Directional tuning of the human presaccadic spike potential.

This study examined the behavior of the presaccadic spike potential (SP) in 20 normal, right-handed subjects for self-paced 10 degree saccades along vertical, horizontal and oblique meridians. The SP was recorded differentially between posterior parietal sites and a linked ear reference. The SP amplitude showed clear directional tuning properties with the maximum response prior to saccades directed in an oblique, downward and contralateral direction. The minimum response was obtained for upward saccades. The data were fitted to a model consisting of 3 components: (1) a constant component reflecting saccadic amplitude; (2) a component with narrow directional tuning that is modeled from properties of frontal eye field neurons in monkeys; and (3) a component with broad directional tuning that is designed to reflect directional tuning properties of potentials originating from horizontal and vertical saccade generators, motoneurons and extraocular muscles. The narrowly tuned mechanism is sufficient to encode the direction of saccades. One surprising finding, though, was that the broadly tuned mechanism was necessary and sufficient to account for differences in SP amplitude as a function of electrode laterality. Application of this model approach to published data for periorbital SP, recorded between periorbital and parietal sites (G. W. Thickbroom and F. L. Mastaglia, Electroencephalogr. Clin. Neurophysiol., 64 (1986) 211-214), suggested that this potential represents a broadly tuned mechanism with directional tuning opposite the parietal SP. These data indicate that the periorbital and parietal SPs represent activation of different networks of central generators.