1. A systematic study of the positional selectivity of visual fixation (VF) neurons of the posterior parietal association cortex (area 7a or PG) was made in seven hemispheres of four alert behaving monkeys. By gaze fixation on a small spot of light in space, 125 units were identified as VF neurons. the position of the fixation target was varied not only in the frontal plane, but also in depth. 2. Microelectrode penetrations were made in the anterior and posterior part of area 7a. The recording sites of 104 VF neurons, determined histologically, were distributed mainly in the posterior part: the caudal part of the posterior bank of the intraparietal sulcus and the caudal third of the anterior bank of the superior temporal sulcus. The following conclusions are based mainly on the observations of the VF neurons in the posterior part of area 7a. 3. Most of the VF neurons examined in the frontal plane (86/93) had a preferred direction of gaze along the horizontal (39/86), vertical (38/86), or diagonal axis (9/86), and their discharge rates were monotonic increasing functions of the angle of deviation from the center. 4. Many VF neurons had selectivity in the depth of fixation. The majority (41/63) were activated more intensely when the fixation point was nearer to the animal (less than 50 cm), whereas a considerable number (18/63) were activated better when the fixation point was further from the animal (100 cm or more). There were a few units (4/63) that discharged maximally at intermediate distances. 5. Thirty-two VF neurons were studied in detail along all three axes: vertical, horizontal, and depth. About half of them (17/32) were found to be selective both in the radial direction and the distance of fixation, while the others displayed selectivity in the direction of gaze alone (11/32) or in depth alone (4/32). 6. The activity of VF neurosis in the dark was compared to that in a lighted room in order to examine their relationship to eye position in detail. Half of the VF neurons tested (25/50) showed almost the same discharge rate in the dark as in the lighted room in spite of the absence of visual stimuli other than the target light in the fovea. Moreover, the discharge rate of the majority of these VF neurons displayed a close correlation with eye position, even in complete darkness. On the other hand nearly half of the VF neurons (22/50) showed a decrease in discharge rate in the dark during fixation at preferred positions suggesting that visual stimuli in the surroundings have some excitatory effects on these neurons. There were also some VF neurons whose activity decreased when the target light was interrputed. Even in such visually sensitive VF neurons, their positional selectivity depended mainly on eye position. 7…
Partially Mixed Selectivity in Human Posterior Parietal Association Cortex
