Effect of saccade size on presaccadic spike potential amplitude.

Presaccadic spike potentials were recorded from electrodes placed at the inner canthus and below the eye in ten normal subjects. The responses to twenty abducting and twenty adducting saccades were back-averaged from the beginning of the eye movement, and separate waveforms were obtained for saccades of 20 degrees, 10 degrees, 7.5 degrees, 5.0 degrees, and 2.5 degrees. The spike onset and peak latencies relative to the beginning of the eye movement and the onset-peak amplitudes were measured. Throughout the range of saccade sizes, the onset and peak occurred earlier with adduction than with abduction, but there was no consistent change in the latency values with saccade size. The amplitude, however, showed an increase with saccade size up to movements of 10 degrees, although statistical analysis showed that these amplitude changes were not consistently significant. Comparison of the 20 degrees saccades to the 10 degrees saccades and the 10 degrees saccades to the 7.5 degrees saccades revealed no statistically significant differences in amplitudes. The difference between the 7.5 degrees and the 5 degrees saccades, however, was statistically significant (P less than 0.05) but only at the inner electrode with abduction; comparison of the 5 degrees and 2.5 degrees results showed a significant difference (P less than 0.01) at the inner electrode with both abduction and adduction. Comparison of the 7.5 degrees and the 2.5 degrees showed a significant difference (P less than 0.01) at both electrode sites with abduction and adduction. These findings are discussed with respect to the previously suggested origin of the spike potential and the choice of the recording electrode site.