Responses of Visual-Tracking Neurons from Cortical Area MST-I to Visual, Eye and Head Motion

Thirty-one neurons which exhibited ocular pursuit-related activity [visual-tracking (VT) neurons] were found clustered within area MST-I (the lateral part of area MST) of two rhesus monkeys. Their responses were studied to determine whether this activity was correlated only with pursuit eye movement or with head movement as well. The latter hypothesis appeared to be preferable since visual, eye movement and head movement inputs were found to be mapped in register onto most of these cells. First, in each cell tested (n=19) the pursuit response persisted even in the absence of retinal image motion, offering clear evidence for non-visual input. Second, 22 of the 31 cells were directionally responsive to moving visual stimuli and in 20 of these the preferred directions for the visual motion and pursuit responses agreed closely. Responses were also obtained from many of the same cells during suppression of both the horizontal and the vertical vestibulo-ocular reflex (VOR). In each case, where directional visual, pursuit and VOR suppression responses were each obtained, vector addition of responses during suppression of the horizontal and vertical VOR resulted in an estimated preferred direction for head rotation which was closely aligned with the preferred direction previously obtained for eye motion or visual motion. In addition, the preferred direction of head movement was conserved even when the VOR was elicited by passive head rotation in complete darkness, although the responses in this instance were, on average, only 62% of those obtained during VOR suppression. Our interpretation is that, at present, MST-I VT neurons are best described as encoding the direction of target motion in space-centred coordinates by integrating inputs reflecting retinal image motion plus eye and head movement.