A patient can identify objects, name them, and describe their properties accurately, but when reaching for them consistently misses by several centimeters despite normal visual acuity. This pattern most likely indicates damage to:
AInferotemporal cortex, which stores visual object representations
BPrimary visual cortex (V1), affecting the resolution of visual input
DPrimary motor cortex, impairing execution of reaching movements
This is the hallmark of optic ataxia — accurate object perception with inaccurate visually guided reaching. The patient's intact identification confirms that the ventral stream and object representations are functioning normally. The deficit is specifically in converting visual target location into motor coordinates, which is the function of posterior parietal cortex (PPC) in the dorsal stream. If primary motor cortex were damaged, the deficit would appear across all reaching regardless of whether targets were visual.
Question 2 Multiple Choice
Why is 'how pathway' a more accurate label for the dorsal stream than the traditional 'where pathway'?
AThe dorsal stream processes high-frequency spatial details, not low-frequency location information
BThe dorsal stream computes real-time metric information for motor control, not conscious spatial representation
CThe dorsal stream does not actually process spatial location — it only processes motion
DThe 'where' label was based on lesion studies in primates that do not generalize to humans
The 'where' label implies the dorsal stream represents spatial location for conscious awareness. But the key function is converting visual input into action-relevant parameters — distance to target, grip aperture, object orientation — in real time and largely outside awareness. These computations are fundamentally different from perceptual spatial representation. You do not consciously calculate grip width when reaching for a cup; the dorsal stream performs these motor-format computations automatically. 'How' captures this functional role more precisely.
Question 3 True / False
A patient with visual form agnosia (ventral stream damage) cannot recognize or identify objects by sight, yet they accurately calibrate their grip aperture when reaching for those objects.
TTrue
FFalse
Answer: True
This is the double dissociation that confirms the independence of vision-for-action and vision-for-perception. The ventral stream handles object identity; the dorsal stream handles action parameters like grip width. When the ventral stream is damaged, perceptual identification fails but the dorsal stream can still compute appropriate grip apertures from the object's physical properties (size, orientation). This demonstrates that the action system does not require conscious object recognition — it operates on visual input processed through a completely separate pathway.
Question 4 True / False
The visuomotor transformations performed by the dorsal stream rely on conscious awareness of the target's location in order to produce accurate reaching movements.
TTrue
FFalse
Answer: False
The dorsal stream operates largely outside conscious awareness. When you reach for your coffee cup, you do not consciously compute the distance from your hand to the cup, calculate required finger aperture, or track the trajectory of your arm. These computations happen automatically and continuously, driven by real-time visual input processed by the dorsal stream and posterior parietal cortex. Conscious spatial awareness is a ventral-stream phenomenon. The dissociation is precisely that action and perception are served by parallel, largely independent systems running on the same visual input.
Question 5 Short Answer
What does the double dissociation between optic ataxia and visual form agnosia demonstrate about the relationship between visual perception and visually guided action?
Think about your answer, then reveal below.
Model answer: The double dissociation shows that vision-for-action and vision-for-perception are functionally and anatomically independent systems. Optic ataxia (dorsal stream damage) impairs reaching despite intact object recognition — demonstrating that the action system is dissociable from the perceptual system. Visual form agnosia (ventral stream damage) impairs object recognition while leaving visually guided action intact — demonstrating that the action system does not depend on conscious perception. Together, these cases confirm that the same visual input is processed in two distinct ways by two distinct pathways for two distinct purposes.
A single dissociation (one direction) could be explained by one system being a component of another, or by one being more 'fragile.' The double dissociation — each system can be independently damaged — is the strong evidence for genuine functional independence. It rules out the simpler explanation that perception and action share a single system, with one being more vulnerable. This finding reshaped our understanding of vision from a single unified system to a collection of specialized parallel processors.