Quick phase programming and saccadic re-orientation in congenital nystagmus

Horizontal single-step stimuli were presented to 19 subjects with congenital nystagmus (CN). When the stimulus jump was in the same direction as the waveform beat, spatial and temporal measurements of the responses revealed that saccades occurring between 90 and 180 msec after the stimulus were directed to an average of the spatial locations of the initial and final target positions. Transition functions of response end position against delay resembled those collected from normal subjects responding to either double-step staircase or pulse-overshoot stimuli, according to the stimulus direction. An analogy is drawn between the response to a single-step stimulus in the presence of CN and responses made by normal subjects to double-step stimuli. These findings suggest that pathways involved in the computation of amplitude of visually guided saccades also participate in programming CN quick phases and that the time taken to programme CN quick phases is similar to that for voluntary saccades. Evidence of saccadic parallel processing was demonstrated in some, but not all, of the subjects. Responses to steps in the same direction as the nystagmus beat frequently overshot the target. The mean overshoot was proportional to the amplitude and intensity of the nystagmus. Responses to stimuli presented in a direction opposite to the nystagmus beat were as accurate as the responses of a normal control group.