Response of the Cerebral Cortex to Spatial 
Information 
Jean-Pierre Roy, M.D. — International Research Scholar 
Dr. Roy is Assistant Professor of Neurology and Neurosurgery at the Montreal Neurological Institute, 
McGill University, and Neurologist at Montreal Neurological Hospital, Montreal, Quebec. After receiving 
his M.D. degree from Laval University in Quebec City, he spent two years in Mircea Steriade's laboratory 
at Laval investigating the origin of rhythmic oscillations in the thalamic relay cells. He then went on to a 
neurology residency at the Montreal Neurological Hospital. This was followed by fellowship studies at 
NIH with Robert Wurtz on neuronal sensitivity to visual stimuli in the cerebral cortex. 
THE main goal of my work is to understand the 
transformation of simple information, such as 
the speed and direction of motion registered by 
individual neurons, into information about the 
motion of the subject and the spatial properties of 
his environment. My approach is to examine the 
response of single cells in one area of a rhesus 
monkey's parietal cortex — the medial superior 
temporal area (MST) — to different visually pre- 
sented stimuli. The neurons in the MST are inter- 
esting in that about half of them respond to mo- 
tion in one direction, for example left, when the 
motion is in front of where the subject is looking, 
and respond to motion in the opposite direction, 
right in this case, when the motion is behind 
where the subject is looking. This corresponds, 
we think, to the apparent motion of the environ- 
ment experienced during self-motion. 
When a subject moves in the environment 
while looking at an object to the side, the objects 
in the foreground will move in the direction op- 
posite to that of self-motion while those in the 
background will move in the same direction as 
the self-motion. The correlation between direc- 
tion of motion and disparity in these neurons sug- 
gests that they detect these environmental mo- 
tions. But more than that, it suggests that these 
cells are involved not so much in indicating the 
movement of the environment as such, but rather 
the movement of the subject himself. Indeed, 
these cells will respond if foreground moves left 
or if background moves right, or both. Each of 
these three conditions will be observed when the 
subject moves to the right. Hence we propose 
that those neurons signal motion of the subject 
himself. 
Within one direction and one disparity re- 
sponse, we have evidence that speed is another 
property the cells record. Indeed, there appears 
to be a preference for lower speeds when the dis- 
parity of the motion is of a small absolute value, 
i.e., when it corresponds to motion close to the 
point of fixation, and a preference for higher 
speeds when the disparity is of a large absolute 
value, i.e., when it corresponds to motion far 
from the point of fixation. This last property is 
very interesting in that it suggests that those cells 
could be signaling not only the direction of the 
self-motion but possibly also the structure of the 
environment. 
This requires elaboration. I am proposing that 
the preliminary evidence of a differential re- 
sponse of cells to different speeds could repre- 
sent a role for them in transforming the speed 
information into depth (or more precisely, rela- 
tive depth) information. 
During the condition described above, objects 
in the environment that are very near or very far 
will move fast, while those that are close to the 
plane where the subject fixates will move slowly. 
Speed, then, contains information about the 
depth of objects in the environment. In order to 
test this hypothesis, individual planes moving in- 
dependently, as were used before, are inade- 
quate. What is needed are multiple planes mov- 
ing simultaneously at different speeds. To 
explore whether MST neurons do respond to 
these stimuli representing speed and disparity 
gradients, we have developed stimuli with ap- 
propriate characteristics. 
With those stimuli, we should be able to exam- 
ine the response of neurons to different simple 
properties of the stimulus. Our prediction is that 
the cells should respond optimally to a stimulus 
that has the correct direction of motion of the 
foreground versus the background, but also a 
speed gradient and possibly a disparity gradient. 
This would suggest that those cells are indeed 
involved in transforming the information they re- 
ceive about speed into information about depth. 
It would not be the ultimate answer, which could 
only come from lesion studies we are planning 
for the future. 
This work is also supported by a grant from the 
Medical Research Council of Canada. 
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