A patient can recognize a coffee cup, describe its shape, and name it correctly, but when reaching for it produces misguided arm movements and incorrect grip aperture. A second patient reaches with accurate trajectory and correctly scaled grip but cannot name the cup or describe its function. Which patient has dorsal stream damage?
AThe first patient — impaired recognition indicates a 'where/how' system failure
BThe second patient — accurate reaching means the dorsal stream is intact, so it must be damaged elsewhere
CThe first patient — the reaching and grasping impairments point to a broken visuomotor system
DBoth patients have dorsal stream damage because both show some visual deficit
The first patient has optic ataxia, the signature of dorsal stream (posterior parietal cortex) damage: recognition is intact (ventral stream works), but the visuomotor transformation that guides reaching and grip scaling fails. The second patient has visual agnosia from ventral stream damage: the dorsal stream correctly guides action without conscious object recognition. This double dissociation is the strongest evidence that the two streams operate independently.
Question 2 Multiple Choice
A visual illusion makes a disk appear 20% larger than it actually is. A person reaches to grasp the disk. Which outcome best demonstrates dorsal-ventral stream dissociation?
AThe person reports the disk looks larger AND opens their grip wider — both streams are fooled by perception
BThe person reports the disk looks larger but grip aperture matches the disk's actual size — grasping uses the dorsal stream's egocentric calculation, not the ventral percept
CThe person reports the disk looks normal AND grip aperture is accurate — the illusion is too weak to affect either stream
DThe person opens their grip wider but reports no size distortion — the dorsal stream overrides ventral perception
Grasping uses the dorsal stream's online egocentric calculations, which are based on the actual retinal geometry, not the ventral stream's perceptual interpretation that the illusion distorts. So the illusion fools conscious perception (ventral) but not grip scaling (dorsal). This is exactly the pattern observed in experiments using the Ebbinghaus illusion — a real-world demonstration that what you 'see' and what your hand 'knows' can diverge.
Question 3 True / False
Dorsal stream spatial representations persist across days, giving the brain a stable allocentric map of where objects are in the environment.
TTrue
FFalse
Answer: False
Dorsal stream representations are egocentric (relative to the observer's body) and transient — they are continuously updated as the observer moves and are discarded once the action is complete. This is unlike ventral stream object memories, which persist across time and viewing angles. A stable allocentric map would be useless for real-time motor control, where what matters is the object's position relative to your current hand location, not some abstract fixed frame.
Question 4 True / False
A patient with posterior parietal cortex damage would likely show difficulty recognizing familiar faces but would be able to reach accurately toward objects.
TTrue
FFalse
Answer: False
Posterior parietal cortex is the core of the dorsal stream, so its damage produces optic ataxia — impaired reaching and grasping despite intact recognition. Face recognition is a ventral stream function (especially fusiform face area). The described pattern is the opposite of what posterior parietal damage produces. A patient with ventral stream damage might show prosopagnosia (face recognition failure) while retaining accurate motor guidance.
Question 5 Short Answer
Why must the dorsal stream encode object location in an egocentric reference frame rather than an allocentric one?
Think about your answer, then reveal below.
Model answer: Motor commands operate in body-centered coordinates: the arm moves relative to the shoulder, the hand opens relative to the wrist. To translate visual information into action, the brain needs to know where the object is relative to the body, not relative to some external landmark. An allocentric representation ('the cup is 10 cm from the plate') cannot directly specify a motor command; an egocentric one ('the cup is 35 cm in front of my shoulder at 15° elevation') can.
This is why egocentric coding is not a limitation of the dorsal stream but its core design feature. The continuous, real-time updating of the egocentric map reflects the fact that every body movement changes the spatial relationship between the observer and the object. The moment you rotate your shoulder, the arm-centered coordinates for the target change — and the dorsal stream recalculates accordingly.