EYE POSITION EFFECTS IN MONKEY CORTEX .1. VISUAL AND PURSUIT-RELATED ACTIVITY IN EXTRASTRIATE AREAS MT AND MST

We studied the effect of eye position on visual and pursuit-related ac tivity in neurons in the superior temporal sulcus of the macaque monke y. Altogether, 109 neurons from the middle temporal area (area MT) and the medial superior temporal area (area MST) were tested for influenc e of eye position on their stimulus-driven response in a fixation para digm. In this paradigm the monitored eye position signal was superimpo sed onto the stimulus control signal while the monkey fixated at diffe rent locations on a screen. This setup guaranteed that an optimized st imulus was moved across the receptive field at the same retinal locati on for all fixation locations. For 61% of the MT neurons and 82% of th e MST neurons the stimulus-induced response was modulated by the posit ion of the eyes in the orbit. Directional selectivity was not influenc ed by eye position. One hundred sixty-eight neurons exhibited directio n-specific responses during smooth tracking eye movements and were tes ted in a pursuit paradigm. Here the monkey had to track a target that started to move in the preferred direction with constant speed from fi ve different locations on the screen in random order. Pursuit-related activity was modulated by eye position in 78% of the MT neurons as wel l as in 80% of the MST neurons tested. Neuronal activity varied linear ly as a function of both horizontal and vertical eye position for most of the neurons tested in both areas, i.e., two-dimensional regression planes could be approximated to the responses of most of the neurons. The directions of the gradients of these regression planes correlated neither with the preferred stimulus direction tested in the fixation paradigm nor with the preferred tracking direction in the pursuit para digm. Eighty-six neurons were tested with both the fixation and the pu rsuit paradigms. The directions of the gradients of the regression pla nes fit to the responses in both paradigms tended to correlate with ea ch other, i.e., for more than two thirds of the neurons the angular di fference between both directions was less than +/-90 degrees. The modu latory effect of the position of the eyes in the orbit proved to balan ce out at the population level for neurons in areas MT and MST, tested with the fixation as well as the pursuit paradigm. Results are discus sed in light of the hypothesis of an ongoing coordinate transformation of the incoming sensory signals into a nonretinocentric representatio n of the visual field.