Saccades to somatosensory targets. I. behavioral characteristics.

1. We compared the properties of saccades to somatosensory and visual targets. This comparison provides insight into the translation of sensory signals coding target location in different sensory coordinate frameworks into motor commands of a common format. Vibrotactile stimuli were delivered to the hands, which were fixed in position and concealed beneath a barrier. Saccades of different directions and amplitudes were elicited by the same somatosensory target from different initial eye positions. Both monkeys and humans served as subjects. 2. Somatosensory saccades were less accurate than visual saccades in both humans and monkeys. When the barrier concealing the hands was removed, somatosensory saccade accuracy improved. While the hands were concealed, the visual frame of reference provided by room illumination did not greatly affect saccade accuracy: accuracy was not degraded in complete darkness for two of three monkeys. 3. The endpoints of saccades to a single somatosensory target varied with initial eye position for the monkeys, but not for the human subjects. 4. We also found evidence of an effect of limb position on somatosensory saccades: when human subjects performed the task with crossed hands, the incidence of curved saccades increased. Saccades often began in the direction of the unstimulated hand and curved markedly toward the stimulated hand. When one subject was required to delay the saccade by 600-1,000 ms after target onset (the delayed saccade task), the saccades were straight. Somatosensory saccades were also straight when the hands were not crossed. 5. The reaction times of somatosensory saccades were longer than the reaction times of visual saccades, and they decreased as a function of saccade amplitude. The delayed saccade task reduced the differences between somatosensory and visual saccade reaction times. The reaction times of saccades to very dim visual targets increased into the range found for saccades to somatosensory targets. When the saccade target was the combination of the somatosensory and visual stimuli at the same location, the reaction time was slightly lower than for visual targets alone. 6. The peak velocities of somatosensory saccades were lower than those of visual saccades of the same amplitude. The velocities of saccades to combined somatosensory and visual targets were indistinguishable from those of saccades to visual targets alone. The differences between somatosensory and visual saccade velocity were maintained in the delayed trial type. These differences suggest that the main sequence or velocity-amplitude relationship characteristic of saccades depends on the modality of the target. 7. The implications of these modality-dependent differences in accuracy, reaction time, and saccade velocity are discussed with regard to models of the saccade generator and the coordinate transformation necessary for somatosensory saccades.