Elaboration et decision des saccades: Adaptation a la trace du stimulus

Abstract

Rapid displacements of the line of sight (saccades) were determined by the occurrence of brief visual stimuli (20–200 msec light flashes) in the peripheral visual field (10°), in normal subjects ( Ss). As the reaction time ( RT) of a saccade (220 msec, mean in control conditions) exceeded duration T1 of the stimulus the saccade was thus completed after the end of the stimulus, i.e. without any possible visual reference. The following data were obtained : (1) In all the Ss, the saccade was correctly oriented. (2) In some Ss (Group 1), the saccade amplitude was constantly adapted-the (virtual) position of the stimulus. RT duration was then similar to that obtained in control conditions. (3) In the other Ss, the saccade was not correctly adapted unless T1 was over a threshold value (60 to 150 msec, depending on the S). When T1 was lower the saccade was too short and stopped before the stimulus position was reached. RT was then a function of T1 duration: it was longer (300 msec) for an infra-threshold T1, than for longer T1 values, where it shortened down to the control conditions value. (4) Results described under 3 allow relating the RT duration and the precision of the saccade: the shorter the RT, the better the adaptation of the saccade. The large RT variability, which has been noted mainly for infra-threshold T1s, determines a variability of the saccades amplitude. These results, together with these from other authors, are suggestive of a model describing the preparation and the execution of a saccade. This model should include: (a) a storage of the information concerning the stimulus position, as a trace related qualitatively to the stimulus position, and quantitatively to its angular distance ; (b) a progressive decision making process tending to open the connection between the trace and the execution mechanisms. In our experimental conditions the increase of the decision time would determine the lack of precision of the saccade, through a partial fading of the trace.