Read Lamme (2000) TINS article for Wednesday. Visual Pathways V1 is, of course, not the only visual area (it turns out it’s not even always “primary”)

Read Lamme (2000) TINS article for Wednesday

Visual Pathways

• V1 is, of course, not the only visual area

• (it turns out it’s not even always “primary”)

• What role does the rest of visual cortex play?

Visual Pathways

• Visual processing “culminates in formatting the representation of the stimulus so it matches (or not) information in memory”

• Alternatively, visual processing formats the representation in a way that can be stored for later comparison to other inputs…visual memory

The Role of “Extrastriate” Areas

• Consider two plausible models:

1. System is hierarchical:– each area performs some elaboration on the input it is given

and then passes on that elaboration as input to the next “higher” area

2. System is analytic and parallel:– different areas elaborate on different features of the input


• Consider two plausible models:

1. System is hierarchical:– each area performs some elaboration on the input it is given

and then passes on that elaboration as input to the next “higher” area

2. System is analytic and parallel:– different areas elaborate on different features of the input

– Already by the retinal ganglion cells we see this “divide-and-conquer” strategy


• Different visual cortex regions contain cells with different tuning properties


• Cells in Parietal lobe have little preference for the size of stimuli


• Cells in Temporal Lobe are sharply tuned to the form of an object


• Each visual area has cells that are specifically tuned for various features or properties in the scene

• E.g. V5 (area MT) has cells that respond to the presence of motion in their receptive fields – these cells are direction

tuned but are non-specific for color

– V5 cells even have velocity specificity!

Unit Recordings


• Functional imaging (PET) investigations of motion and colour selective visual cortical areas

• Zeki et al.

• Subtractive Logic– stimulus alternates between two scenes that differ only in

the feature of interest (i.e. colour, motion, etc.)


• Identifying colour sensitive regions

Subtract Voxel intensities during these scans…

…from voxel intensities during these scans

…etc.Time ->


• result– voxels are identified that are preferentially selective for

colour

– these tend to cluster in anterior/inferior occipital lobe


• similar logic was used to find motion-selective areas

Subtract Voxel intensities during these scans…

…from voxel intensities during these scans

…etc.Time ->

MOVING STATIONARY MOVING STATIONARY


• result– voxels are identified that are preferentially selective for

motion

– these tend to cluster in superior/dorsal occipital lobe near TemporoParietal Junction

– Akin to Human V5


• Thus PET studies doubly-dissociate colour and motion sensitive regions


• Visual areas elaborate on low-level input

– in Enigma pattern, motion is perceived, but the low-level input is stationary

– functional imaging reveals V5 but not V1 activation associated with viewing this stimulus


• V4 and V5 are doubly-dissociated in lesion literature:



– achromatopsia (color blindness): • there are many forms of color blindness• cortical achromatopsia arises from lesions in the area of V4• singly dissociable from motion perception deficit - patients with

V4 lesions have other visual problems, but motion perception is substantially spared



– akinetopsia (motion blindness): • bilateral lesions to area V5 (extremely rare)• severe impairment in judging direction and velocity of motion -

especially with fast-moving stimuli• visual world appeared to progress in still frames• similar effects occur when M-cell layers in LGN are lesioned in

monkeys

• Taken together, achromatopsia and akinetopsia constitute a double-dissociation of the functions of V4 and V5

Dual Pathways

• V4 and V5 are key parts of two larger functional pathways:

– Dorsal or “Where” pathway

– Ventral or “What” pathway

– Ungerleider and Mishkin (1982)

• Magno and Parvo dichotomy arose at the retina and gives rise to two distinct cortical pathways

Dual Pathways

• Why “What” and “Where”?

– monkey lesion experiments– human lesions– differences in tuning

properties of cells– neuroimaging

Dual Pathways

• Pohl (1973) Early dissociations of Temporal and Parietal functions

• Landmark task:

– Monkeys trained to find reward in well near a landmark

– once they get the task the contingency is switched

– #errors until relearning indicates ability to use the spatial relationship information to perform task

Dual Pathways


• Landmark task:

– Dissociates Parietal and Temporal lobes

– Parietal lesions impair relearning of landmark task

Dual Pathways


• Object task:

– Reward location is indicated by one of two objects

– contingency is switched

– # errors to relearn indicates ability to use object distinction to perform task

Dual Pathways


• Object task:

– Adding this task doubly dissociates Parietal and Temporal lesions

– Temporal lesions impair object task

Documents

Read Lamme (2000) TINS article for Wednesday. Visual Pathways V1 is, of course, not the only visual area (it turns out it’s not even always “primary”)