Is the duration of an eye fixation a sufficient criterion referring to information input?

Abstract

In experiments on eye movements duration of eye fixations is frequently used as a criterion. In different studies mean duration of eye fixations is relatively constant, amounting approximately to .30 sec., even though duration of a single fixation can vary within a quite large range (cf. Fig. 1). An eye fixation in reference to information-input (Purkinje, 1825) may mean distinguishing between fixations devoted to exploring, processing, and staring according to their durations which should be respectively short, at about average, and long. Even though strict limits were never given, this statement has influenced recent research. If this statement is true, then the distribution of durations must be represented at least by a bimodal vs trimodal curve. For testing this hypothesis the durations of a total of 960 eye fixations, observed on six drivers while steering a car on a straight road, were analyzed as described, elsewhere (Cohen & Fischer, 1977). The mean duration was 0.36 sec. with a standard deviation of 0.22 and a skewness of 2.18; the respective distribution as well as the cumulative frequency are shown in Fig. 1. This figure indicates clearly a unimodal curve. The observed distribution does not allow distinction berween types of eye fixations by their durations only. Furthermore, about 70% of the eye fixations might be approximated by a linear relationship between their cumulative frequency and the respective durations. These data suggest that duration of eye fixation alone is not a sufficient criterion for distinguishing among the three possible purposes of single eye fixations. The duration of a fixation might also depend on the interactions among prior fixations, the am litude of eye movements, characteristics of the fixated point, the present schema of the suiject, as well as his capacity to process information. It might be assumed that detailed information is picked up through foveal vision, while parafoveal vision could be devoted simultaneously to exploration of further points of special interest in the environment to determine the next fixation point prior to beginning the eye movement. This dual function of the eye excludes also a strict distinction between eye fixations devoted only to processing or to scanning.