GAIN ADAPTATION OF EYE AND HEAD MOVEMENT COMPONENTS OF SIMIAN GAZE SHIFTS

To investigate the site of gaze adaptation in primates, we reduced the gain of large head-restrained gaze shifts made to 50 degrees target s teps by jumping the target 40% backwards during a targeting saccade an d then tested gain transfer to the eye-and head-movement components of head-unrestrained gaze shifts. After several hundred backstep trials, saccadic gain decreased by at least 10% in 8 of 13 experiments. which were then selected for further study. The minimum saccadic gain decre ase in these eight experiments was 12.8% (mean = 18.4%). Head-unrestra ined gaze shifts to ordinary 50 degrees target steps experienced a gai n reduction of at least 9.3% (mean = 14.9%), a mean gain transfer of 8 1%. Both the eye and head components of the gaze shift also decreased. However, average head movement gain decreased much more (22.1%) than eye movement gain (9.2%). Also, peak head velocity generally decreased significantly (20%), but peak eye velocity either increased or remain ed constant (average increase of 5.6%). However, the adapted peak eye and head velocities were appropriate for the adapted, smaller gaze amp litudes. Similar dissociations in eye and head metrics occurred when h ead-unrestrained gaze shifts were adapted directly (II = 2). These res ults indicated that head-restrained saccadic gain adaptation did not p roduce adaptation of eye movement alone. Nor did it produce a proporti onal gain change in both eye and head movement. Rather, normal eye and head amplitude and velocity relations for a given gaze amplitude were preserved. Such a result could be explained most easily if head-restr ained adaptation were realized before the eye and head commands had be en individualized. Therefore, gaze adaptation is most likely to occur upstream of the creation of separate eye and head movement commands.