Lecture 23 - Visual cortex: Principles in coding stimulus features (TV) Flashcards
Anatomical evidence for convergence between parallel pathways in the striate cortex
1) Cells in 4Calpha (M stream) and 4CBeta (P stream) project to both blob and interblob areas
2) Cells in 4B (M stream) project to blob areas
3) K cells synapse onto blobs Thus interblob areas in layer 2/3 get both P and M inputs and the blob areas get P, M and K inputs
Physiological evidence for convergence between parallel pathways in the striate cortex
1) Orientation sel. varies across blob and interblob areas - with little difference in orientation tuning between blob and interblob areas (no r/ship b/w the degree of orientation sel. and colour sel)
2) No relationship between receptive field properties and cell’s location relative to cytochrome oxidase-rich regions
3) Single cell responses in V1 exhibit signatures of both M and P inputs
What are the multiple input streams to Area MT?
V1 (4Calpha and 4C beta -> 4B) -> V2 Thick stripes -> V3 -> MT AND V2 Thick stripes -> MT directly -Koniocellular region of LGN
What is blindsight?
Damage to PVC (high metabolic demand) caused by hypoxia. Though perceptually blind in a part of the visual field, people perform above chance when forced to guess (forced-choice situation), say the location of a flash of light. The reason they can still predict it is because the Dorsal Stream (Where) is also mediated by inputs to MT that bypass V1. It still receives input from the earlier stages of visual processing in the retina -> SC -> Pulv -> MT -> PPC AND LGN -> PPC
Ungerleider and Mishkin - Temporal lobe (WHAT) lesions exp showed that?
Lesions in the temporal lobe interfered with object discrimination
Ungerleider and Mishkin - Parietal lobe (WHERE) lesions exp showed that?
Parietal lobe lesions interfered with landmark discrimination - where monkeys had to pickup object (ball) closer to the landmark (e.g. cylinder)
In lesions of the dorsal stream (WHERE) - a patient will have difficult putting their hand in a slot
In lesions of the dorsal stream (WHERE) - a patient will have difficult putting their hand in a slot
Other functions of posterior parietal cortex
-Directing attention to spatial locations -Helping bind different attributes of an object together -Gating inputs into ventral stream -Important for reading? (link to dyslexia)
Visual Neglect
Usually due to damage to right parietal cortex - patient unware of what is happening in left visual field. e.g. only draw one half of clock face. Lesions not always the same in different people BUT ALWAYS overlap in inferior posterior parietal region.
Why is visual neglect more common in right parietal cortex?
Right parietal cortex seems to be able to control tension on both visual hemifields BUT the left parietal cortex only controls tension on the right hemifield (Right hemisphere can still view both sides, no neglect present)
Area F5 in frontal cortex is unique as?
It responds when they see a particular action. When the subject does the action, the neurons also neurons. AKA mirror neurones aka Gandhi AKA empathy neurons - seeing someone sad makes you feel sad
Difference between the old and emerging view of parallel pathways
Old view: Strict segregation of parallel pathways Emerging view: It is now known that there are specific lines of cross-talk b/w parallel pathways. M signals invade the other streams at different levels. Is it because they are faster and prepare the various cortical areas for the slower signals?
