LOCAL FEEDBACK SIGNALS ARE NOT DISTORTED BY PRIOR EYE-MOVEMENTS - EVIDENCE FROM VISUALLY EVOKED DOUBLE SACCADES

Recent experiments have shown that the amplitude and direction of sacc ades evoked by microstimulation of the monkey superior colliculus depe nd systematically on the amplitude and direction of preceding visually guided saccades as well as on the postsaccade stimulation interval. T he data are consistent with the hypothesis that an eye displacement in tegrator in the local feedback loop of the saccadic burst generator is gradually reset with a time constant of similar to 45 ms. If this is true, similar effects should occur during naturally evoked saccade seq uences, causing systematic interval-dependent errors. To test this pre diction in humans, saccades toward visual single- and double-step stim uli were elicited, and the properties of the second saccades were inve stigated as a function of the intersaccadic interval (ISI). In 15-20% of the saccadic responses, ISIs fell well below 100 ms. The errors of the second saccades were not systematically affected by the preceding primary saccade, irrespective of the ISI. Only a slight increase in th e endpoint variability of second saccades was observed for the shortes t ISIs. These results are at odds with the hypothesis that the putativ e eye displacement integrator has a reset time constant >10 ms. Instea d, it is concluded that the signals involved in the internal feedback control of the saccadic burst generator reflect eye position and/or ey e displacement accurately, irrespective of preceding eye movements.