Gaze field properties of eye position neurones in areas MST and 7a of the macaque monkey.

The activity of parietal cortex neurones primarily related to eye position (EP neurones) was studied in macaque monkeys with the aim of precisely defining the neurones' gaze fields (GF) and comparing them in two functionally different areas, MSTd and 7a. Discharge rates of single neurones in the inferior parietal lobule and in the underlying cortex of the superior temporal sulcus were recorded in two Java monkeys while the animals fixated a steady visual target positioned at several different points on a video screen. The GFs were then drawn as a regression surface fitting the mean discharge rates. Cells tonically influenced by the angle of gaze were found in both areas. The GFs most often took the form of a nearly planar surface best characterized as a ramp tilted towards a hemifield or quadrant of the visual field, shifted eccentrically with respect to the straight ahead (primary position), and with a midpoint centred between 0 deg and 20 deg of gaze eccentricity and saturation between 10 deg and 35 deg. In a minority of cases, the discharge rate was nearly maximal at the primary position and decreased to a minimum within 35 deg of eccentricity. In other instances, the GFs were peaked surfaces, limited to a restricted part of visual space. EP neurones, while showing similar gaze fields in areas MST and 7a, were found intermingled with functionally different types of cells. The results suggest that EP neurones similar to those already described in several areas of the monkey parietal cortex are present also in area MST. These cells, by signalling the degree of gaze eccentricity from the primary position, encode gaze position in an orbito-centered frame extending up to 30-35 deg from the straight-ahead. The role of EP neurones might be to supply contiguous elements with a gaze eccentricity signal required for visuo-motor processes such as the control of tracking movements.