Introduction to Physiological Psychologyksweeney/pdfs/9.pdfDeuteranopia: no green cones – red and green hues confused Tritanopia: blue cones lacking or faulty – world seen in reds

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VisionVision

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Introduction to Introduction to

Physiological PsychologyPhysiological Psychology

Light-a part of the

spectrum of

Electromagnetic

Energy

(the part that’s

visible to us!)

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In a vacuum, light travels at a constant speed of ~186,000

miles/sec. So if the frequency of the oscillation varies, the

distance between peaks (or wavelength) also varies.

(vertical)(vertical) Route within the retinaRoute within the retina

�� Rods and ConesRods and Cones

�� Bipolar CellsBipolar Cells

�� Ganglion CellsGanglion Cells

�� The axons of the The axons of the

ganglion cells form ganglion cells form

the optic nervethe optic nerve

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Eye movementEye movement

�� Although each Although each

fixation generates a fixation generates a

different different sensationsensation at at

the level of the the level of the

retina, the brain retina, the brain

creates a single creates a single

perceptionperception

Yarbus, 1967

Why have two of them?Why have two of them?

�� ConvergenceConvergence ::–– eyes must turn slightly inward to eyes must turn slightly inward to focusfocus when when

objects are closeobjects are close

�� Binocular disparityBinocular disparity::–– difference between the images on the two difference between the images on the two

retinasretinas

�� Both are Both are greatergreater when objects are close when objects are close ––provides brain with a 3provides brain with a 3--D image and D image and distance informationdistance information

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Sensory neurons for visionSensory neurons for vision

�� RODS and CONES:RODS and CONES:

–– Specialized neurons Specialized neurons

that respond to light that respond to light

with changes in their with changes in their

membrane potentialmembrane potential

Photoreceptors: Rods and ConesPhotoreceptors: Rods and Cones

�� RODSRODS::–– ~120 million rods~120 million rods

–– Scotopic Vision Scotopic Vision (skotos=darkness)(skotos=darkness)

–– Sensitive to brightness, but Sensitive to brightness, but

not color (shades of gray)not color (shades of gray)

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�� RODSRODS::–– ManyMany rods converge onto rods converge onto one one

retinal ganglion cellretinal ganglion cell

–– Responsible for Responsible for lowlow--light light

visionvision

–– Not present Not present at all at all in foveain fovea


�� CONES:CONES:–– ~6 million cones~6 million cones

–– PhotopicPhotopic Vision Vision (photos=light)(photos=light)

–– Sensitive to colorSensitive to color

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�� CONES:CONES:–– A A single retinal ganglion cell single retinal ganglion cell

receives signals from one (or receives signals from one (or

few) cones.few) cones.

–– Responsible for Responsible for high acuity high acuity

vision vision (fine detail)(fine detail)

–– Fovea contains Fovea contains only only conescones

Rods and ConesRods and Cones

�� The outer segment of a photoreceptor The outer segment of a photoreceptor

contains hundreds of contains hundreds of lamellae.lamellae.

�� Within the lamellae you find Within the lamellae you find

photopigmentsphotopigments-- molecules that contain molecules that contain

an an opsinopsin and a and a retinal.retinal. (E.g(E.g. . rhodopsinrhodopsin))

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RhodopsinRhodopsin

�� Rhodopsin Rhodopsin is a receptor that responds to is a receptor that responds to

lightlight instead of to instead of to neurotransmitters neurotransmitters (photons bind to it)(photons bind to it)

�� When rhodopsin is exposed to light, it When rhodopsin is exposed to light, it

breaks down and the breaks down and the opsinopsin bleachesbleaches..

�� The effect of the bleaching is a change in The effect of the bleaching is a change in

the release of NTthe release of NT

–– Not the way you might think!Not the way you might think!

Strange but true…Strange but true…

�� The effect of light is to turn receptor The effect of light is to turn receptor

cells cells ““OFFOFF””… … darknessdarkness turns them turns them ““ONON””!!

