Characteristics of saccadic intrusions

Primary fixation is never perfectly stable, but is frequently interrupted by slow drifts, microsaccades and saccadic intrusions (SI). SI are involuntary, conjugate movements which take the form of an initial fast movement away from the desired eye position and followed after a short duration, by either a return secondary saccade or a drift. The purpose of this study was to examine the prevalence and metrics of SI in a population of 50 healthy subjects. Using both one and two dimensional recordings we find that all 50 members of the subject group exhibited SI. The SI were bilateral, conjugate and horizontal. No purely vertical SI were detected when examined in three subjects. SI amplitude mean and range was 0.6° ± 0.5°, 0.1°-4.1°; SI frequency mean and range was 18.0 ± 14.3 per min, 1.0-54.8 per min; SI duration mean and range was 225 ± 150, 20-870 ms. The mean SI amplitude and frequency when SI < 0.5° were removed was 0.97° ± 0.56° and 7.0 ± 11.4 per min respectively. Age was positively correlated with SI amplitude (p < 0.01), but there was no correlation between age and SI frequency. Three of four types of SI monophasic square wave intrusions (MSWI), biphasic square wave intrusions (BSWI) and double saccadic pulses (DSP) were found to be exclusively saccadic, whilst the fourth type, the single saccadic pulses (SSP), were confirmed to exhibit a slow secondary component. MSWI were the most frequently observed SI occurring in 47 out of 50 (94%) of the subjects with a mean amplitude, frequency and duration of 0.7° ± 0.5°, 11.5 ± 11.6 per min, and 255 ± 147 ms respectively. Mean amplitudes and frequencies for BSWI (n = 20), SSP (n = 11) and DSP (n = 34) were found to be 0.50° ± 0.2°, 1.2 ± 2.5 per min; 0.40° ± 0.20°, 0.4 ± 1.0 per min and 0.3° ± 0.4°, 5.0 ± 8.7 per min respectively. No differences in MSWI characteristics were found between binocular and monocular viewing. Possible explanations for SI occurrence include experimental viewing conditions, subject fatigue and covert shifts in attention.