Extra-striate cortex: perception & action Flashcards
how much % of the human cerebral cortex is specialised for different aspects of visual function
33%
what is the extra-striate cortex important for
visually guided actions
what is the composition of the hyper column
is input from one eye to the other eye, in a point in space, in form of ocular dominance columns
what are the reason for hyper columns
all possible stimulus, colours, forms (orientations) & directions of motion are represented for each point in space via both the left and right eyes
what are in each ocular dominance columns
is a cell which contains cytochrome oxidase deposited as cytochrome oxidase blobs and blob cells interested in processing colour information and can perceive each individual colour in that particular visual field, and inter blob zones surrounding the blobs are capable of encoding every conceivable stimulus orientation that falls in that region of space & occupying layer 4B are a bunch of cells direction selective and capable of encoding every conceivable direction of motion of the stimulus in that point in space
how is the arrangement within the hyper column
parallel image processing to & within area v1 & v2 cortex, beginning with different retinal ganglion cells in the retina
what property in area v2 does the midget RGCs which take up the parvo pathway where layer 4C beta goes on to blob cells and inter blob cells in area v1 form
thin stripe cells: colour
what property in area v2 does the parasol RGCs which take up the magno pathway where layer 4C alpha goes on to inter blob and layer 4B in area v1 form
inter blob cells: pale stripe cells: form
Layer 4B: thick stripe cells: motion
what property in area v2 does the small bi stratified RGCS which take up the konio pathway which goes straight to blob cells
thin stripe cells: colour
what does parallel processing that goes up in area v1 continues outputs of area v1 to adjoining extra striate cortex v2 also contains
unique cytochrome oxidase and has cells within it that has different levels of co staining & deposited in orderly & systematic fashions
is the image processing mediated by the cells in v1 & v2 cortex sufficient to identify the object as what it is
it is not as the RFs of cells in the hyper columns of these two early cortical areas only analyse each small region of the object like individual pieces of a jigsaw
what is required to receive a whole image
further processing by cells with much larger and more complex RFs that are able to see the big picture, which are found in higher extra striate cortical areas of the visual cortex
in area v1 what form of the image is present
still has a very pixelated image 1000 pixels, photoreceptors slightly rebuild from this in the retina which pixelate the image into tiny spots of light = 75 mega pixels
what was the traditional view of the human visual cortex
was thought to be restricted to the occipital lobe with the primary visual area (v1 striate cortex, area 17) responsible for mediating all visual sensations, since damage to v1 was known to result in anoxia in the opposite hemifield.
striate cortex was assumed to send information (via direct anatomical connections) to surrounding ‘visual association areas’ in occipital cortex (i.e. brodmann areas 18 & 19) where a unified percept was constructed from the form, colour and motion sensations related to an object & by associating these with memories of the same or similar objects from past visual experience
in the traditional view of the human visual cortex, what was the primary/striate cortex (area 17) considered to be
the visuo-sensory area
in the traditional view of the human visual cortex, what was area 18 & 19 considered to be
visuo-psychic areas associating the sensations into a unified visual and memory based percept and is extended up to bordmann areas 37, 20 & 21
what is the modern view of the human visual cortex
v1 is just one, the largest of several (8-10) other discrete visual areas in the occipital lobe each with a separate function, with area v2 adjoining it & area v3 running parallel to v2, plus others located more dorsally (V3A, V5/MT, V7) or ventrally (V4, V8, LOC), each of which contains it own retinotopic map of the opposite hemi-field
also the entire posterior parental (PP) cortex (dorsally: BA 7, 5, 39& 40) and inferior occipito-temporal (IOT) plus adjoining inferior temporal (IT) cortex (ventrally: BA 37, 20, & 21) are also mainly involved in vision, with these regions each containing at least 10 additional visual areas
what is the functional specialisation (zeki), general theory of functional organisation of multiple extra-striate areas
- areas v1 & v2 ‘sub contract’ the labour of processing specific object properties to specialised, high occipital cortical areas, via the anatomically & functionally segregated i.e. parallel pathways emanating from these early cortical areas
- so that each is concerned with processing just one object attribute
