Scheduling mental operations in a multiple-response sequence: Modeling the effects of a strategy to minimise variance in the timing of saccades

Common tasks in daily life are often accomplished by a sequence of actions. For highly practiced tasks, such as reading or piano playing, the eyes routinely move to the next item in sequence before the response to the current item is made (eye hand span), evidence of overlap in the mental processing of successive stimuli. We (Wu & Remington, 2004) have previously presented data from a typing-like task following Pashler (1994) requiring separate choice responses to a series of five stimuli. We found a consistent pattern of results in both motor and ocular timing, and examined behavior under perceptual and central difficulty factors. Here we report a model of that task, which demonstrates how the observed timing of eye movements to successive stimuli could result from attempts to produce regular rhythmic output. The model makes two key assumptions about the generation of an eye movement to the next stimulus in a sequence. The first, dependence on perceptual encoding, asserts that the eyes remain fixated on the current stimulus until perceptual encoding is completed. The second, minimization of inter-movement time variability, asserts that people try to achieve a rhythmic regularity in motor performance, which includes the timing of eye movements. We discuss the conditions under which variance minimization might apply, and the implications for accounts that tie eye movements more directly to internal cognitive, perceptual, and motor operations.