was significant correlation only

between the results from the emotional/social

subset of the SP test and the CARS scores

(r¼ 0.818; p¼ 0.047), which verifies that emotions

and social interactions are important for determining

the severity of autism.

26 B. Guclu et al.

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Psychophysical data

The psychophysical thresholds of the subjects are

given individually to investigate the detection and the

masking mechanism (Figure 3). The white bars are

the average thresholds obtained without masking

when the stimulus frequency was 40Hz. The 40-Hz

unmasked thresholds of the normal subjects

(Figure 3A) are 6.3, 7.6, 12.8, 9.8, 11.0, and

23.2 dB, respectively, for N1–6. The 40-Hz

unmasked thresholds of the autistic subjects are

20.7, 13.3, 5.8, 10.4, 4.0, and 9.2 dB, respectively,

for A1–6. There is no statistical difference between

the 40-Hz unmasked thresholds of the normal and

autistic children (t-test; p¼ 0.743). The average

40-Hz unmasked threshold among subjects is

14.0 dB for the normal group and 12.5 dB for the

autistic group (Figure 4). The 40-Hz unmasked

thresholds of the normal children were compared to

the data obtained from normal adults (Guclu and

Bolanowski 2005; Kalkanc| and Guclu 2005) and no

significant difference could be found (t-test;

p¼ 0.634). Without masking data (see below), it is

impossible to determine if the 40-Hz unmasked

thresholds are mediated by the P or NP I channel

(Guclu and Bolanowski 2005).

The 250-Hz unmasked thresholds are mediated by

the P channel because of the high sensitivity of this

psychophysical channel at 250Hz (Bolanowski et al.

1988; Gescheider et al. 2001). The 250-Hz

unmasked thresholds are �14.7, �21.6, �11.2,

�16.2, �8.8, and �8.3 dB for N1–6, respectively.

On the other hand, the 250-Hz unmasked thresholds

are �5.8, �10.7, �17.0, �9.4, �15.8, and �10.5 dB

for A1–6, respectively. There is no statistical

difference between the 250-Hz thresholds of

normal and autistic children (t-test; p¼ 0.507). The

average 250-Hz unmasked threshold among subjects

is �12.4 dB for the normal group and �10.7 dB for

the autistic group (Figure 4). No significant differ-

ence was found (t-test; p¼ 0.829) between the

thresholds of normal children and normal adults

(Guclu and Bolanowski 2005; Kalkanc| and Guclu

2005).

In order to determine the threshold of the NP I

channel at 40Hz, the sensitivity of the P channel

should be elevated by masking. Since the masking

effects may change across subjects, the level of the

0

5

10

15

20

25(A) (B)

0 5 10 15 20 25SP emotional/social positive score

0 5 10 15 20 25SP emotional/social positive score

SP

touc

h po

sitiv

e sc

ore

r = 0.715p = 0.009

0

10

20

30

40

50

60

70

TI r

aw sc

ore

r = 0.822p = 0.001

0

10

20

30

40

50

60

70(C)

0 5 10 15 20 25

SP touch positive score

TI r

aw s

core

0

10

20

30

40

50

60

70(D)

TI r

aw s

core

r = 0.780p = 0.003

0 20 40 60 80 100 120

SP overall positive score

r = 0.753p = 0.005

Figure 2. Correlations between the following questionnaire data are presented including all subjects: (A) SP touch subsetscore and SP emotional/social subset score, (B) TI raw score and SP emotional/social subset score, (C) TI raw score and SPtouch subset score, and (D) TI raw score and SP entire set score. Correlation coefficients and p-values are given on the plots.SP¼Sensory Profile questionnaire; TI¼Touch Inventory for Elementary-School-Aged Children.

Tactile sensitivity of normal and autistic children 27

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250-Hz masking stimulus used for each subject was

determined individually (Guclu and Bolanowski

2005). However, this did not pose a problem,

because the conclusions were not based on how

effective the P channel was masked. It was only

ensured that the P channel was masked at least

5–15 dB to eliminate its contribution to detection.

The purpose of this procedure was to selectively

measure the threshold of the NP I channel.

Appreciable masking was obtained for each subject.

The threshold shifts obtained by testing a 250-Hz

stimulus applied immediately after the masking

stimulus are 22.8, 29.6, 25.5, 39.8, 10.9, and

36.3 dB for N1–6, respectively. For autistic children,

the masking levels were set lower than the normal

children to minimize attentional lapses which may

occur during the experiment. Therefore, lower

threshold shifts were obtained for the autistic

children. The threshold shifts are 6.5, 14.9, 12.1,

11.4, 14.6, and 10.3 dB for A1–6, respectively. Note,

however, that autistic children displayed a masking

phenomenon indistinguishable from normal

children.