- this ‘division of labour’ is both necessary and efficient because different cellular mechanisms (algorithms) are required to process form vs colour vs motion vs depth information, and it would not be possible to house all the different mechanisms in the same cortical area
in addition, these specialised areas represent all-purpose processors of the specific object attribute & each sends its their own parallel outputs to even higher ‘satellite’ PP or IT areas mega-specialised for processing specific ‘sub-modalities’ of each object property e.g. V5/MT to different areas of MST concerned with motion in depth or optic flow processing
in the functional specialisation (zeki), general theory of functional organisation of multiple extra-striate areas, cells of hyper columns of V1 & V2 have parallel pathways that come out of pyramidal cells with long axons, sends specific types of information to specific areas of extra-striate cortex which are
form - lateral occipital area (LOC)
colour - V4
motion - V5/MT
depth - V3A/V7
in area V1 & V2 what cells are segregated in different parts of the hyper column
cells with receptive field (RF) properties showing selectivity for the orientation (form), wavelength (colour) or direction of motion of visual stimuli
these different cells send parallel projections (outputs) to different retinotopic occipital areas at the next stage of visual processing in extra striate cortex
give examples of the similarities of the distribution of RF properties among V1 & V2 cells
75% orientation selective
40% colour coded
25% direction selective
what do the similarities of the distribution of RF properties among V1 & V2 cells suggest
that V1 and V2 have similar/overlapping functions
what area is functionally specialised for colour processing and what does it contain to show this
area V4
it contains mainly: 80% wavelength or colour sensitive cells & very few 5% that are direction-selective (i.e. not interested in motion)
what area is functionally specialised for motion processing and what does it contain to show this
area V5/MT
it contains 90% direction selective cells & very few 5% interested in colour or form of the moving stimulus
if there was a lesion made to primary V1 cortex, what are the 5 separate areas infront of it that contain patches of degeneration i.e. that it makes connections with
5 areas in the extra striate cortex 1 in area V2 right next to V1 1 in area V3 1 in area V3A 1 in area V4 1 in area V5
another pattern was these connections were retinotopically organised with a retinotopic map of the opposite hemi-field
the proportion or orientation selective cells, colour coded cells & direction coded cells found in area v1 & v2, what are the % of neurons in monkey visual cortex v1 & v2
orientation selective 75%
colour selective 40%
direction & motion 25%
which have similar proportions in area v2 but different beyond in higher cortical extra striate areas
what are the proportion or orientation selective cells, colour coded cells & direction coded cells found in area v4, what are the % of neurons in monkey visual cortex v4
wavelength colour selective 80%
direction & motion selective 5%
orientation selective 25%
what are the proportion or orientation selective cells, colour coded cells & direction coded cells found in area v5/MT, what are the % of neurons in monkey visual cortex v5/MT
orientation 25%
direction & motion 90%
wavelength colour selective 5%
what is the ‘what’ verses ‘where’ or ‘how’ cortical processing streams (mishkin & ungerleider/goodale & milner), general theory of the functional organisation of multiple extra-striate areas
- the pathways emanating from areas v1 & v2 involve continuations of the parvo vs magno systems, with 2 parallel processing streams passing through 2 different sets of specialised extra striate areas concerned with really key differences in object attributes
- ventral, perceptual & mainly parvo pathway via v3, v4, v8 & LOC to infra temporal (IT) cortex = what objects are: with each area specialised for aspects or form & or colour processing
- dorsal, spatial/action & mainly magno pathway via V5/MT, V3A & V7 to posterior parietal (PP) cortex = where objects are and how to interact with them for controlling visually guided actions with each area specialised for aspects of motion & or 3D spatial processing
what does the ventral, perceptual & mainly parvo pathway go via to reach the infra temporal (IT) cortex
via v3, v4, v8 & LOC
what is the ventral, perceptual & mainly parvo pathway interested in
WHAT objects are, with each area specialised for aspects of form & or colour processing
what does the dorsal, spatial/action & mainly magno pathway go via to reach the posterior parietal (PP) cortex
V5/MT, V3A & V7
what is the dorsal, spatial/action & mainly magno pathway interested in
WHERE objects are and how to interact with them for controlling visually guided actions with each area specialised for aspects of motion & or 3D spatial processing
