Inhibition of saccade return (ISR): spatio-temporal properties of saccade programming

Tasks such as reading or visual search consist of series of saccades. We ha ve investigated to what extent saccades that are made within a series of se lf-paced movements are influenced by preceding movements. The present paper concerns an analysis of the duration of the fixations preceding saccades. We tested human subjects in a paradigm where they had to fixate two to four targets in a fixed order as fast as they could. We found that fixations be fore so-called 'return saccades' (saccades returning to the previously fixa ted position) are considerably longer (up to 40%) than other fixations. Thi s phenomenon, which we call 'Inhibition of Saccade Return' (ISR), is presen t when return and regular saccades are mixed in one trial, and seems to be reset after each saccade. ISR is strongest at the previously fixated target , and decreases gradually from there. The radius of the area where ISR is f ound is about 4 degrees. The relation between ISR and 'Inhibition of Return ' of spatial attention [Posner & Cohen, 1984] is discussed, as well as the neurophysiological basis of ISR. (C) 2000 Elsevier Science Ltd. All rights reserved.