Dynamic representation of eye position in the parieto-occipital sulcus

Area V6A, on the anterior bank of the parieto-occipital sulcus of the monkey brain, contains neurons sensitive both to visual stimulation and to the position and movement of the eyes. We examined the effects of eye position and eye movement on the activity of V6A neurons in monkeys trained to saccade to and fixate on target locations. Forty-eight percent of the neurons responded during these tasks. The responses were not caused by the visual stimulation 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 positions, whereas others had more complex and distributed eye-position fields. None of these eye-related neurons responded before or during saccades. They all responded postsaccadically during fixation on the target location. However, the neurons did not simply encode the static position of the eyes. Instead most (88%) responded best after the eye saccaded into the eye-position 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 presence or the movement of the eye within the eye-position held. Area V6A therefore contains a representation of the position of the eye in the orbit, but this representation appears to be dynamic, emphasizing the arrival of the eye at a new position.