�� Remember that receptors have a Remember that receptors have a

spontaneous firing rate:spontaneous firing rate:

–– They do NOT fire action potentials, but They do NOT fire action potentials, but

graded graded potentialspotentials

–– The effect of receptors firing is The effect of receptors firing is inhibition inhibition of of

the bipolar cellsthe bipolar cells

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Transduction: how light becomes Transduction: how light becomes

neural signalsneural signals

�� A cone or rod actually releases A cone or rod actually releases

LESSLESS neurotransmitter when neurotransmitter when

stimulated by light!stimulated by light!

–– RhodopsinRhodopsin molecules are bleached molecules are bleached

by light, causing by light, causing hyperpolarization hyperpolarization

of rods. of rods.

–– Thus, inhibition: Thus, inhibition: lessless release release

of neurotransmitterof neurotransmitter (glutamate)(glutamate)

–– Result is: Result is: depolarizationdepolarization of of

bipolar cell (= more release of bipolar cell (= more release of

neurotransmitter)neurotransmitter)

–– Ganglion cell is Ganglion cell is more more likely to likely to

fire (generally)fire (generally)

The effect of a bleached The effect of a bleached

photopigment…photopigment…

�� … is that a the photoreceptor… is that a the photoreceptor’’s s membrane potential changes.membrane potential changes.

�� ReceptorReceptor’’s membrane potential affects s membrane potential affects

release of NT onto bipolar cells.release of NT onto bipolar cells.

�� Bipolar cells Bipolar cells ‘‘speakspeak’’ to ganglion cells, to ganglion cells,

which bring information to the brain. which bring information to the brain.

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So in the So in the darkdark……

�� Photoreceptors release enough NT to Photoreceptors release enough NT to

prevent prevent bipolar cells from triggering bipolar cells from triggering

ganglion cells.ganglion cells.

–– Ganglion cells, by NOT firing, report to the Ganglion cells, by NOT firing, report to the

brain: brain: ““no lightno light””

�� And in the light?And in the light?

Cone and Rod VisionCone and Rod Vision

�� Only cones are Only cones are

found at the found at the

fovea!!fovea!!

Distribution of rods and cones

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Cone and Rod VisionCone and Rod Vision

�� Less convergence Less convergence

in cones, in cones,

increasing increasing acuityacuity

while decreasing while decreasing

sensitivitysensitivity

�� More convergence More convergence

in rod system, in rod system,

increasing increasing

sensitivitysensitivity while while

decreasing decreasing acuityacuity

So we have a response from a So we have a response from a

ganglion cell… now what?ganglion cell… now what?

�� Bundle of ganglion cell axons exiting the Bundle of ganglion cell axons exiting the

eye: blind spot eye: blind spot

�� No receptors where No receptors where

information exits information exits

the eye:the eye:–– Visual system uses Visual system uses

information from cells information from cells

around the blind spot for around the blind spot for

““completion,completion,”” filling in filling in

the blind spotthe blind spot

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From the Eyes to the Visual CortexFrom the Eyes to the Visual Cortex

Lateral Geniculate NucleusLateral Geniculate Nucleus

�� A nucleus within the thalamus A nucleus within the thalamus ((““relay centerrelay center””) )

–– receives information from the retina and receives information from the retina and

projects to primary visual cortex.projects to primary visual cortex.

�� Contains six Contains six ““layerslayers”” of neuronsof neurons

–– each layer receives information from only 1 each layer receives information from only 1

eye.eye.

�� First two layers: magnocellularFirst two layers: magnocellular

�� Next four layers: parvocellularNext four layers: parvocellular

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M and P channelsM and P channels

�� MagnocellularMagnocellular

–– Larger cell bodiesLarger cell bodies

–– Responsive to movementResponsive to movement

–– Input primarily from rodsInput primarily from rods

�� ParvocellularParvocellular

–– Small cell bodiesSmall cell bodies

–– Responsive to color, fine Responsive to color, fine

detailsdetails

–– Input primarily from conesInput primarily from cones

M and P channelsM and P channels

�� Layers 1, 4, 6Layers 1, 4, 6-- contracontra

�� Layers 2, 3, 5Layers 2, 3, 5-- ipsiipsi

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From the Eyes to the Visual CortexFrom the Eyes to the Visual Cortex

�� The visual system is The visual system is

organized organized

retinotopically: retinotopically:

–– The left hemiretina of each The left hemiretina of each eye (right visual field) eye (right visual field) connects to the right lateral connects to the right lateral geniculate nucleus (LGN)geniculate nucleus (LGN)

–– the right hemiretina (left the right hemiretina (left visual field) connects to the visual field) connects to the left LGN left LGN

Coding of Visual Coding of Visual

InformationInformation

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Coding of information in the retinaCoding of information in the retina

�� For any sensory neuron, a receptive field For any sensory neuron, a receptive field

is the is the ‘‘placeplace’’ in which a stimulus will in which a stimulus will

cause the neuron to fire.cause the neuron to fire.

