Introduction to Physiological Psychology Vision

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Light- a part of the spectrum of Electromagnetic Energy (the part that’s visible to us!)

1 In a vacuum, 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) Route within the

 Rods and Cones  Bipolar Cells  Ganglion Cells

 The axons of the ganglion cells form the

2 movement

 Although each fixation generates a different sensation at the level of the retina, the brain creates a single

Yarbus, 1967

Why have two of them?

 Convergence :: –– must turn slightly inward to focus when objects are close  Binocular disparity :: –– difference between the images on the two  Both are greater when objects are close –– provides brain with a 33--DD image and distance information

3 Sensory neurons for vision

 RODS and CONES: –– Specialized neurons that respond to light with changes in their membrane potential

Photoreceptors: Rods and Cones

 RODS :: –– ~120 million rods –– Scotopic Vision (skotos=darkness) –– Sensitive to brightness, but not color (shades of gray)

4 Photoreceptors: Rods and Cones

 RODS :: –– Many rods converge onto one –– Responsible for lowlow--lightlight vision –– Not present at all in fovea

Photoreceptors: Rods and Cones

 CONES: –– ~6 million cones –– (photos=light) –– Sensitive to color

5 Photoreceptors: Rods and Cones

 CONES: ––A single retinal ganglion cell receives signals from one (or few) cones. –– Responsible for high acuity vision (fine detail) –– Fovea contains only cones

Rods and Cones  The outer segment of a photoreceptor contains hundreds of lamellae.  Within the lamellae you find photopigmentsphotopigments--moleculesmolecules that contain an opsin and a retinal. (E.g(E.g.. ))

6 Rhodopsin  Rhodopsin is a receptor that responds to light instead of to neurotransmitters (photons bind to it)  When rhodopsin is exposed to light, it breaks down and the opsin bleachesbleaches..

 The effect of the bleaching is a change in the release of NT –– Not the way you might think!

Strange but true…

 The effect of light is to turn receptor cells ““OFFOFF””…… darkness turns them ““ONON””!!

 Remember that receptors have a spontaneous firing rate: –– They do NOT fire action potentials, but graded potentials –– The effect of receptors firing is inhibition of the bipolar cells

7 Transduction: how light becomes neural signals

 A cone or rod actually releases LESS neurotransmitter when stimulated by light! –– Rhodopsin molecules are bleached by light, causing hyperpolarization of rods. –– Thus, inhibition: less release of neurotransmitter (glutamate) –– Result is: depolarization of bipolar cell (= more release of neurotransmitter) –– Ganglion cell is more likely to fire (generally)

The effect of a bleached …

 … is that a the photoreceptor ’’s membrane potential changes.  ReceptorReceptor’’ss membrane potential affects release of NT onto bipolar cells.  Bipolar cells ‘‘speak ’’ to ganglion cells, which bring information to the brain.

8 So in the dark ……

 Photoreceptors release enough NT to prevent bipolar cells from triggering ganglion cells. –– Ganglion cells, by NOT firing, report to the brain: ““nono lightlight””

 And in the light?

Cone and Rod Vision

Distribution of rods and cones

 Only cones are found at the fovea!!

9 Cone and Rod Vision

 Less convergence in cones, increasing acuity while decreasing sensitivity

 More convergence in rod system, increasing sensitivity while decreasing acuity

So we have a response from a ganglion cell… now what?

 Bundle of ganglion cell axons exiting the eye: blind spot  No receptors where information exits the eye: –– Visual system uses information from cells around the blind spot for ““completion, ”” filling in the blind spot

10 From the Eyes to the

Lateral Geniculate Nucleus

 A nucleus within the ((““relay center ””) –– receives information from the retina and projects to primary visual cortex.  Contains six ““layers ”” of neurons –– each layer receives information from only 1 eye.  First two layers: magnocellular  Next four layers: parvocellular

11 M and P channels

 Magnocellular –– Larger cell bodies –– Responsive to movement –– Input primarily from rods  Parvocellular –– Small cell bodies –– Responsive to color, fine details –– Input primarily from cones

M and P channels

 Layers 1, 4, 66--contracontra  Layers 2, 3, 55--ipsiipsi

12 From the Eyes to the Visual Cortex

 The visual system is organized retinotopically:

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

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

Coding of Visual Information

13 Coding of information in the retina

 For any sensory neuron, a receptive field is the ‘‘place ’’ in which a stimulus will cause the neuron to fire.  The receptive fields in the fovea are smaller than in the rest of the retina.

Receptive Fields  Many ganglion cells have receptive fields with a centercenter--surroundsurround organization: excitatory and inhibitory regions separated by a circular boundary  Some cells are ““onon-- center ”” and some are ““offoff--centercenter ””

14 What does color get us?

What does color get us?

15  Why can you visualize red (imagine a firefire-- truck)… and you can imagine a reddish yellow… but it is difficult (impossible?) to imagine a reddish green…  … or a bluishbluish--yellow?!yellow?!

