Presaccadic 'spike' potential: investigation of topography and source.

Approximately 15-30 ms before a saccade, a large-amplitude, relatively short duration potential whose source has not previously been determined with certainty, can be recorded from the scalp in man. We have measured the surface topography of this potential during horizontal saccades in normal subjects and have found it to be maximal near the eye on the side ipsilateral to the direction of the saccade. Dipole modelling predicts a source near the eye ipsilateral to the direction of gaze, and the application of source derivation supports such an origin. Measurements of the potential close to the orbits in normal subjects and in patients with a unilateral abducens nerve palsy, exenterated orbit or ocular prosthesis suggest that it is a summation potential derived from the ipsilateral lateral rectus and contralateral medial rectus muscles. It is hypothesized that the potential may arise as a result of the synchronized recruitment of motor units in the extraocular muscles prior to the commencement of the saccade.