�� The receptive fields in the fovea are The receptive fields in the fovea are

smaller than in the rest of the retina. smaller than in the rest of the retina.

Receptive Receptive

FieldsFields�� Many ganglion cells Many ganglion cells

have have receptive receptive fields fields with a with a centercenter--surround surround organization: organization: excitatory and excitatory and inhibitory regions inhibitory regions separated by a separated by a circular boundarycircular boundary

�� Some cells are Some cells are ““onon--centercenter”” and some and some are are ““offoff--centercenter””

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What does color get us?What does color get us?

What does color get us?What does color get us?

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�� Why can you visualize red (imagine a fireWhy can you visualize red (imagine a fire--

truck)… and you can imagine a reddish truck)… and you can imagine a reddish

yellow… but it is difficult (impossible?) to yellow… but it is difficult (impossible?) to

imagine a reddish green…imagine a reddish green…

�� … or a bluish… or a bluish--yellow?!yellow?!

Color Mixing vs. Pigment MixingColor Mixing vs. Pigment Mixing

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Color vision theoriesColor vision theories

�� Trichromatic theory:Trichromatic theory: there are 3 different there are 3 different

receptors (types of cones) in the eye, each receptors (types of cones) in the eye, each

sensitive to a single hue (red, green, blue)sensitive to a single hue (red, green, blue)

Color vision theoriesColor vision theories

�� Trichromatic theory:Trichromatic theory: there are 3 different there are 3 different

receptors (types of cones) in the eye, each receptors (types of cones) in the eye, each

sensitive to a single hue (red, green, blue)sensitive to a single hue (red, green, blue)–– Because Young noted that any color could be accounted for by Because Young noted that any color could be accounted for by

mixing just 3 mixing just 3 lightslights in various proportionsin various proportions

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Trichromatic TheoryTrichromatic Theory

�� At the level of the retina, At the level of the retina, conescones code for code for three wavelengths of light (different opsins): three wavelengths of light (different opsins):

�� ShortShort (S), (S), MediumMedium (M), (M), LongLong (L): blue, green, red(L): blue, green, red

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�� The precise The precise

distribution of distribution of

cones varies from cones varies from

person to person, person to person,

but generally but generally

speaking speaking ““blueblue””

sensitive cones are sensitive cones are

less common than less common than

““redred”” and and ““greengreen””

conesconesImage from David Williams, U of Rochester

Color BlindnessColor Blindness

� Protanopia: no red cones

– see yellow and blue, red and green

hues confused

� Deuteranopia: no green cones

– red and green hues confused

� Tritanopia: blue cones lacking

or faulty

– world seen in reds and greens, no

blue

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�� Those with normal color Those with normal color vision should read the vision should read the number 8. number 8.

�� Those with redThose with red--green color green color vision deficiencies vision deficiencies (protanopia, deuteranopia) (protanopia, deuteranopia) should read the number 3. should read the number 3.

�� Total color blindness should Total color blindness should not be able to read any not be able to read any numeral.numeral.

The trichromatic theory The trichromatic theory

doesndoesn’’t tell the whole story…t tell the whole story…

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The trichromatic theory The trichromatic theory

doesndoesn’’t tell the whole story…t tell the whole story…�� The retinal ganglion cells code for The retinal ganglion cells code for

complementary colors.complementary colors.

