Dynamic representation of eye position in the parieto-occipital sulcus

Area V6A, on the anterior bank of the parieto-occipital sulcus of the monke y brain, contains neurons sensitive both to visual stimulation and to the p osition and movement of the eyes. We examined the effects of eye position a nd eye movement on the activity of V6A neurons in monkeys trained to saccad e to and fixate on target locations. Forty-eight percent of the neurons res ponded during these tasks. The responses were not caused by the visual stim ulation of the fixation light because extinguishing the fixation light had no effect. Instead the neurons responded in relation to the position of the eye during fixation. Some neurons preferred a restricted range of eye posi tions, whereas others had more complex and distributed eye-position fields. None of these eye-related neurons responded before or during saccades. The y all responded postsaccadically during fixation on the target location. Ho wever, the neurons did not simply encode the static position of the eyes. I nstead most (88%) responded best after the eye saccaded into the eye-positi on field and responded significantly less well when the eye made a saccade that-was entirely contained within the eye-position field. Furthermore, for many eye-position cells (45%), the response was greatest immediately after the eye reached the preferred position and was significantly reduced after 500 ms of fixation; Thus these neurons preferentially encoded the initial arrival of the eye into the eye-position held rather than the continued pre sence or the movement of the eye within the eye-position held. Area V6A the refore contains a representation of the position of the eye in the orbit, b ut this representation appears to be dynamic, emphasizing the arrival of th e eye at a new position.