A patient suffers a stroke affecting their ventral visual stream. Which deficit would you most expect?
AInability to perceive motion, particularly in the peripheral visual field
BInability to guide reaching movements accurately toward objects
CInability to recognize objects, faces, or identify what an object is
DLoss of depth perception due to disrupted binocular disparity processing
The ventral stream (V1 → V2 → V4 → inferotemporal cortex) processes object identity — shape, color, and faces — answering 'what is it?' Damage produces visual agnosia, where patients can see clearly but cannot recognize or identify objects. Options A and B describe dorsal stream deficits: the dorsal stream (V1 → V2 → MT → parietal cortex) processes motion and spatial location — answering 'where is it?' Optic ataxia (impaired visually guided reaching) and motion perception deficits result from dorsal stream damage. The double dissociation between the two streams is one of the strongest pieces of evidence for their functional independence.
Question 2 Multiple Choice
Why are retinal ganglion cells with center-surround receptive fields described as 'contrast detectors' rather than 'light detectors'?
ABecause they only respond to colored light, not white light
BBecause uniform illumination activates center and surround equally, producing little net response, while edges produce strong differential responses
CBecause they fire in proportion to the total number of photons hitting the retina
DBecause they are inhibited by any light stimulus, firing maximally in darkness
Center-surround organization means that a ganglion cell is excited by light in its center and inhibited by light in its surround (or vice versa for off-center cells). When uniform light illuminates the entire receptive field, the excitatory and inhibitory inputs partially cancel, yielding a weak response. But at an edge — where one side is bright and the other is dark — the center-surround imbalance is maximized, producing a strong response. This is why the retina emphasizes spatial discontinuities (edges, contours) and why you can read in dim light: contrast structure, not absolute brightness, drives perception.
Question 3 True / False
Damage to the dorsal visual stream would most likely impair a patient's ability to recognize faces.
TTrue
FFalse
Answer: False
Face recognition is a function of the ventral stream, specifically inferotemporal (IT) cortex. The dorsal stream (projecting through MT/V5 to parietal cortex) processes spatial location, motion, and the visual guidance of action — answering 'where is it and how do I interact with it?' Impairment of the dorsal stream produces optic ataxia (difficulty directing hand movements to objects) or motion perception deficits, not face blindness. Prosopagnosia (face recognition failure) is a ventral stream deficit. The two-stream distinction is clinically confirmed by patients who can recognize objects but cannot reach for them, and vice versa.
Question 4 True / False
Retinal ganglion cells are maximally responsive to local contrast rather than to absolute light levels, allowing the visual system to function across a wide range of illumination conditions.
TTrue
FFalse
Answer: True
Center-surround receptive fields implement a form of local adaptation: what matters is not the total light falling on the retina but the difference between brightness in the center and surround. This explains a well-known phenomenon: the same gray patch looks lighter or darker depending on its surrounding context. The retina sends a contrast-encoded, edge-emphasized signal to the brain, not a raw brightness map. This design compresses the enormous dynamic range of natural scenes into a manageable neural signal while preserving the spatial structure needed for object recognition.
Question 5 Short Answer
Explain how the two major visual processing streams — ventral and dorsal — differ in function, and give an example of the kind of deficit that results from damage to each.
Think about your answer, then reveal below.
Model answer: The ventral stream (V1 → V4 → IT cortex) processes object identity — what something is, including shape, color, and faces. Damage causes visual agnosia: a patient can see but cannot identify objects or recognize faces despite intact basic vision. The dorsal stream (V1 → MT → parietal cortex) processes spatial location and visually guided action — where something is and how to interact with it. Damage causes optic ataxia: a patient can recognize objects but cannot accurately reach or point to them. The double dissociation shows these are functionally distinct systems for 'what' vs 'where/how.'
The two-stream model explains why vision is not a single unified sense but a collection of parallel specialized processes. The dissociation becomes most apparent in neurological patients — someone with ventral damage can reach accurately for objects they cannot name, while someone with dorsal damage can name objects they cannot reach. This implies the streams operate with considerable independence, each extracting different aspects of the visual scene for different purposes.