�� This is known as This is known as opponentopponent--process codingprocess coding

�� Another type of ganglion cell only Another type of ganglion cell only

encodes brightness: encodes brightness: ‘‘blackblack--whitewhite’’

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Opponent Process Theory

�� Ganglion cellsGanglion cells

–– Three typesThree types

�� Red/green, yellow/blue, black/whiteRed/green, yellow/blue, black/white

–– Each cell represents an Each cell represents an opponent process opponent process

systemsystem

�� Resting behavior in red/green cells is midResting behavior in red/green cells is mid--level level

rate of responserate of response

�� For R+GFor R+G--, rate , rate increasesincreases when red is present, when red is present,

decreasesdecreases when green is present when green is present (opposite for R(opposite for R--G+)G+)

�� Yellow/blue (Y+BYellow/blue (Y+B--) ) increasesincreases when both red and when both red and

green are present, green are present, decreasesdecreases when blue is when blue is

presentpresent

Opponent Process TheoryOpponent Process Theory

Opposing retinal processes enable color visionOpposing retinal processes enable color vision

““ONON”” ““OFFOFF””

red red greengreen

green green red red

blue blue yellow yellow

yellowyellow blue blue

black black whitewhite

whitewhite blackblack

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�� ReddishReddish--green? Bluishgreen? Bluish--Yellow?Yellow?

�� You canYou can’’t imagine them because ganglion t imagine them because ganglion

cells that signal red or green (or yellow cells that signal red or green (or yellow

or blue) can only or blue) can only increase increase or or decrease decrease

rate of firing, they canrate of firing, they can’’t do both at t do both at

once!!once!!

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�� The The complementary aftercomplementary after--effect effect is caused by the fact is caused by the fact

that after adaptation, locations stimulated by green that after adaptation, locations stimulated by green

light will be less sensitive to green than to red, and light will be less sensitive to green than to red, and

vice versa. Since vice versa. Since white lightwhite light contains all colors and contains all colors and

stimulates all photoreceptors equally, those that have stimulates all photoreceptors equally, those that have

been been ““green adaptedgreen adapted”” will fire will fire ‘‘redred’’ to white light to white light

(and vice versa)(and vice versa)-- a larger a larger ““redred”” than than ““greengreen”” signal will signal will

be generated. Itbe generated. It’’s the s the locallocal imbalance between the red imbalance between the red

and green inputs to the opponent mechanism that and green inputs to the opponent mechanism that

generates the (relatively weak) color aftergenerates the (relatively weak) color after--effects.effects.

We havenWe haven’’t even reached the t even reached the

cortex yet!cortex yet!�� Primary visual cortex (Striate Cortex, V1)Primary visual cortex (Striate Cortex, V1)

�� Visual Association cortex (extrastriate)Visual Association cortex (extrastriate)

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Primary Visual Cortex (V1, Striate Cortex)Primary Visual Cortex (V1, Striate Cortex)

~140 million neurons just in V1!

Retinotopic OrganizationRetinotopic Organization

�� Information received at adjacent portions Information received at adjacent portions of the retina remains adjacent in V1.of the retina remains adjacent in V1.

�� More cortex is devoted to areas of high More cortex is devoted to areas of high acuity. acuity. (Just like the disproportionate representation of (Just like the disproportionate representation of sensitive body parts in somatosensory cortex!)sensitive body parts in somatosensory cortex!)

�� About 25% of primary visual cortex is About 25% of primary visual cortex is dedicated to processing input from the dedicated to processing input from the fovea.fovea.

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Striate CortexStriate Cortex

�� Six principal layers of striate cortexSix principal layers of striate cortex

Processing in Striate CortexProcessing in Striate Cortex

�� Layers 2 and 3 receive information from Layers 2 and 3 receive information from

the parvocellular layers and koniocellular the parvocellular layers and koniocellular

layers of the LGN.layers of the LGN.

�� Cells are grouped together in Cells are grouped together in ““blobsblobs””

–– Cells Cells within within blobs are sensitive to colorblobs are sensitive to color

–– Cells Cells outsideoutside blobs are sensitive to blobs are sensitive to

orientation, movement, binocular disparityorientation, movement, binocular disparity

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Orientation and MovementOrientation and Movement

�� Most neurons in V1 are Most neurons in V1 are sensitive to orientation: sensitive to orientation: –– if a line or edge appears in if a line or edge appears in

their receptive field, they their receptive field, they respond best when it is at a respond best when it is at a certain anglecertain angle

