Visual System 3: Cortical Areas & Functions Flashcards
what is the extrastriate cortex?
all visual cortex beyond primary visual cortex is called _extrastriate cortex
There are submodalities in vision any one of which can serve to segment the visual scene into surfaces and objects: color, motion, depth, texture
how is V2 organized?
V2 is retinotopically organized and represents the entire contralateral visual field.
V2 also has modules within it and these are defined by the CO staining. Instead of the discrete CO blobs seen in V1, V2 is
characterized by parallel bands defined on the basis of the staining, all running perpendicular to the V1-V2 border.
There are 3 types of bands in V2 called thick stripes, thin stripes, and interstripes (or pale stripes).
what do the CO bands in V2 represent?
distinct compartments (modules) defined on the basis of their connections with other cortical areas and the properties of the RFs of cells found in each.
how many visual cortical areas are there in brain?
many visual cortical areas (estimated 30), most with a complete retinotopically organized representation of the contralateral hemifield.
Collectively, these areas fill all of the occipital lobe and much of the parietal and temporal lobes (>40% of the neocortex in man).
how were ventral and dorsal pathways discovered?
monkey experiment with object discrimination and location discrimination - lesions in different areas prevented the monkey from learning one of the tasks, while the other ability remained intact
posterior parietal cortex –> location
inferotemporal cortex –> object identification
where does the dorsal path go?
The dorsal stream arises from layer IVB in striate cortex, proceeds through the thick CO stripes in V2 and proceeds through a series of visual areas including MT to end in the posterior parietal cortex.
where does the ventral pathway go?
arises from layers II-III in striate cortex, proceeds through the pale stripes in V2 and a series of areas including V4 to end in the inferotemporal cortex
what does dorsal pathway do?
specialized for representing the scene in a way suitable for guiding action (motion, depth).
what does ventral pathway do?
specialized for neural representation of objects both animate and inanimate.
how big is MT?
the size of a dime
what are the supporting reasons for MT as the homunuculus for motion? 9 reasons
All MT cells are direction but not orientation selective
-Direction is represented in a columnar system
-Lesions of MT produce selective deficits in detecting/discriminating motion –> akinotopsia_-
-Motion thresholds of cells in MT and the whole monkey are the same
-Lesions of MT produce errors in smooth pursuit eye movements
-Electrical stimulation within a direction column biases the monkeys judgement of the direction of motion in predictable ways
-Lesions of MT abolish the ability to perceive structure from motion
-Cells in MT solve a high level problem in motion perception _ the aperture problem
akinotopsia
inability to understand motion in the visual environment
what is the aperture problem?
The motion of an edge cannot be determined by a single orientation selective cell. (in class showed that box moving over the circle)
MT cells combine the outputs of multiple V1 cells in such a way that MT cells are rendered sensitive to the true direction of motion regardless of the orientation of the edges. Thus MT cells and the networks they are embedded in solve the aperture problem
from where do extrastriated cortices receive inputs?
V1 but also from thalamic nuclei (the pulvinar, not the LGN).
what is the main ventral stream area?
v4
