Latency and accuracy of saccades to somatosensory targets

To examine the accuracy and latency of reflexive saccades to vibratory stimulation of the fingertips made by normal human subjects and to compare the findings to those of visually guided saccades.

Eye movements were recorded using infrared oculography. Stimuli were presented via an array of audiometric bone vibrator transducers driven at 250 Hz and positioned at eye level in a darkened room. Target locations were at 0 and +/-5, 10, and 15 deg. Visual stimuli were green LEDs. Saccades were analysed interactively off-line and latency and amplitude measured for both types of saccade.

Saccades to tactile stimuli had longer latencies and showed less accurate final eye positions than those to equivalent visual targets; they were unaffected by subject age. Error magnitude for the tactile saccades increased monotonically with increasing target eccentricity; because the fingers remained in a fixed position throughout the testing, this also meant that error was lowest for the thumb and increased with progression outwards towards the ring finger. Visually guided refixations were accurate and differed less than 0.2 deg across target locations. Subject age had no effect on performance.

Human subjects may make relatively accurate refixations to tactile targets in the absence of visual cues, but only to certain locations/fingers; others were localised very poorly. Further studies are needed to determine whether finger selection or target location is the primary determinant of accuracy in this task.