CH 6: The Visual System

Question 1

Explain how the PUPIL can affect the image that falls on the retina.

What occurs during high illumination vs low?

Answer 1

• Light enters eye via PUPIL = hole in iris
• Pupil size adjusts IRT changes in illumination
• HIGH illumination
• -> sensitivity unimportant
• -> pupils constrict
• -> image sharpens on retina
• -> ^depth of focus
• LOW illumination
• -> pupils dilate
• -> ^light in
• -> sacrifice clarity & focus

Question 2

Explain how the LENS can affect the image that falls on the retina.

Looking at something near vs far?

Answer 2

• LENS = behind each pupil = focuses incoming light on retina
• Look at something NEAR:
• -> Lens assumes natural cylindrical shape
• -> ^Ability of lens to refract light
• -> Brings close objects into sharp focus
• Look at something FAR:
• -> Lens flattens
• -> Adjusts lens to bring image into focus on retina = accommodation

Question 3

Explain why some vertebrates have 1 eye on each side of their head whereas other vertebrates have their eyes mounted side by side on front of their heads.

Answer 3

• Having 1 eye on each side of head = see in almost every direction w/o moving heads
• Having both eyes on front of head = simultaneously view what’s in front
• -> Creates 3D perceptions from 2D retinal images

Question 4

Define BINOCULAR DISPARITY

Answer 4

• The difference in the position of the same image on the 2 retinas
• Is greater for close objects vs distant objects
• Visual sys uses degree of binocular disparity to construct 1 3D perception from 2 2D retinal images

Question 5

Describe the functions of the retina (3).

Answer 5

1. Converts light to neural signals
2. Conducts the neural signals toward CNS
3. Participates in processing of the neural signals

Question 6

Name the cell types that make up the retina (5).

Answer 6

1. RECEPTORS = cells specialized to receive chem, mech, or radiant signals from environ.
2. HORIZONTAL Cells = specialized function = lateral communication
3. BIPOLAR Cells = bipolar neurons that form in middle layer of retina
4. AMACRINE Cells = specialized function = lateral communication
5. RETINAL GANGLION Cells = neurons whose axons leave the eyeball & form optic nerve

Question 7

Define LATERAL COMMUNICAITON

Retinal neurons communicate:

1. ________ via ______
2. ________ via ______

Answer 7

• Communication across major channels of sensory input

Retinal neurons communicate:

1. CHEMICALLY via synapses
2. ELECTRICALLY via gap junctions

Question 8

Describe the list of events that occur after light reaches receptor layer (3).

Answer 8

• Light reaches receptor layer only after passing through other 4 layers
• -> Activate receptors
• -> Neural message transmitted back out through retina layers to retinal ganglion cells
• -> Message exits eyeball via ganglion cells’ axons

Question 9

Describe the DUPLEXITY THEORY of Vision.

Answer 9

• Cones & rods mediate diff kinds of vision
• Sp active only @day = CONE-only retinas
• Sp active only @night = ROD-only retinas

Question 10

Explain the differences b/w the photopic & scotopic systems.

Answer 10

PHOTOPIC Vision:
= cone-mediated vision
= predominantly in good lighting
= provides high acuity (finely detailed), COLOURED perceptions of world

• Few cones converge on each retinal ganglion cell
• -> Effects of same dim light applied to sheet of cones can’t add to same degree
• -> Retinal ganglion cells may not respond to light

SCOTOPIC Vision:
= rod-mediated vision
= predominantly in dim lighting
= not enough light to readily excite cones
= NO sensitivity bc lacks both detail & colour

• Output of hundreds of rod converges on a single retinal ganglion cell
• -> Effects of dim light simultaneously stimulates many rods
• -> Add to influence the firing of the retinal ganglion cell onto which the output of the stimulated rods converges
• -> Pays for ^sensitivity via low acuity

Question 11

Explain difference b/w sensitivity & acuity.

Answer 11

• SENSITIVITY = ability to detect presence of dimly lit objects
• ACUITY = ability to see details of objects

Question 12

Define SPECTRAL SENSITIVITY CURVE

Answer 12

• Graph of the relative brightness of lights of same intensity presented at diff wavelengths

Question 13

Explain the difference b/w the photopic & scotopic spectral sensitivity curves.

