Metrics of saccade responses to visual double stimuli: two different modes.

Earlier studies using visual double stimuli along the horizontal axis have revealed a strong averaging tendency in the saccadic system. This study shows averaging also for equally eccentric double stimuli with a modest difference in direction (delta phi = 30 deg). When the difference is enlarged (delta phi = 90 deg) the response pattern becomes bistable; i.e. the eye jumps near either one or the other stimulus. This bistable response mode is reflected also in saccade amplitude when double stimuli along the horizontal axis have a large difference in eccentricity. It is concluded that the saccadic system response mode to double stimuli depends on interstimulus spacing. Furthermore, both types of response can be shown to exist with double stimuli confined to one visual half-field.