The latency of target elicited saccadic eye movements

Abstract

In 1967 M.G. Saslow found that latencies of target elicited saccades were significantly reduced when the target onset was preceded shortly by the offset of a fixation point (Saslow, 1967). This result has subsequently been replicated by various authors, and has provided the basis for a number of investigations into the properties of the mechanisms of saccadic control. In 1983 B. Fischer and R. Boch reported the discovery of a second effect. Using the same basic experimental methods and using monkeys as subjects, they found a population of saccades with extremely short reaction times in addition to the general reduction in saccade latencies previously reported. They termed this population 'express saccades' (Fischer and Boch, 1983).Various models have been proposed to explain both the reduction in saccade latencies reported by Saslow, and the occurrence of 'express saccades’ reported by Fischer et al. This thesis provides an explicit, quantitative framework against which these models can be compared. Although the phenomenon of express saccades has been well established for monkeys, the evidence for their occurrence in humans appears less convincing. This thesis tests in a rigorous manner for a population of saccades in humans equivalent to the express saccades found for monkeys. Chapter One reviews the experimental factors that affect the latencies of target elicited saccades. The validity of the 'when/where' distinction in models of saccadic control is discussed in Chapter Two. In Chapter Three the reduction in saccade latencies found by Saslow, and express saccades, are discussed in greater detail together with models proposed in explanation. The fourth chapter gives the rationale for experiments designed to test these models, and in Chapter Five these experiments are described and their results and implications for models of saccadic latency are discussed. Conclusions to the thesis are given in Chapter Six.