Lastly, the thresholds were measured at 40Hz with

forward masking (gray bars in Figure 3). If masked

thresholds at 40Hz are higher than unmasked

thresholds at 40Hz, the masked thresholds are

considered to be mediated by the NP I channel

and the unmasked thresholds should be mediated by

the P channel (Guclu and Bolanowski 2005).

The 40-Hz masked thresholds of normal children

are 18.5, 24.0, 31.1, 29.9, 17.8, and 39.2 dB for

N1–6, respectively (Figure 3A). For all normal

subjects, the gray bars are significantly higher than

the white bars (t-test; p50.001). This shows that the

white bars are the thresholds of the P channel at

40Hz. The 40-Hz masked thresholds of autistic

children are 26.2, 26.0, 14.6, 15.1, 18.6, and 13.9 dB

for A1–6, respectively (Figure 3B). For all autistic

subjects, the gray bars are significantly higher than

the white bars (t-test; p50.047). This implies that

the detection mechanism found for the normal

subjects is present for the autistic subjects as well.

Furthermore, there is no statistical difference

between the NP I thresholds (gray bars) of normal

subjects and autistic subjects (t-test; p¼ 0.142). The

average NP I thresholds across subjects is 30.1 dB for

the normal group and 20.7 dB for the autistic group

(Figure 4). Although there is about 9.4 dB difference

between the two subject groups, this difference is not

statistically significant because of inter-subject var-

iance. The average NP I threshold of normal adults is

19.6 dB (Guclu and Bolanowski 2005; Kalkanc| andGuclu 2005). The thresholds of normal adults are

significantly lower than the thresholds of normal

children (t-test; p¼ 0.019), but not different than the

thresholds of autistic children (t-test; p¼ 0.613).

−5

5

15

25

35

45

55

−5

5

15

25

35

45

55

(A)

(B)

N1 N2 N3 N4 N5 N6

Thr

esho

ld (

dB r

e 1

µm p

eak)

No masking

Masking

A1 A2 A3 A4 A5 A6

Thr

esho

ld (

dB r

e 1

µm p

eak)

No masking

Masking

Figure 3. Psychophysical thresholds—of normal (A) andautistic (B) children. The white bars are the 40-Hzunmasked thresholds and they are mediated by thePacinian channel (see text). The gray bars are the 40-Hzmasked thresholds and they are assumed to be mediated bythe Non-Pacinian I channel. The error bars are thestandard errors of the mean.

−20

−10

0

10

20

30

40

10 100 1000Frequency (Hz)

Thr

esho

ld (

dB r

e 1

µm p

eak)

Normal P Normal NP IAutistic P Autistic NP I

Figure 4. The average thresholds of the Pacinian andNon-Pacinian I (NP I) channels of normal and autisticchildren, at 40 and 250Hz. Note that the thresholds of theNP I channel could not be measured at 250Hz, becausethe sensitivity of the NP I channel is very low compared tothe Pacinian channel at 250Hz. The error bars are thestandard errors of the mean.

28 B. Guclu et al.

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Correlations between questionnaire data and psycho-

physical data

The analyses in the previous subsections established

that there are no differences between normal and

autistic children regarding the spread of qualitative

sensory problems and the psychophysical tactile

thresholds. However, there seems to be a correlation

between emotional and tactile problems. There may

also be correlations between qualitative sensory

problems and psychophysical thresholds for each

subject group, or when both groups are combined.

This would imply that the qualitative sensory

problems may originate from perceptual deficits,

although this cannot be strictly proven because

correlation does not show a causal relationship.

First, each subject group was treated separately.

