On the origin of the presaccadic spike potential.

The spike potential (SP) accompanying the onset of saccadic eye movements has been reported to originate near the orbital region. Its dependence on saccade size, however, does not correlate with the behaviour of any of the possible sources of the potential available in the orbital region. The exact size dependence cannot be studied from results obtained with classical ENG methods to detect the saccadic onset and furthermore without removal of the artefact in the signal caused by the corneo-retinal potential. We recorded the SP by monitoring the eye movements with an infrared scleral reflection method (IRIS), and carefully studied the SP amplitude as a function of saccade size, and revealed a more realistic function. Furthermore, removal of the artefact of the corneo-retinal potential revealed a biphasic wave shape of the SP instead of the usually observed monophasic peak. These results support the hypothesis that this electrical activity accompanying the onset of saccadic eye movements originates in the oculomotor neurones innervating the ocular muscle units.