RELEASE OF FIXATION FOR PURSUIT AND SACCADES IN HUMANS - EVIDENCE FORSHARED INPUTS ACTING ON DIFFERENT NEURAL SUBSTRATES

1. In three human subjects, we measured the latency of pursuit and sac cadic eye movements made to an eccentric target after a fixated centra l target was extinguished. In one set of experiments, we varied the ti me interval between the extinction of the central target and the appea rance of the eccentric target (''gap duration''). In a second set of e xperiments, we varied the eccentricity at which the second target appe ared. 2. Varying the gap duration produced similar changes in the late ncies of pursuit and saccades. Gaps as short as 30 ms caused significa nt decreases in latency; progressively longer gaps produced shorter la tencies, reaching a minimum for gaps of 150-200 ms. Over the range of gap durations used, the latencies of pursuit and saccades displayed th e same dependence on gap duration. 3. Varying the eccentricity of the second target produced different effects on the latencies of pursuit a nd saccades. Saccade latencies increased when the eccentricity of the second target was decreased from 4 degrees to 0.5 degrees, whereas pur suit latencies were not consistently altered. Despite these difference s in the dependence on retinal eccentricity between pursuit and saccad es, imposing a 200-ms gap between the extinction of the fixation point and appearance of the second target still reduced the latency of both . 4. Our results are consistent with the idea that the mechanisms unde rlying the release of fixation fur pursuit and saccades have shared in puts but a different neural substrate. The common dependence on gap du ration may indicate that a single preparatory input coordinates bath t ypes of movements. The different dependence on retinal eccentricity in dicates that there are differences in the spatial organization of the premotor circuits that trigger the onset of the two types of movements .