Correlations were calculated between the psycho-

physical thresholds (40-Hz Pacinian, 250-Hz

Pacinian, 40-Hz NP I) and questionnaire results

(TI score, SP score, SP touch subset score,

SP emotional/social subset score). Although all

combinations of psychophysical data and

questionnaire data were tested, no significant corre-

lations could be found. For the normal group, the

highest (r¼�0.545; p¼ 0.264) correlation is

between the SP touch subset score and 40-Hz NP I

threshold (Figure 5A; n¼ 6). For the autistic group,

the highest (r¼�0.807; p¼ 0.052) correlation is

between the TI score and 250-Hz Pacinian threshold

(Figure 5B; n¼ 6). Then, all subject data were

pooled and every combination of questionnaire and

psychophysical data was tested. However, no sig-

nificant correlations were found in this analysis

either. If all subjects are included in the analysis,

the highest (r¼�0.547; p¼ 0.066) correlation is

between TI score and 250-Hz Pacinian threshold

(Figure 5C; n¼ 12). Regarding the emotional/social

subset of the SP test, the highest correlation

(r¼�0.152; p¼ 0.638) is for the 40-Hz NP I

thresholds (Figure 5D; n¼ 12) if all subjects are

included. Additionally, no significant correlations

were found between the psychophysical thresholds

and CARS scores for the autistic subjects. These

analyses show that there is probably no causal

0

5

10

15

20

25(A)

(C) (D)

0 20 40 60 80 100

40-Hz NP I threshold (µm)

SP

touc

h po

sitiv

e sc

ore

r = −0.545p = 0.264Normal

0

10

20

30

40

50

60

70(B)

0 0.1 0.2 0.3 0.4 0.5 0.6

250-Hz Pacinian threshold (µm)

TI r

aw s

core

s

r = −0.807p = 0.052Autistic

0

10

20

30

40

50

60

70

0 0.1 0.2 0.3 0.4 0.5 0.6

250-Hz Pacinian threshold (µm)

TI r

aw s

core

s

r = −0.547

p = 0.066

0

5

10

15

20

25

0 20 40 60 80 100

40-Hz NP I threshold (µm)

SP

em

otio

nal/s

ocia

l sco

res

r = −0.152p = 0.638

Figure 5. The highest correlations between the psychophysical and questionnaire data are presented (including all subjectsunless otherwise noted), but there are no significant correlations (see p-values). (A) SP touch subset score and 40-Hz NP Ithreshold, (B) TI raw score and 250-Hz Pacinian threshold, (C) TI raw score and 250-Hz Pacinian threshold, and (D) SPemotional/social subset score and 40-Hz NP I threshold. Correlation coefficients and p-values are given on the plots.SP¼Sensory Profile questionnaire; TI¼Touch Inventory for Elementary-School-Aged Children; NP I¼Non-Pacinian Itactile channel.

Tactile sensitivity of normal and autistic children 29

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relationship between qualitative sensory problems

identified by the questionnaires and psychophysical

thresholds.

Nevertheless, there is some decreasing tendency

(negative slopes) on the plots given in Figure 5.

In other words, as the thresholds increase,

the questionnaire scores decrease, that is, more

hypo-responsivity. As a final attempt to investigate

a possible relationship between questionnaire results

and psychophysical thresholds, the subjects were

arranged into tactile defensive (hyper-responsive:

N1, N2, A3, A4, A5) and not-defensive (hypo-

responsive: N4, A1, A6) groups according to the

analyses presented above. The psychophysical

thresholds of these two groups were compared to

test for the hypothesis that hyper-responsive

subjects have lower thresholds than the thresholds

of hypo-responsive subjects. No statistically signifi-

cant effects could be found for the three thresholds

measured (40-Hz Pacinian: p¼ 0.068; 40-Hz NP I:

p¼ 0.075; 250-Hz Pacinian: p¼ 0.095). Similarly,

the subjects were regrouped into emotional (hyper-

responsive: N1, N2, N3, A1, A3, A4, A5) and non-

emotional (hypo-responsive: N4) groups according

to questionnaire data. Again, no statistically signifi-

cant effects could be found under the same test

hypothesis mentioned above (40-Hz Pacinian:

p¼ 0.574; 40-Hz NP I: p¼ 0.100; 250-Hz

Pacinian: p¼ 0.702). Therefore, there is no evidence

that hyper-responsive subjects have lower thresholds

than the thresholds of hypo-responsive subjects.

Discussion

The most important result in this study is that no

relationship could be found between psychophysical

thresholds and qualitative reports of sensory

problems by the subjects and subjects’ parents.

This result was further supported by the lack of

statistical difference between the thresholds of

normal and autistic children. There was about

9.4 dB difference between the NP I thresholds of

the normal and the autistic group at 40Hz, but this

difference was not statistically significant. If the

means and the variances stay constant while increas-

ing the sample size, such a difference will be expected

to be significant. However, it is also likely that the

mean threshold of the autistic subjects may approach

the mean threshold of the normal subjects if more

subjects are recruited.

