Sg. Lisberger et Vp. Ferrera, VECTOR AVERAGING FOR SMOOTH-PURSUIT EYE-MOVEMENTS INITIATED BY 2 MOVING TARGETS IN MONKEYS, The Journal of neuroscience, 17(19), 1997, pp. 7490-7502
The visual input for pursuit eye movements is represented in the cereb
ral cortex as the distributed activity of neurons that are tuned for b
oth the direction and speed of target motion. To probe how the motor s
ystem uses this distributed code to compute a command for smooth eye m
ovements, we have recorded the initiation of pursuit for 150 msec pres
entations of two spots moving at different speeds and/or in different
directions. With equal probability, one of the two spots continued to
move at the same speed and in the same direction and became the tracki
ng target, whereas the other disappeared and served as a distractor. W
e measured eye acceleration in the interval from 110 to 206 msec after
the onset of spot motion, within both the open-loop interval for purs
uit and the interval during which eye motion was affected by the two s
pots. Our results demonstrate that weighted vector averaging is used t
o combine the responses to two moving spots. We found only a minute nu
mber of responses that were consistent with either vector summation or
winner-take-all computations. In addition, our data show that it is d
ifficult for the monkey to defeat vector averaging without extended tr
aining on the use of an explicit cue about which spot will become the
target. We argue that our experiment reveals the computations done by
the pursuit system in the absence of attentional bias and that vector
averaging is normally used to read the distributed code of image motio
n when there is only one target.