B&C: Vision 2: Colour, motion & Shape Flashcards
What kind of damage leads to cortical colour blindness and what does it consist of? How does this differ to other colour blindness?
Lesions to V4/V8 and it is known as achromatopsia because all colour is gone. This is very different from retinal blindness where one colour is gone
What is akinetopsia?
a condition where someone only sees stationary states, with objects flashing from one position to the next.
How it Akinetopsia caused?
Bilateral MT, MST or STS lesion
How can one attempt to replicate akinetopsia?
Virtual lesion of MT using TMS
How do we perceive shapes?
Low level perception areas may interpret the basic elements such as lines and orientations then higher level areas may form shapes with these starting components
As well as colour what else is V4 important for? How can this be demonstrated when lesioned there?
Shapes, the achromatopsia patient also had deficits in discriminating complex shapes.
What is the LOC?
Visual area in the brain responding more strongly to shapes (regardless if they are familiar or not) than to scrambled objects
Visual area OC responds to shapes even……
When defined by second order cues such as contour from motion stimuli (while not responding to the motion itself)
Where are ‘face cells’ located?
inferotemporal cortex
Whats the problem with the hierarchal coding theory?
You cannot have a cell for every possible object- combinatorial explosion
What is a potential solution to the explosion problem?
Ensemble coding theory- set of neurons signalling different aspects
What other computational problem do we have in vision?
Object constancy: recognising objects from any position, under different lighting and even when they’re partly covered (in all these cases the image on the retina-the distribution of light and contrast- is highly different.
What two types of models are there for object recognition?
View dependent frame of reference:
we match visual input with many different stored representations of an object > heavy burden on memory, more difficult to recognize objects in unusual views
View independent frame of reference:
we match visual input with a canonical and rather abstract memory representation of an object > less burden on memory, objects more easily confused
How did they test for repetition priming in the human visual cortex?
Repitition priming: in the fMRI when showing the same object immediately after showing it, the signal the second time will be weaker. They attempted to show in different ways to see would the signal be weaker (would the brain still recognise it)
What were the results of the experiment?
You only get the weaker signal in the earlier areas when the image is exactly the same (no invariance.) However if you go to higher you get RP for same objects of different sizes (size invariant coding.) Another visual area shows RP for identical objects in different viewpoints (viewpoint invariant coding.)
Left inferior frontal gyro (Brocas area) show RP for objects in the same class, possibly because you say the name internally. (exemplar invariant coding.)
