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.

The trajectories of saccadic eye movements evoked electrically from many brain structures are dependent to some degree on the initial position of the eye. Under certain conditions, likely to occur in stimulation experiments, local feedback models of the saccadic system can yield eye movements which behave in this way. The models in question assume that an early processing stage adds an internal representation of eye position to retinal error to yield a signal representing target position with respect to the head. The saccadic system is driven by the difference between this signal and one representing the current position of the eye. Albano & Wurtz (1982) pointed out that lesions perturbing the computation of eye position with respect to the head can result in initial position dependence of visually evoked saccades. It is shown here that position-dependent saccades will also result if electrical stimulation evokes a signal equivalent to retinal error but fails to effect a complete addition of eye position to this signal. Also, when multiple or staircase saccades are produced, as during long stimulus trains, they will have identical directions but decrease progressively in amplitude by a factor related to the fraction of added eye position.

Saccades evoked electrically from the deep layers of the superior colliculus have been examined in the alert cat with its head fixed. Amplitudes of the vertical and horizontal components varied linearly with the starting position of the eye. The slopes of the linear-regression lines provided an estimate of the sensitivity of these components to initial eye position. In observations on 29 sites in nine cats, the vertical and horizontal components of saccades evoked from a given site were rarely influenced to the same degree by initial eye position. For most sites, the horizontal component was more sensitive than the vertical component. Sensitivities of vertical and horizontal components were lowest near the representations of the horizontal and vertical meridians, respectively, of the collicular retinotopic map, but otherwise exhibited no systematic retinotopic dependence. Estimates of component amplitudes for saccades evoked from the center of the oculomotor range also diverged significantly from those predicted from the retinotopic map. The results of this and previous studies indicate that electrical stimulation of the cat's superior colliculus cannot yield a unique oculomotor map or one that is in register everywhere with the sensory retinotopic map. Several features of these observations suggest that electrical stimulation of the colliculus produces faulty activation of a saccadic control system that computes target position with respect to the head and that small and large saccades are controlled differently.