Answer 13

PHOTOPIC Spectral Sensitivity Curve:
= graph of sensitivity of CONE-mediated vision to diff wavelengths of light
- Under photopic conditions, visual sys is max sensitive to wavelengths of ~560nm
–> Light at 500nm would have to be ^^intensive than one at 560nm to be seen as equally bright

SCOTOPIC Spectral Sensitivity Curve:
= graph of sensitivity of ROD-mediated vision to diff wavelengths of light
- Under scotopic conditions, visual sys is max sensitive to wavelengths of ~500nm
–> Light at 560nm would have to be ^^intensive than one at 500nm to be seen as equally bright

Question 14

Explain how the difference b/w the photopic & scotopic spectral sensitivity curves can account for the Purkinjie effect

Answer 14

• Bc of these differences in sensitivity, can observe visual effect during transition from photopic to scotopic vision

PURKINJIE EFFECT:

• In intense light, red & yellow wavelengths look brighter than blue or green wavelengths of equal intensity
• In dim light, blue & green wavelengths look brighter than red & yellow wavelengths of equal intensity

Question 15

Define FIXATIONAL EYE MOVEMENTS

Answer 15

• Involuntary movements that occur when person tries to stare at a point

Question 16

List the 3 types of fixational eye movements.

Answer 16

1. Tremors
2. Drifts
3. Saccades = small, jerky movements/flicks

Question 17

Explain what happens when all eye movements are blocked.

Answer 17

• Main effect of eye movements = movement of images on retina
• When eye movements blocked, visual objects begin to fade & disappear
• -> bc most visual neurons respond only to CHANGING images
• -> If retinal images are artificially stabilized (kept form moving on retina), image disappears & reappears
• -> THUS eye movements enable us to see during fixation by keeping images moving on retina

Question 18

Define TRANSDUCTION

Answer 18

• Conversion of 1 form of E to another

Question 19

Define VISUAL TRANSDUCTION

Answer 19

• Conversion of light to neural signals via visual receptors

- see pg 143-144 for more details

Question 20

Describe the components & layout of the retina-geniculate-striate system.

Answer 20

RETINA-GENICULATE-STRITATE PATHWAY:

• Conducts signals from each retina to the primary visual cortex (=stritate cortex) via the lateral geniculate nuclei of the thalamus
• 90% of axons of retinal ganglion cells = part of retina-geniculate striate pathways
• *see pg 145 Figure 6.13**
• all signals from (L) visual field reach (R) primary visual cortex
• all signals from (R) visual field reach (L) primary visual cortex
• Each lateral geniculate nucleus has 6 layers
• -> Each lateral geniculate nucleus receives visual input only from the contralateral field
• -> 3 layers receive input from 1 eye, and 3 from the other

Question 21

WRT the retina-geniculate striate system, explain’s what’s meant by RETINOTOPIC.

Answer 21

• The retina-geniculate striate sys = RETINOTOPIC = each level of sys is organized like map of retina
• aka 2 stimuli presented to adjacent areas of the retina excite adjacent neurons at all levels of the sys
• see pg 145 for more details

Question 22

Describe the P Channels

Answer 22

P channels:
= Parvocellular Layers
= the top 4 layers of the lateral geniculate nuclei that are composed of neurons w/ SMALL cell bodies

• Parvocellular neurons = responsive to colour, fine pattern details, & stationary/slow objects
• CONES provide majority of input here

Question 23

Describe M Channels

Answer 23

M channels:
= Magnocellular Layers
= the bottom 2 layers of the lateral geniculate nuclei that are composed of neurons w/ LARGE cell bodies

• Magnocellular neurons = responsive to movement
• RODS provide majority of input here

Question 24

Define RECEPTIVE FIELD of a visual neuron

Answer 24

• Area of visual field in within which it’s possible for a visual stimulus to influence the firing of that neuron

Question 25

Describe the methods used by David Hubel & Torsten Wiessel to map the receptive fields of visual system neurons.

Answer 25

- Position tip of micro electrode near single neuron in part of visual sys under investigation - Block eye movements via paralyzing eye muscles - Images on screen in front of subject are focussed sharply on retina via adjustable lens - Identify receptive field of neuron - Record responses of neuron to various stimuli w/in its receptive field IOT characterize the types of stimuli that most influence activity - -> Repeat entire procedure for each neuron - Strategy = study neurons near receptors & gradually work up through 'higher' levels of sys in effort to understand the ^complexity of neural responses at each level

Question 26

Describe the characteristics of the receptive fields of: - Retinal ganglion cells - Lateral geniculate neurons - Striate neurons of lower layer IV What are 4 commonalities?

Answer 26

- Found little change in receptive fields as worked through the levels - Found 4 communities: 1. At each level, receptive fields in the foveal area of retina were smaller than those at periphery -- consistent w/ fact that fovea mediates high-acuity vision 2. All neurons (retinal ganglion cells, lateral geniculate neurons, & lower layer IV neurons) had CIRCULAR receptive fields 3. All neurons were MONOCULAR = each neuron had a receptive field in 1 eye but not the other 4. Many neurons at each of the 3 levels of the retina-geniculate-striate sys had receptive fields that compromised an excitatory area & inhibitory area separated by a circular boundary **see pg 149 for important details**

Question 27

Describe the characteristics of the receptive fields of SIMPLE cortical cells.