There is a link between the emotional and tactile

responses in the questionnaires, which suggests that

the subjects are more likely to have sensory problems

if they have emotional problems. The parsimonious

explanation based on these findings is that the

sensory problems reported in questionnaires or in

clinical evaluation are not based on low-level

perceptual deficits. The correlation between emo-

tional and sensory questionnaire data implies that

there may be higher-order, emotional processes

involved in the sensory problems. Since the sense

of touch has not been studied much in autism as

compared to the modalities of vision and hearing,

this conclusion is rather important.

Although the sample size used in this study is small

(six normal and six autistic children), the psycho-

physical procedures are very robust and have been

frequently used in basic somatosensory research

(Verrillo 1963, 1971; Gescheider et al. 1983, 1994;

Bolanowski et al. 1988; Lamore and Keemink 1988;

Makous et al. 1996; Guclu and Bolanowski 2005).

Two-interval forced-choice task avoids interaction of

subject’s decision criterion and provides objective

threshold measurements even with small sample

sizes. However, the conclusion presented here is

tentative because responses obtained from the

questionnaires are subjective. Nevertheless, such

questionnaires are helpful in the clinical setting to

discriminate developmentally delayed children with

and without tactile defensiveness (Larson 1982),

to discriminate sensory experiences in pervasive

developmental disorders and attention-deficit/

hyperactivity disorder (Ermer and Dunn 1998) and

for many other purposes which do not require much

detailed information on the underlying neural

processes (McCracken 1975; Bauer 1977; Royeen

1986; Dunn 1994). In future studies, emotional

responses of a larger sample of subjects may be

modulated and measured during tactile stimulation

to test the conclusion presented here. However, this

is an ambitious goal because autistic children, even

if they are higher functioning, may probably have

attentional and cognitive difficulties in such complex

tasks.

It is also important to note that the SP and TI tests

have not been done in large populations of normal

and autistic subjects parallel with measuring psycho-

physical thresholds. Future studies may find a link

between the questionnaires and the thresholds,

which would imply that sensory defensiveness

reflects an independent additional impairment.

At this time, however, that statement is speculative,

and the purpose of the current study was not to

support or disprove it. On the other hand, the lack of

statistical difference between the SP scores of normal

and autistic children may be because of the small

sample size.

Cognitive theories on autism

Three non-exclusive theories have been proposed to

explain the cognitive functioning in autism

(Nurcombe and Stone 2000; Hill and Frith 2003).

Impaired theory of mind explains the inability of

30 B. Guclu et al.

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autistic children to recognize and understand other

people’s mental states (Baron-Cohen 1989). The

executive dysfunction theory stresses on the frontal-

lobe dysfunction of autistic children which is

observed by their cognitive inflexibility and perse-

veration (Hughes et al. 1994). The weak central

coherence theory discusses the tendency of autistic

children to focus on local, rather than global aspects

of objects (Frith and Happe 1994). The latter theory

attempts to explain experimental data by suggesting

poor connectivity between basic perceptual processes

and top-down modulating processes. The influence

of top-down modulating processes has been recog-

nized to be important in developmental disorders

long ago (Ayres 1964, 1979). Hypo- and hyper-

responsivity to sensory input have been attributed to

sensory modulation dysfunction. The neural activity

originating from sensory input is thought to be not

regulated properly in relation with the other func-

tions of the nervous system. The limbic system, and

therefore, emotions may be closely associated with

this problem.

Emotional modulation of the sensory response

Children with sensory modulation dysfunction show

larger electrodermal responses (McIntosh et al.

1999). The orbitofrontal–amygdala circuit is impli-

cated in self-regulation of social–emotional behavior

in autism (Bachevalier and Loveland 2006). The

facial-affect expressions of autistic children are

abnormal (Yirmiya et al. 1989), and attention-

deficit/hyperactivity, mood, and anxiety disorders

may accompany autism as comorbid conditions

(Nurcombe and Stone 2000). Therefore, it is

certainly possible that emotions can modulate

hypo- and hyper-responsivity to tactile sensory

input in autism and other developmental disorders.

However, studies which target this relationship are

scarce in the literature (Baranek and Berkson 1994;

Parush et al. 1997; Mangeot et al. 2001) and they do

not provide clear evidence of the modulatory effect.

