Presaccadic spike potentials with large horizontal eye movements.

Presaccadic spike potentials were recorded from electrodes at the inner canthus and below the eye in 10 normal subjects for a range of horizontal saccades (5 degrees, 10 degrees, 20 degrees and 40 degrees). An eye movement trace was recorded for 20 abducting and 20 adducting saccades to determine the beginning of the saccade, and the spike potential back-averaged from this point. The latencies of the spike potential onset and peak were found with respect to the start of the eye movement and the amplitude from the onset to the peak was measured for each saccade size. The latency values remained constant throughout the range of eye movement sizes, although adducting saccades showed an earlier onset and peak latency than abducting saccades. The amplitude data, however, showed a definite relationship between saccade size and spike potential amplitude, with a significant increase in the amplitude for saccades between 10 degrees and 40 degrees (inner electrode abduction, lower electrode abduction and adduction P less than 0.01; inner electrode adduction P less than 0.05). A possible explanation of this increase in amplitude is hypothesised from a computer model of the action potential activity that may occur in the extraocular muscles before an eye movement.