Answer 27

SIMPLE CELLS: = Neurons in visual cortex that respond max to straight-edge stimuli of a particular width & orientation = Like lower layer IV neurons, have receptive fields that can age divided into antagonistic 'on' & 'off' regions = Unresponsive to diffuse light = Like lower layer IV neurons, are MONOCULAR = respond to stimuli of only 1/2 of the eyes - Difference = borders b/w the 'on' & 'off' regions of cortical receptive fields of simple cells are STRAIGHT LINES rather than circles

Question 28

Describe the characteristics of the receptive fields of COMPLEX cortical cells. Compare & contrast w/ simple cells (3).

Answer 28

COMPLEX CELLS: = Neurons in visual cortex that respond max to straight-edge stimuli in certain orientation in ANY part of their receptive field - Like simple cells: 1. Have rectangular receptive fields 2. Respond best to straight-line stimuli in specific orientation 3. Unresponsive to diffuse light - Differs from simple cells: 1. Have larger receptive fields 2. Not possible to divide the receptive fields of complex cells into static 'on' & 'off' regions - -> bc complex cells respond to particular straight-edge stimuli of particular orientation regardless of its position w/in its receptive field of that cell 3. Are BINOCULAR = respond to stimulus of either eye

Question 29

Define the main function of the PRIMARY VISUAL CORTEX

Answer 29

= part of cerebral cortex which processes visual info

Question 30

Describe the organization of the primary visual cortex (3).

Answer 30

1. Is organized into functional VERTICAL (=at right angles to the cortical layers) columns - All neurons in same column: > Respond to stimuli applied to same area of retina > Are dominated by same eye > 'Prefer' same straight-line angles 2. Location of various functional columns in primary visual cortex is influenced by: - Location on the retina of the column's visual fields - Dominant eye of the column - Column's preferred straight-line angle 3. The 'preferences' of neurons become ^complex as studies progressed from retina, to thalamus, to lower layer IV of visual cortex, to simple cortical cells, to complex cortical cells - Bc neurons w/ simpler preferences converged on neurons w/ ^complex preferences

Question 31

Describe the changing view of visual sys receptive fields (Initial vs. current assumptions).

Answer 31

- The response of a visual cortex neuron depends not only on the stimuli in its receptive field, but on the larger scene in which these stimuli are embedded INITIAL ASSUMPTION: = a visual neuron's receptive field is a property of the neuron resulting from hard-wired conference of neural circuits NOW: = a neuron's receptive field is a plastic property of the neuron that's continuously fine-tuned on the basis of changing signals from the context

Question 32

Describe the COMPONENT Theory of colour vision.

Answer 32

- There are 3 kinds of colour receptors (cones), each w/ a diff spectral sensitivity - The colour of a particular stimulus is presumed to be encoded by the ratio of activity in the 3 kinds of receptors

Question 33

Describe the OPPONENT-PROCESS Theory of colour vision.

Answer 33

- There are 2 diff classes of cells in the visual system for encoding colour & another class for encoding brightness - ie) complementary colours = pairs of colours that prod white/grey when equally combined - **see pg 153 for details**

Question 34

Describe Land's COLOUR CONSTANCY

Answer 34

- = Tendency of an object to appear the same colour despite major changes in the wavelengths of light that it reflects - ie) my blue shirt is still same blue in night vs morning despite the various diff lights that it reflects during day - Improves our ability to tell objects apart in memorable way - -> can respond appropriately to them - Our ability to recognize objects is lessened if their colour changed every time there's change in illumination

Question 35

Describe Land's RETINEX THEORY of colour vision.

Answer 35

- = The colour of an object is determined by its reflectance - REFLECTANCE = the proportion of light of diff wavelengths that a surface reflects - Although the wavelengths of light reflected by a surface ^^change w/ changes in illumination, the efficiency w/ which a surface absorbs each wavelengths & reflects the unabsorbed portion dos NOT change - If the perception of colour depends on the analysis of contrast b/w adjacent areas of the visual field, then some colour neurons must be responsive to colour contrast - **see pg 155 for details**

Question 36

List the 3 classes of visual cortex.

Answer 36

1. Primary visual cortex 2. Secondary visual cortex 3. Visual Association cortex

Question 37

Describe the Primary Visual Cortex & its location in the brain.

Answer 37

- Receives most of its input from visual relay nuclei of thalamus (ie. from the lateral geniculate nuclei) - Located in posterior region of occipital lobes

Question 38

Describe the Secondary Visual Cortex & its location in the brain.