The reason for that may be due to the lack of critical

control of the sensory input. In order to study the

effects of emotions on the sensory response, the

state of the tactile psychophysical channels should

be carefully controlled as in the psychophysical

experiments described here. If the low-level neural

processing is accurately determined, a top-down

modulatory effect may be less ambiguous. Note,

however, that a careful control of emotional states is

also essential, and there is no unique best paradigm

for that. Salmond et al. (2003) studied the emotional

modulation of the startle response in autistic children

who had abnormalities in the amygdala as observed

from MRI scans, but could not find an effect.

Similarly, Bernier et al. (2005) showed that autistic

children have normal responses to a fear–potential

startle paradigm.

There may also be a reverse effect between

emotions and sensory input for autistic children.

Some children with developmental disorders benefit

from receiving many sensory stimuli during sensory

integration therapy (Bundy et al. 2002; Sidney Chu,

personal communication) and touch therapy (Field

et al. 1997). Although such interventions are con-

troversial (Baranek 2002), more studies with criti-

cally controlled stimuli are required before finding

definite answers.

It is also interesting to note that normal children

responded similarly to the questionnaires in the

presented study, although they do not suffer from

clinical sensory problems. This may be explained

by using the top-down modulation hypothesis.

The sensory responses tested by the questionnaires

may be distributed uniformly across the general

population. Normal children, who may have normal

emotional modulation, do not display the same

sensory responses as strongly as seen at a clinical

level, or they may tolerate sensory modulation

dysfunctions at a higher cognitive level. Autistic

children may have more serious modulation dysfunc-

tions and/or less capability to tolerate them. In future

studies, normal children who give more positive

responses to questionnaires can be retested in

experiments with emotional modulation.

Low-level sensory processing in autism

Visual stimuli have been used frequently in autism,

but the experiments largely focused on attentional or

cognitive mechanisms and not on lower-level neural

and perceptual processes (Swettenham et al. 2003).

There have been more studies of basic neural

processing in the hearing modality. Brain stem

auditory evoked responses were found to be abnor-

mal in autism (Thivierge et al. 1990). By using

transiently evoked otoacoustic emissions, Khalfa

et al. (2001) reported a difference in the efferent

activity of the medial olivocochlear system.

In addition to genetic factors, brain stem injury due

to environmental factors during the earliest stages of

brain development was hypothesized for some

anomalies associated with autism (Rodier and

Hyman 1998; Rodier 2002). Damage to brain stem

nuclei may be the reason for some of the sensory

abnormalities (Patricia Rodier and Susan Hyman,

personal communication).

In addition to the brain stem, many other

structural abnormalities are associated with autism

(Nurcombe and Stone 2000), especially in

cerebellum, amygdala, hippocampus, and cerebral

cortex. Disruption of minicolumns in cerebral

centers was reported by Casanova et al. (2002).

Tactile sensitivity of normal and autistic children 31

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Cortical dysfunction has also been documented. Less

activation was observed in cortical speech-related

areas (Boddaert et al. 2004). There seems to be a

reorganization of left–right hemisphere functions of

cortical auditory areas (Bruneau et al. 2003). Basic

auditory perception is affected. Plaisted et al. (2003)

found abnormally broad psychophysical auditory

filters in autistic children. In addition, smaller

auditory dynamic ranges were found in autism, as

well as increased perception of loudness (Khalfa

et al. 2004).

Low-level tactile processing has not captured

attention in the literature despite the many tactile

problems mentioned in this paper. Therefore,

the study presented here may be an important

contribution. If the brain stem is abnormal in

autism, cuneate and gracile nuclei may be affected

and the tactile afferent input may be altered.

However, the structural abnormalities are not homo-

geneous across the autistic population. Interestingly,

a very recent study by Blakemore et al. (2006) shows

a slight decrease in the 200-Hz tactile thresholds of

adults having the Asperger syndrome (a class of

autism spectrum disorders) as compared to normals,

but there is no difference at 30Hz. Perhaps, the

neural changes underlying tactile sensory problems

do not affect absolute thresholds seriously, but

modify intensity discrimination or magnitude

estimation in autism. Another possibility is the

contribution of the anterolateral system for crude

touch, which has direct connections with the

reticular formation. These questions, among many

others, remain to be tested for this enigmatic

disorder.

Acknowledgements

This work was supported by Bogazici University

Research Fund (04HX101) and TUB_ITAK

(104S228) to Dr Guclu. We thank Cigdem Oztek

for help in experiments with normal subjects, Patricia

Rodier for discussions on brain stem injury in

autism, Susan Hyman for discussions on infantile

autism, and Sidney Chu for comments on the

sensory integration theory. We also appreciate the

editing by Ronald Verrillo.

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