Human spike potentials prior to saccades and optokinetic nystagmus fast phases: Effects of instructions, eye movement direction and electrode laterality

Abstract

Presaccadic potentials in humans consist of 3 components, a slow negative shift, a positive antecedent potential and a spike potential (SP). This study demonstrated the presence of the SP before another class of rapid eye movements, fast phase of optokinetics nystagmus (OKN), in 11 right-handed, normal human subjects and compared the amplitude of the averaged SP for OKN fast phases, recorded at P 3 and P 4 (International 10/20 System), with the SP amplitude for 10° self-paced saccades between two red light-emitting diodes. In particular, the effects of electrode laterality, rapid eye movement direction and two sets of instruction during the OKN task were assessed. Subjects were told to either ‘look at the screen’ (OKN1 paradigm) or to ‘try to slow or stop the motion of the pattern’ (OKN2 paradigm) during presentation of optokinetic stimulation. As in the case of saccades, the OKN fast phase velocity spike was used to trigger the signal averager (Nicolet (CA-1000)). There were two significant differences in SP amplitude prior to saccades and OKN fast phases. First, 3-way analysis of variance (ANOVA) and Newman-Keuls comparisons revealed that the SP was attenuated significantly for OKN for either eye movement direction over either left or right recording sites( F = 16.045, P < 0.001). This effect was not related to amplitude differences in eye movements in the different tasks, although a contribution of variance in the OKN fast phase amplitudes cannot be excluded. Second, the significant inverse relationship between P 4 and P 3 directional indices (defined for each recording site as: [SP amplitude for contralateral eye movement-SP amplitude for ipsilateral eye movement]/[SP amplitude for contralateral eye movement + SP amplitude for ipsilateral eye movement]) for saccades 2 was absent for OKN fast phases. This indicates that while the SP is larger at both P 4 and P 3 for saccades in the same direction, the directional properties at these sites are uncorrelated for the pre-OKN fast phase SP. Furthermore, there were differences in SP amplitude as a function of the two OKN paradigms, reflected in significant paradigm × eye movement direction( F = 5.857, P < 0.05) and paradigm × eye movement direction × electrode laterality( F = 4.40, P < 0.03) interaction effects. Newman-Keuls comparisons demonstrated that the SP amplitude for right and left fast phases, independent of electrode laterality, was affected differentially in the two OKN paradigms and that a larger SP for contralateral rapid eye movements was present for all paradigms and recording sites except for the right recording site in OKN2. These data suggest that there are significant differences in patterns of activity in common pathways prior to saccades and OKN fast phases.