The frequency of catch-up and micro saccades reacts to and predicts stimulus events

[摘要] Smooth pursuit and fixation are generally regarded as independent systems. But are the small saccades that occur during pursuit (catch-up saccades) and the small saccades that occur during fixation (microsaccades) generated by different mechanisms? Here we test this by capitalizing on the curious phenomenon that microsaccades subside around the time of visual events. Subsidence occurs either in reaction to a visual cue or in anticipation of a task-relevant stimulus. During anticipatory subsidence, saccades almost completely stop. We ask if a similar phenomenon occurs for catch-up saccades during ocular pursuit. Observes pursued a linear array of 15 small alphanumeric characters (0.28° × 0.5°) and performed a character discrimination task on them (Lovejoy et al., 2009). A linear array of '8's appeared stationary for a random fixation duration, then translated either leftward or rightward across the screen at 8°/s, 12°/s or 16°/s for 1640–2440 ms. The pursuit target was always the middle character of the array, which was cued 320 ms after fixation onset. Following the fixation period, at a random time (1040–1440 ms), the character array changed from '8's to '2's and '5's except for a single probe character that changed to a '3' or an 'E'. After 200 ms, all characters changed back to '8's. Observers identified the '3' or 'E' with a keypress. We found that catch-up saccade frequency decreased following the pursuit target cue, and also in anticipation of the identification task. The time course of the reactive and anticipatory subsidence was virtually identical during pursuit and fixation. The results provide evidence that the mechanism generating catch-up saccades during pursuit also generates microsaccades during fixation. We hypothesize that small saccades subside during pursuit and fixation to allow clear vision when high acuity is required.

[学科分类] 眼科学