Answer 38

- Receives most of its input from primary visual cortex - Most located in 2 general regions: 1. PRESTRIATE Cortex = band of tissue in occipital lobe that surrounds primary visual cortex 2. INFEROTEMPORAL Cortex = cortex of the inferior temporal lobe

Question 39

Describe the Visual Association Cortex &a; its location in the brain.

Answer 39

- Receives input from areas of secondary visual cortex & secondary areas of other sensory systems

Question 40

Describe the major flow of visual info in the cortex.

Answer 40

- Most visual info enters primary visual cortex via the lateral geniculate nuclei - -> Then segregated into multiple pathways that project separately to various functional areas of secondary, & then association, visual cortex - Primary visual Cortex - -> areas of secondary visual cortex - -> areas of association cortex - ***As move up hierarchy, neurons have larger receptive fields & neurons respond to ^specific & complex nuclei

Question 41

Explain what is produced when an area of primary visual cortex is damaged.

Answer 41

- Produces a SCOTOMA in corresponding area of contralateral visual field of both eyes - Scotoma = area of blindness - Patient not consciously aware of their deficits bc COMPLETION - ie) when looking at complex figure, part of which lies in scotoma, often report seeing complete image

Question 42

Define BLINDSIGHT

Answer 42

- Ability to respond to visual stimuli in a scotoma w/ no conscious awareness of them - ie) responding to things to your (L) visual field even though blind in (L) visual field - Sometimes displayed by patients w/ scotomas resulting from damage to primary visual cortex

Question 43

Explain the difference b/w the DORSAL & VENTRAL Streams.

Answer 43

DORSAL Stream: = Group of visual pathways flowing from primary visual cortex to dorsal prostrate cortex to posterior parietal cortex - Visual cortex neurons here respond most robustly to spatial stimuli --> ie) indicates location or direction of movement VENTRAL Stream: = Group of visual pathways flowing from visual cortex to ventral prostate cortex to inferotemporal cortex - Visual cortex neurons here respond to characteristics of objects --> ie) colour, shape

Question 44

List the 2 theories of the functional differences b/w dorsal vs. ventral stream.

Answer 44

1. 'Where' vs. 'What Theory | 2. 'Control of Behaviour' vs. 'Conscious Perception' Theory

Question 45

Explain the 'Where' vs 'What Theory. What is predicted to occur from damage to the dorsal vs. ventral stream?

Answer 45

- Dorsal stream involved in perception of 'WHERE' objects are - Ventral stream involved in perception of 'WHAT' objects are Predicts: - Damage to dorsal stream disrupts visual spatial perception - Damage to ventral stream disrupts visual pattern

Question 46

Explain the 'Control of Behaviour' vs. 'Conscious Perception' Theory. What is predicted to occur from damage to the dorsal vs. ventral stream?

Answer 46

- Dorsal stream primary function = direct behavioural interactions w/ objects - Ventral stream primary function = mediate the conscious perception of objects Predicts: - Damage to dorsal stream disrupts visually guided behaviour but not conscious visual perception - Damage to ventral stream damages conscious visual perception but not visually guided behaviour

Question 47

Describe the phenomenon of PROSOPAGNOSIA.

Answer 47

- The visual agnosia for faces - Visual agnosia = things can be seen but not recognized/identified - Acquired either during development or result of brain injury

Question 48

Describe the 1st theoretical issue ass. w/ prosopagnosia.

Answer 48

1. IS PROSOPAGNOSIA SPECIFIC TO FACES? - Diagnosed to patients who can't recognize particular faces but can identify other objects ie) chair, dog - -> Flawed evidence bc ability to recognize chair is irrelevant - -> Relevance = recognizing which chair, which pencil, etc.

Question 49

Describe the 2nd theoretical issue ass. w/ prosopagnosia.

Answer 49

2. WHAT BRAIN PATHOLOGY IS ASS. W/ PROSOPAGNOSIA? - Prosopagnosia via damage to ventral surface of brain at boundary b/w occipital & temporal lobes = FUSIFORM FACE AREAS - -> Has parts selectively activated by human faces

Question 50

Describe the 3rd theoretical issue ass. w/ prosopagnosia.

Answer 50

3. CAN PROSPAGNOSICS PERCEIVE FACES IN THE ABSENCE OF CONSCIOUS AWARENESS? - The fact that prosopagnosia results from bilateral damage to ventral sys suggests that dorsal-stream function may be intact - **see pg 162 for details**

Question 51

Describe the phenomenon of AKINETOPSIA.

Answer 51

- Deficiency in ability to see movement progress in a normal smooth fashion - Can be triggered by high doses of certain antidepressants - Ass. w/ damage to MID-TEMPORAL AREA of cortex - Mid-temporal area function = perception of motion

Question 52

Describe the theoretical issues associated w/ Akinetopsia.

Answer 52

**see pg 163**