PERFORMANCE OF MEMORY-GUIDED SEQUENCES OF SACCADES BY NORMAL SUBJECTS

We studied the performance of memory-guided sequences of saccades in 2 0 healthy subjects. Subjects were presented sequences of two, three, f our, or five subsequent visual targets at a frequency of one target pe r second, which they had to perform after a delay of three seconds. Th us, the temporal order of the presented targets had to be memorized in this paradigm. The mean percentage of correctly performed sequences o f saccades was 97% 86%, 52%, and 32% for two, three, four, and five se quentially presented targets, respectively. The intersubject performan ce was variable, with some subjects not being able to perform quintupl e memory-guided sequences at all. A multiple regression analysis revea led that greater numbers of clustered targets (i.e., targets closer th an 2.5 degrees) led to an increased rate of erroneously performed memo ry-guided sequences of saccades. This study demonstrates the relativel y small capacity of the working memory for chronological order, an asp ect of working memory which was not studied systematically until now. Furthermore, there is a remarkable intersubject variability for longer sequences. Lastly, visuospatial properties of the target presentation such as the number of clustered targets are crucial for the performan ce of such saccade sequences, whereas neither the hemifield of the pre sentation of the targets nor the number of changes in the direction of the saccades seem to influence the performance. These results have im plications for clinical research using such sequences, since longer se quences and the effect of cluster presentation of the visual targets c ould bias the study.