A patient with posterior parietal cortex (PPC) damage is shown a coffee cup on a table. She describes it accurately — 'a white ceramic mug with a handle' — but when she reaches for it, her hand misses by several centimeters and she cannot correct the trajectory. What best explains this pattern?
AThe PPC damage impaired visual recognition, so she cannot actually see the cup clearly
BThe PPC damage disrupted the coordinate transformation that converts visual location into body-centered reaching commands, leaving recognition intact
CThe motor cortex was also damaged, preventing accurate finger movements
DShe has learned to reach inaccurately through motor habit and needs retraining
This is the classic presentation of optic ataxia: intact recognition (ventral stream is undamaged) combined with impaired visually guided reaching (dorsal stream/PPC is damaged). The PPC is not responsible for recognizing what objects are — it converts their retinal location into the body-centered coordinates the motor system needs to reach for them. The patient can describe the cup because the ventral stream is fine; she cannot reach for it because the retinocentric-to-body-centered coordinate transformation is broken.
Question 2 Multiple Choice
A researcher wears goggles that shift the visual field 15 degrees to the right. After many practice reaches, the reaches become accurate again. The goggles are then removed. What do we expect, and what does it reveal about motor learning?
AReaches will be immediately accurate again, because the original motor program is restored
BReaches will overshoot to the left, because the internal sensorimotor transformation was recalibrated and now applies a 15-degree correction to normal vision
CReaches will still go to the right, because the arm muscles were trained to move rightward
DReaches will be random, because removing the goggles confuses the visual system
The leftward aftereffect is the key evidence that motor learning updated the *internal model* — the coordinate transformation itself — not just trained the muscles to move in a particular direction. If only muscle output had changed, removing the goggles would simply restore the original calibration. Instead, the transformation now applies a correction that overshoots in the opposite direction when the original visual input returns. This demonstrates that motor learning is calibration of the sensorimotor mapping, explaining how stroke patients can re-learn reaching by recalibrating around damaged circuitry.
Question 3 True / False
The posterior parietal cortex computes object location in head-centered and body-centered coordinates by combining retinal position with signals about eye orientation and head position.
TTrue
FFalse
Answer: True
This is exactly the function of PPC in sensorimotor integration. Visual input arrives in a retinocentric frame — where on the retina the image falls. The PPC integrates this with proprioceptive signals and efference copies about eye and head orientation to progressively construct a body-centered representation that the motor system can work with. This multi-signal integration is why PPC lesions specifically impair reaching even when vision is intact.
Question 4 True / False
Prism adaptation aftereffects are caused by retraining arm muscles to fire in a new pattern, which should then be un-trained when the prisms are removed.
TTrue
FFalse
Answer: False
The aftereffect reveals that it is the *coordinate transformation* — the internal model — that was recalibrated, not the muscles themselves. If muscles were retrained to move rightward, removing the goggles would simply restore normal muscle output. Instead, the system overshoots leftward because it is now applying a corrective transformation to unshifted visual input. Motor learning in reaching is learning a new sensorimotor mapping, not conditioning specific muscle outputs.
Question 5 Short Answer
Why does prism adaptation produce an aftereffect in the opposite direction when the goggles are removed, and what does this tell us about how the brain implements motor learning for reaching?
Think about your answer, then reveal below.
Model answer: The aftereffect occurs because motor learning updated the internal model — the coordinate transformation itself — to compensate for the visual shift. When the goggles are removed, that updated transformation is still in place and applies an unnecessary correction, causing the overshoot in the opposite direction. This shows that motor learning is not about training muscles to move in a new direction but about recalibrating the sensorimotor transformation that maps visual locations to movement commands.
This is the central insight about motor learning as model calibration. The transformation — not the muscles — is what changes. This same mechanism explains why stroke patients can partially recover reaching ability through rehabilitation: the brain relearns the correct sensorimotor mapping to compensate for changed neural circuitry, not by rebuilding the damaged circuits but by updating the transformations implemented in surviving ones.