Generation of braking saccades in congenital nystagmus

Abstract

Objectives: We examined eye movement records of two congenital nystagmus (CN) subjects, whose waveforms contained braking saccades, to test the hypothesis that eye velocity, rather than eye position, is the more important criterion for braking saccade generation. Specifically, we wished to determine the criteria and timing used by their ocular motor systems in triggering these unique saccades. Materials and methods: For the records analyzed, eye movements were measured by either scleral search coil or IR limbic reflection and data were sampled at rates of 200–488 Hz with a resolution of 12 bits for analysis by custom software. Both position and velocity were used to determine critical timing points in CN cycles, including saccadic onset, duration, offset, and magnitude. Phase planes at various times (between 40 and 70 ms) prior to saccade onset helped determine (using foveation window criteria for best acuity) the conditions to generate a braking saccade. Results: Braking saccades do brake CN slow phases, with average slowing (unrelated to braking-saccade size) of 62 and 119% for the two waveforms studied. At 70 ms prior to braking saccades, both eye position and velocity usually still satisfied the criteria for good acuity established in the previous foveation period; by 40 ms, velocity no longer did. Thus, high eye velocity was the only criterion that could be used for saccade generation. Braking saccades result in longer foveation times per second for CN waveforms. Conclusions: Eye velocity is the main criterion used to trigger braking saccades and the determination to trigger them occurs closer to 40 than 70 ms prior to their execution. Braking saccades can increase the nystagmus acuity function and allow better acuity.