Color Mixing vs. Pigment Mixing

16 theories

 Trichromatic theory: there are 3 different receptors (types of cones) in the eye, each sensitive to a single hue (red, green, blue)

Color vision theories

 Trichromatic theory: there are 3 different receptors (types of cones) in the eye, each sensitive to a single hue (red, green, blue) –– Because Young noted that any color could be accounaccountetedd for by mixing just 3 in various proportions

17 Trichromatic Theory

 At the level of the retina, cones code for three wavelengths of light (different opsins):  Short (S), Medium (M), Long (L): blue, green, red

18 19  The precise distribution of cones varies from person to person, but generally speaking ““blueblue”” sensitive cones are less common than ““redred””andand ““greengreen”” cones Image from David Williams, U of Rochester

Color 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

20  Those with normal color vision should read the number 8.  Those with redred--greengreen color vision deficiencies (protanopia, deuteranopia) should read the number 3.  Total should not be able to read any numeral.

The trichromatic theory doesn ’’t tell the whole story…

21 The trichromatic theory doesn ’’t tell the whole story…  The retinal ganglion cells code for complementary colors.  This is known as opponentopponent--processprocess coding

 Another type of ganglion cell only encodes brightness: ‘‘blackblack--whitewhite ’’

22 Opponent Process Theory

 Ganglion cells –– Three types  Red/green, yellow/blue, black/white –– Each cell represents an opponent process system  Resting behavior in red/green cells is midmid--levellevel rate of response  For R+GR+G--,, rate increases when red is present, decreases when green is present (opposite for RR--G+)G+)  Yellow/blue (Y+B(Y+B--)) increases when both red and green are present, decreases when blue is present

Opponent Process Theory

Opposing retinal processes enable color vision

““ONON ”” ““OFF ”” red green green red blue yellow yellow blue black white white black

23  ReddishReddish--green?green? BluishBluish--Yellow?Yellow?

 You can ’’t imagine them because ganglion cells that signal red or green (or yellow or blue) can only increase or decrease rate of firing, they can ’’t do both at once!!

24  The complementary afterafter--effecteffect is caused by the fact that after , locations stimulated by green light will be less sensitive to green than to red, and vice versa. Since white light contains all colors and stimulates all photoreceptors equally, those that have been ““green adapted ”” will fire ‘‘redred ’’ to white light (and vice versa)versa)--aa larger ““redred ”” than ““green ”” signal will be generated. It ’’s the local imbalance between the red and green inputs to the opponent mechanism that generates the (relatively weak) color afterafter--effecteffects.s.

We haven ’’t even reached the cortex yet!  Primary visual cortex (Striate Cortex, V1)  Visual Association cortex (extrastriate)

25 Primary Visual Cortex (V1, Striate Cortex)

~140 million neurons just in V1!

Retinotopic Organization

 Information received at adjacent portions of the retina remains adjacent in V1.  More cortex is devoted to areas of high acuity. (Just like the disproportionate representation of sensitive body parts in somatosensory cortex!)  About 25% of primary visual cortex is dedicated to processing input from the fovea.

26 Striate Cortex

 Six principal layers of striate cortex

Processing in Striate Cortex

 Layers 2 and 3 receive information from the parvocellular layers and koniocellular layers of the LGN.  Cells are grouped together in ““blobs ”” –– Cells within blobs are sensitive to color –– Cells outside blobs are sensitive to orientation, movement, binocular disparity

27 Orientation and Movement

 Most neurons in V1 are sensitive to orientation: –– if a line or edge appears in their receptive field, they respond best when it is at a certain angle

Receptive Fields in Striate Cortex

 Most neurons in V1 are either –– Simple ––receptivereceptive fields are rectangular with ““onon ”” and ““offoff ”” regions, or –– Complex ––alsoalso rectangular, larger receptive fields, respond best to a particular stimulus anywhere in its receptive field

28 Receptive Fields in Striate Cortex

SIMPLE COMPLEX  Rectangular  Rectangular  ““OnOn ”” and ““offoff ””  Larger receptive regions, like cells fields in layer IV  Do not have static  Orientation and ““onon ”” and ““offoff ”” location sensitive regions  All are monocular  Not location sensitive  Motion sensitive  Many are binocular

Orientation and Movement

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

29 Orientation and Movement

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

Orientation and Movement

 Hypercomplex cellscells--respondrespond best to a particular orientation, but have inhibitory region: they code for ends of lines!

30 Beyond Striate Cortex

 Fundamentally, the coding in striate cortex is for features : color, orientation, spatial frequency, retinal disparity  Perception requires the combination of these features into an integrated whole!  This occurs in extrastriate cortex

Dorsal and Ventral Streams

 Dorsal 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 stream : striate cortex  ventral prestriate cortex  inferotemporal cortex – The “what ” pathway (color and shape), or – Pathway for conscious perception of objects

31 6363

PET study of where/what dichotomy

32 Not a fixed feedfeed--forwardforward system!

Image from Wagner and Kline

33 Visual

 Deficits in visual form perception  NOT blindness!  Caused by damage to visual association areas in ventral stream

 Video….

Prosopagnosia

 Damage to the fusiform face area (FFA) results in 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)

34  The lateral occipital complex is activated in response to a wide variety of objects.  It seems possible that different categories of objects are processed at least in part in different subregions.

 Also in the ventral stream is the extrastriate body area –– Seems to be particularly responsive to body parts

35 Perception of Movement

36