The human pre-saccadic spike potential: Influences of a visual target, saccade direction, electrode laterality and instructions to perform saccades

Abstract

Three components of pre-saccadic evoked potentials have been identified in humans: a slow negative shift (SNS), a positive antecedent potential (AP) and a spike potential (SP). This study examined the influences of: (1) instructions to the subject to make saccades; (2) the presence of a visual target: and (3) the direction of the saccades on the amplitude of the averaged SP, which was recorded from P 3 and P 4 (International 10/20 System) in 20 normal, right-handed subjects. Recordings were made for spontaneous saccades prior to receiving instructions in six subjects. Twenty subjects performed self-paced saccades in the presence of a 10° visual target (two red LEDs) and while blindfolded in a dark room. The SP was either absent or grossly altered (broadened) for spontaneous saccades in an illuminated room; it was robust for self-paced saccades in light or darkness. Three-way analysis of variance revealed a highly significant cortical laterality (P 3 vs P 4) × saccade direction interaction ( P < 0.001), relecting that for a given saccade direction, the SP was larger over the contralateral recording site for the self-paced light (SPL) paradigm (Newman-Keuls test). In the self-paced dark (SPD) paradigm, though, this was only true for saccades to the right. By contrast, scatter plots of the directional indices (D.I.= [(SP for collateral) ± (SP for ipsilateral)]/[SP for contalateral) + (SP for ipsilateral saccades)] for left (P 3) and right (P 4) recording sites from individual subjects revealed a significant negative correlation for both SPL ( r = 0.74) paradigms. This implies that the SP for an individual subject tends to be larger at P 3 and P 4 for saccades in one direction rather than for contralateral saccades. Thus, instructions to perform saccades, saccade direction and left vs right electrode sites all affect the pre-saccadic SP in humans.