Receptive Fields in Striate Cortex Receptive Fields in Striate Cortex

�� Most neurons in V1 are eitherMost neurons in V1 are either–– Simple Simple –– receptive fields are rectangular with receptive fields are rectangular with ““onon”” and and ““offoff”” regions, orregions, or

–– Complex Complex –– also rectangular, larger receptive also rectangular, larger receptive fields, respond best to a particular stimulus fields, respond best to a particular stimulus anywhere in its receptive fieldanywhere in its receptive field

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Receptive Fields in Striate CortexReceptive Fields in Striate Cortex

SIMPLESIMPLE

�� RectangularRectangular

�� ““OnOn”” and and ““offoff””regions, like cells regions, like cells in layer IVin layer IV

�� Orientation and Orientation and location sensitivelocation sensitive

�� All are monocularAll are monocular

COMPLEXCOMPLEX

�� RectangularRectangular

�� Larger receptive Larger receptive fieldsfields

�� Do not have static Do not have static ““onon”” and and ““offoff””regionsregions

�� Not location sensitiveNot location sensitive

�� MotionMotion sensitivesensitive

�� Many are binocularMany are binocular


�� Simple cells: receptive fields are Simple cells: receptive fields are rectangular with rectangular with ““onon”” and and ““offoff”” regions, regions, organized in an opponent fashionorganized in an opponent fashion

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�� Complex cells: also Complex cells: also rectangular, larger rectangular, larger receptive fields, respond best to a receptive fields, respond best to a particular stimulus particular stimulus anywhereanywhere in its in its receptive field, especially if there is receptive field, especially if there is movement movement in the right direction in the right direction (no (no inhibitory surround)inhibitory surround)


�� HypercomplexHypercomplex cellscells-- respond best to a respond best to a particular orientation, but have inhibitory particular orientation, but have inhibitory region: they code for region: they code for endsends of lines!of lines!

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Beyond Striate CortexBeyond Striate Cortex

�� Fundamentally, the coding in striate Fundamentally, the coding in striate

cortex is for cortex is for featuresfeatures: color, orientation, : color, orientation,

spatial frequency, retinal disparityspatial frequency, retinal disparity

�� PerceptionPerception requires the combination of requires the combination of

these features into an integrated whole!these features into an integrated whole!

�� This occurs in This occurs in extrastriate cortexextrastriate cortex

Dorsal and Ventral StreamsDorsal and Ventral Streams

�� Dorsal streamDorsal stream: : striate cortex � dorsal prestriate

cortex � posterior parietal cortex

– The “where” pathway (location and movement), or

– Pathway for control of behavior (e.g. reaching)

�� Ventral streamVentral stream: striate : striate cortex � ventral prestriate

cortex � inferotemporal cortex

– The “what” pathway (color and shape), or

– Pathway for conscious perception of objects

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6363

PET study of where/what dichotomyPET study of where/what dichotomy

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Not a fixed feedNot a fixed feed--forward system!forward system!

Image from Wagner and Kline

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Visual AgnosiaVisual Agnosia

�� Deficits in visual form perceptionDeficits in visual form perception

�� NOT NOT blindness!blindness!

�� Caused by damage to visual association Caused by damage to visual association

areas in ventral streamareas in ventral stream

�� Video….Video….

ProsopagnosiaProsopagnosia

�� Damage to the fusiform face area (FFA) Damage to the fusiform face area (FFA)

results in results in prosopagnosia. prosopagnosia.

Diffusion tensor imaging (DTI) tractography reveals a reduction in the volume of the inferior longitudinal fasciculus in the brains of 6 patients with congenital prosopagnosia (top). (From Thomas et al 2008)

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�� The lateral occipital complex is activated The lateral occipital complex is activated

in response to a wide variety of objects.in response to a wide variety of objects.

�� It seems possible that different It seems possible that different

categories of objects are processed at categories of objects are processed at

least in part in different subregions. least in part in different subregions.

�� Also in the ventral stream is the Also in the ventral stream is the

extrastriate body areaextrastriate body area

–– Seems to be particularly responsive to body Seems to be particularly responsive to body

partsparts

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Perception of MovementPerception of Movement

Introduction to Physiological Psychologyksweeney/pdfs/9.pdfDeuteranopia: no green cones – red and green hues confused Tritanopia: blue cones lacking or faulty – world seen in reds

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