EVIDENCE FOR NONRETINAL FEEDBACK IN COMBINED VERSION-VERGENCE EYE-MOVEMENTS

Recently, a quantitative model for the generation of rapid eye movemen ts in direction and depth was proposed. In this scheme, the saccadic a nd the vergence system share a common initiation system and are contro lled by local feedback loops based on efference copy signals. We have used a remembered-target double-step paradigm to test the idea that bo th subsystems are guided by extraretinal signals. The subject was inst ructed to move the binocular point of fixation to the remembered posit ions indicated by a double-step movement of the target, in direction a nd depth. Since both binocular refixations were made in complete darkn ess, correct execution of this task requires information about both th e stored visual coordinates of the final target and the coordinates of the first movement. Binocular eye movements from five subjects were c ompared with predictions from two feed-forward models and a feedback m odel. Analysis of the pooled direction data showed that the feedback m odel performed best and fitted well. Qualitatively the same result was obtained in the vergence component, but in this case the goodness of fit was considerably less. These results, confirmed in each individual subject, show that the saccadic and vergence subsystem can use nonret inal information about a prior movement in direction and depth. Furthe r analysis showed that the gain of the direction response of the secon d movement was, on average, roughly correct. By contrast, the vergence component of these responses was only about 60% of the required ampli tude. Since the fit procedure gave the same weighting factors to the s econd target and to the first movement, we propose that the low vergen ce gain reflects mechanisms operating after the calculation of the mot or error signal, possibly at the execution stage. Finally, we discuss the possibility of a central control stage keeping track of the ongoin g movement sequence, based on a comparison of desired and current eye position signals.