CH 6: The Visual System Flashcards
Explain how the PUPIL can affect the image that falls on the retina.
What occurs during high illumination vs low?
- 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
Explain how the LENS can affect the image that falls on the retina.
Looking at something near vs far?
- 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
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.
- 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
Define BINOCULAR DISPARITY
- 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
Describe the functions of the retina (3).
- Converts light to neural signals
- Conducts the neural signals toward CNS
- Participates in processing of the neural signals
Name the cell types that make up the retina (5).
- RECEPTORS = cells specialized to receive chem, mech, or radiant signals from environ.
- HORIZONTAL Cells = specialized function = lateral communication
- BIPOLAR Cells = bipolar neurons that form in middle layer of retina
- AMACRINE Cells = specialized function = lateral communication
- RETINAL GANGLION Cells = neurons whose axons leave the eyeball & form optic nerve
Define LATERAL COMMUNICAITON
Retinal neurons communicate:
- ________ via ______
- ________ via ______
- Communication across major channels of sensory input
Retinal neurons communicate:
- CHEMICALLY via synapses
- ELECTRICALLY via gap junctions
Describe the list of events that occur after light reaches receptor layer (3).
- 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
Describe the DUPLEXITY THEORY of Vision.
- Cones & rods mediate diff kinds of vision
- Sp active only @day = CONE-only retinas
- Sp active only @night = ROD-only retinas
Explain the differences b/w the photopic & scotopic systems.
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
Explain difference b/w sensitivity & acuity.
- SENSITIVITY = ability to detect presence of dimly lit objects
- ACUITY = ability to see details of objects
Define SPECTRAL SENSITIVITY CURVE
- Graph of the relative brightness of lights of same intensity presented at diff wavelengths
Explain the difference b/w the photopic & scotopic spectral sensitivity curves.
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
Explain how the difference b/w the photopic & scotopic spectral sensitivity curves can account for the Purkinjie effect
- 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
Define FIXATIONAL EYE MOVEMENTS
- Involuntary movements that occur when person tries to stare at a point
List the 3 types of fixational eye movements.
- Tremors
- Drifts
- Saccades = small, jerky movements/flicks
Explain what happens when all eye movements are blocked.
- 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
Define TRANSDUCTION
- Conversion of 1 form of E to another
Define VISUAL TRANSDUCTION
- Conversion of light to neural signals via visual receptors
- see pg 143-144 for more details
Describe the components & layout of the retina-geniculate-striate system.
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
WRT the retina-geniculate striate system, explain’s what’s meant by RETINOTOPIC.
- 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
Describe the P Channels
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
Describe M Channels
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
Define RECEPTIVE FIELD of a visual neuron
- Area of visual field in within which it’s possible for a visual stimulus to influence the firing of that neuron
Describe the methods used by David Hubel & Torsten Wiessel to map the receptive fields of visual system neurons.
- 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
Describe the characteristics of the receptive fields of:
- Retinal ganglion cells
- Lateral geniculate neurons
- Striate neurons of lower layer IV
What are 4 commonalities?
- 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
- All neurons (retinal ganglion cells, lateral geniculate neurons, & lower layer IV neurons) had CIRCULAR receptive fields
- All neurons were MONOCULAR = each neuron had a receptive field in 1 eye but not the other
- 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
Describe the characteristics of the receptive fields of SIMPLE cortical cells.
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
Describe the characteristics of the receptive fields of COMPLEX cortical cells.
Compare & contrast w/ simple cells (3).
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
Define the main function of the PRIMARY VISUAL CORTEX
= part of cerebral cortex which processes visual info
Describe the organization of the primary visual cortex (3).
- 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 - 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 - 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
Describe the changing view of visual sys receptive fields (Initial vs. current assumptions).
- 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
Describe the COMPONENT Theory of colour vision.
- 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
Describe the OPPONENT-PROCESS Theory of colour vision.
- 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
Describe Land’s COLOUR CONSTANCY
- = 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
Describe Land’s RETINEX THEORY of colour vision.
- = 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
List the 3 classes of visual cortex.
- Primary visual cortex
- Secondary visual cortex
- Visual Association cortex
Describe the Primary Visual Cortex & its location in the brain.
- Receives most of its input from visual relay nuclei of thalamus (ie. from the lateral geniculate nuclei)
- Located in posterior region of occipital lobes
Describe the Secondary Visual Cortex & its location in the brain.
- 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
Describe the Visual Association Cortex &a; its location in the brain.
- Receives input from areas of secondary visual cortex & secondary areas of other sensory systems
Describe the major flow of visual info in the cortex.
- 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
Explain what is produced when an area of primary visual cortex is damaged.
- 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
Define BLINDSIGHT
- 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
Explain the difference b/w the DORSAL & VENTRAL Streams.
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
List the 2 theories of the functional differences b/w dorsal vs. ventral stream.
- ‘Where’ vs. ‘What Theory
2. ‘Control of Behaviour’ vs. ‘Conscious Perception’ Theory
Explain the ‘Where’ vs ‘What Theory.
What is predicted to occur from damage to the dorsal vs. ventral stream?
- 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
Explain the ‘Control of Behaviour’ vs. ‘Conscious Perception’ Theory.
What is predicted to occur from damage to the dorsal vs. ventral stream?
- 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
Describe the phenomenon of PROSOPAGNOSIA.
- The visual agnosia for faces
- Visual agnosia = things can be seen but not recognized/identified
- Acquired either during development or result of brain injury
Describe the 1st theoretical issue ass. w/ prosopagnosia.
- 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.
Describe the 2nd theoretical issue ass. w/ prosopagnosia.
- 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
Describe the 3rd theoretical issue ass. w/ prosopagnosia.
- 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
Describe the phenomenon of AKINETOPSIA.
- 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
Describe the theoretical issues associated w/ Akinetopsia.
see pg 163
