Lecture 32: Visual System 3: Extrastriate cortex Flashcards
What are submodalities in vision?
Color, motion, depth, texture
Where are these submodalities represented?
Extrastriate cortex or anywhere that’s beyond V1
What is achromatopsia?
Normal color vision at the retinal vision but yet the patient can’t see any color (cortical deficit)
What is akinotopsia?
Where you can’t see motion
Example: man who fears crossing traffic because people keep disappearing and reappearing in new and unexpected locations
What is Brodman’s area 17?
Striate cortex
What is Brodman area 18?
Second visual area (V2)
-locate surrounding V1
How is V2 (extrastriate) organized?
They are retinotopically organized
- divided into Ventral and Dorsal pathways
- has modules with in it like the ChromaOxidase Blob
- blobs and inerblobs just differently oriented
- three types of bands…thick stripe, inner stripe and thin stripe
Where is the dorsal pathway of the extrastriate? What is its function?
It is in the caudal parietal lobe
Function: landmark discrimination…where shit is
Where is the ventral pathway of the extrastriate? What is its function?
Inferotemporal lobe
Function: Object Discrimination
Recognizes the “What”
What are the two areas of the extrastriate we need to be familiar with?
- MT in V2
2. V4 in extrastriate
What is MT? Function?
Abbreviation for middle temporal lobe
-most important part of extra striate
Function: Allows you to see motion
Palmer asserts it is the “homounculus for motion”
Why is the MT the “homunculus for motion”?
All MT cells are direction but not orientation selective
-represent a columnar system
-lesions of MT produce selective deficits in detecting/discriminating motion
Example: akinotopsia…you cant pour tea properly because you can’t see that shit fill up
What is the mechanism for MT to keep track of motion?
Allows for smooth pursuit eye movements
- lesions of MT abolish ability to perceive structure from motion and conduct smooth pursuit eye movements
- cells in MT solve a high level problem in motion perception – the aperture problem
What is the aperture problem? And how does MT solve it?
Having a lesion to the MT is like looking at a room through an aperture (a small ass hole)
Consequence: if a big object moves past the aperture, you can’t tell which direction it is traveling because hole is too small
MT solves this by having multiple apertures and combining outputs of V1 cells
What do lesions in MT lead to?
Loss of ability to detect motion
What is V4? Function?
Area in extrastriate in the inferotemporal lobe
Quintessential area for ventral stream (used for recognition)
-majority of cells are COLOR-coded
-orientation selective, color-coded cell for first time
-allows you to understand differences in BRIGHTNESS, even though the item maintains the same color…thereby solving the color constancy problem
What is the Color constancy problem?
The phenomenon in which color can be changed by a. the source of emittance and b. the change that occurs upon reflection
However, brain makes guess about illumination of object and corrects for this change…therefore can accurately determine color
What do lesions in V4 produce?
Produces achromatopsia in man
What are the symptoms of a Posterior parietal lesion?
Contralateral NEGLECT (neglect of space) -old lady who was always hungry (because she only ate from half her plate)
What are the symptoms of an Inferotemporal lesion?
Agnosia: can’t tell the difference between objects
Prosopagnosia: can’t recognize faces…can’t recognize own face in mirror
How does object and face recognition relate with one another in the cortex?
Objects and face recognition area are in competition with one another
-so you can be good at identifying one while not great at identifying another
What is the pathway for the dorsal stream (the Where)?
M ganglion cells (M-cells) in the retina M-cells in LGN Layer IVC(alpha) in V1 Layer IV(beta) thick stripes in V2 MT in extrastriate cortex Posterior Parietal Area
What is the pathway for the ventral stream (the what/who)?
P ganglion cells (P-cells) in the retina LGN layer IVC(beta) in V1 CO blobs in V1 thin stripes V4 ITC
What is the Posterior parietal area (PPA)?
The final point in the dorsal stream pathway
What is the inferotemporal cortex (ITC)?
The final point in ventral stream pathway
What is the ultimate role of P-cells? What pathway is it involved with?
To distinguish the “what” of the world
Ventral Pathway
High spatial/color resolution
What is the ultimate role of M-cells? What pathway is it involved with?
To detect motion
Dorsal pathway
What is MT in humans?
V5