what do magno cells of the LGN go onto, and what is it interested in
layer 4B
cells interested in depth & direction selective
what are cells in area v4 interested in
colour & form (some depth, no motion)
what are cells in area V5/MT interested in
motion & 3D space (some form, no colour)
where is area v4 located
lingual gyrus, inferior-medial occipital cortex
what is area v4 selectively activated by
coloured stimuli
what does localised damage to area v4 result in
selective deficit in colour perception
what is the name for selective deficit in colour perception
achromatopsia
where is the lateral occipital cortical (LOC) area
posterior inferior temporal gyrus
what is the lateral occipital cortical area LOC selectively activated by
shapes & objects
what does localised damage to the lateral occipital cortical area LOC result in
selective deficit in object perception
what is the name for selective deficit in object perception
form agnosia (unable to perceive what objects are)
where is the fusiform area (FFA) located
posterior fusiform gyrus
what is the fusiform area FFA selectively activated by
faces
what does localised damage to the fusiform area FFA result in
selective deficit in face perception
what is the name for selective deficit in face perception
prosopagnosia (inability to identify faces)
what stimuli are conscious things which = what pathway
colour, object & face = what pathway
so where & action control are unaffected therefore patients with ventral/perceptual deficits can still localise objects in space & interact with them
what do the cells in primary v1 cortex equally respond well to
real & scrambled images
what is the lateral occipital part of the cortex activated by
real objects
what happens to a patient with form agnosia, v1 is intact but LOC is bilaterally damaged
px cannot copy simple drawings of familiar objects, because she can only see bits (text on page) of the object, but not the whole thing (though her memory for what these objects should look like seem intact)
but she can also accurately reach out and pick up a real apple or book even though she doesn’t know what they are
why can a px with form agnosia accurately reach out and pick up a real apple or book even though she doesn’t know what they are
their dorsal (where & how) pathway is working perfectly
which part of the brain is involved in hand & arm movements/reach out and grasp objects
superior parietal lobule
what is the inferior parietal lobule part of the brain involved in
eye movements
where in the brain is area V5/MT located
junction of inferior occipital & superior temporal sulci
what is area V5/MT selectively activated by
moving stimuli
what does localised damage to area V5/MT result in
a selective deficit in motion perception
what is the name for a selective deficit in motion perception
akinetopsia (can’t see moving objects, can see colour & stationary objects, good Vas and depth)
where is the inferior parietal lobule located
in & below intra-parietal sulcus
what is the inferior parietal lobule selectively activated during
particular eye movements
what does localised damage to the inferior parietal lobule result in
disruption of particular eye movements
where is the superior parietal lobule located
in & above the intra-parietal sulcus
what is the superior parietal lobule selectively activated during
reaching & grasping actions
what does localised damage to the superior parietal lobule result in
selective deficit in hand movement control
what is the name for selective deficit in hand movement control
optic ataxia (can’t reach and grasp)
what can patients with dorsal/spatial-action deficits still do
still perceive what objects are
what happens when a patient with optic ataxia: v1 intact, but left superior parietal lobule SPL damaged (opposite right hand)
px can correctly describe (perceive) the target orientation as vertical, but cannot control her moving hand when attempting to insert it into the slot.
Two error types:
- incorrect hand orientation
- spatial mis-localisation (correct orientation but missed the slot)
what is double dissociation
people with form agnosia are useless at perceiving orientation of the stimuli, but are fine at posting hand in a slot whereas people with optic ataxia are good at perceiving orientation of an object but are useless at posting hand in different orientations
what does damage to the ventral system do
knocks out perception but leaves action intact
what does damage to the dorsal stream do
knocks out visually guided movements but leaves perception intact
in the extra striate cortex, what do thin stripe cells in V2 receive direct input from
inter blob cells of primary v1 cortex
in the extra striate cortex, damage to area v4 results in
selective deficit in colour perception
in the extra striate cortex, area V5/MT is considered to be part of which processing pathway
magnocellular
in the extra striate cortex, what is associated with the control of eye movements
the inferior parietal lobule
