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

Tasks such as reading or visual search consist of series of saccades. We have investigated to what extent saccades that are made within a series of self-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 before so-called 'return saccades' (saccades returning to the previously fixated position) are considerably longer (up to 40%) than other fixations. This phenomenon, which we call 'Inhibition of Saccade Return' (ISR), is present 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 found 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.