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psyc 304 > CH 6: The Visual System > Flashcards

CH 6: The Visual System Flashcards

1
Q

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

What occurs during high illumination vs low?

A
  • 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
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2
Q

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

Looking at something near vs far?

A
  • 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
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3
Q

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.

A
  • 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
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4
Q

Define BINOCULAR DISPARITY

A
  • 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
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5
Q

Describe the functions of the retina (3).

A
  1. Converts light to neural signals
  2. Conducts the neural signals toward CNS
  3. Participates in processing of the neural signals
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6
Q

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

A
  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
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7
Q

Define LATERAL COMMUNICAITON

Retinal neurons communicate:

  1. ________ via ______
  2. ________ via ______
A
  • Communication across major channels of sensory input

Retinal neurons communicate:

  1. CHEMICALLY via synapses
  2. ELECTRICALLY via gap junctions
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8
Q

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

A
  • 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
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9
Q

Describe the DUPLEXITY THEORY of Vision.

A
  • Cones & rods mediate diff kinds of vision
  • Sp active only @day = CONE-only retinas
  • Sp active only @night = ROD-only retinas
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10
Q

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

A

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
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11
Q

Explain difference b/w sensitivity & acuity.

A
  • SENSITIVITY = ability to detect presence of dimly lit objects
  • ACUITY = ability to see details of objects
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12
Q

Define SPECTRAL SENSITIVITY CURVE

A
  • Graph of the relative brightness of lights of same intensity presented at diff wavelengths
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13
Q

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

A

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

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14
Q

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

A
  • 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
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15
Q

Define FIXATIONAL EYE MOVEMENTS

A
  • Involuntary movements that occur when person tries to stare at a point
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16
Q

List the 3 types of fixational eye movements.

A
  1. Tremors
  2. Drifts
  3. Saccades = small, jerky movements/flicks
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17
Q

Explain what happens when all eye movements are blocked.

A
  • 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
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18
Q

Define TRANSDUCTION

A
  • Conversion of 1 form of E to another
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19
Q

Define VISUAL TRANSDUCTION

A
  • Conversion of light to neural signals via visual receptors

- see pg 143-144 for more details

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20
Q

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

A

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
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21
Q

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

A
  • 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
22
Q

Describe the P Channels

A

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
23
Q

Describe M Channels

A

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
24
Q

Define RECEPTIVE FIELD of a visual neuron

A
  • Area of visual field in within which it’s possible for a visual stimulus to influence the firing of that neuron
25
Q

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

A
  • 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
26
Q

Describe the characteristics of the receptive fields of:

  • Retinal ganglion cells
  • Lateral geniculate neurons
  • Striate neurons of lower layer IV

What are 4 commonalities?

A
  • 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
  1. All neurons (retinal ganglion cells, lateral geniculate neurons, & lower layer IV neurons) had CIRCULAR receptive fields
  2. All neurons were MONOCULAR = each neuron had a receptive field in 1 eye but not the other
  3. 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

27
Q

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

A

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
28
Q

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

Compare & contrast w/ simple cells (3).

A

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
29
Q

Define the main function of the PRIMARY VISUAL CORTEX

A

= part of cerebral cortex which processes visual info

30
Q

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

A
  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
31
Q

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

A
  • 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

32
Q

Describe the COMPONENT Theory of colour vision.

A
  • 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
33
Q

Describe the OPPONENT-PROCESS Theory of colour vision.

A
  • 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
34
Q

Describe Land’s COLOUR CONSTANCY

A
  • = 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
35
Q

Describe Land’s RETINEX THEORY of colour vision.

A
  • = 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
36
Q

List the 3 classes of visual cortex.

A
  1. Primary visual cortex
  2. Secondary visual cortex
  3. Visual Association cortex
37
Q

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

A
  • Receives most of its input from visual relay nuclei of thalamus (ie. from the lateral geniculate nuclei)
  • Located in posterior region of occipital lobes
38
Q

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

A
  • 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
39
Q

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

A
  • Receives input from areas of secondary visual cortex & secondary areas of other sensory systems
40
Q

Describe the major flow of visual info in the cortex.

A
  • 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
41
Q

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

A
  • 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
42
Q

Define BLINDSIGHT

A
  • 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
43
Q

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

A

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

44
Q

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

A
  1. ‘Where’ vs. ‘What Theory

2. ‘Control of Behaviour’ vs. ‘Conscious Perception’ Theory

45
Q

Explain the ‘Where’ vs ‘What Theory.

What is predicted to occur from damage to the dorsal vs. ventral stream?

A
  • 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
46
Q

Explain the ‘Control of Behaviour’ vs. ‘Conscious Perception’ Theory.

What is predicted to occur from damage to the dorsal vs. ventral stream?

A
  • 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
47
Q

Describe the phenomenon of PROSOPAGNOSIA.

A
  • The visual agnosia for faces
  • Visual agnosia = things can be seen but not recognized/identified
  • Acquired either during development or result of brain injury
48
Q

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

A
  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.
49
Q

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

A
  1. 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
50
Q

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

A
  1. 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
51
Q

Describe the phenomenon of AKINETOPSIA.

A
  • 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
52
Q

Describe the theoretical issues associated w/ Akinetopsia.

A

see pg 163

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