Visual localization and estimation of extent of target motion during ocular pursuit: a common mechanism?

The ability to localize a visual target and to estimate the distance through which it moves was studied during ocular pursuit. In the first experiment observers had to localize the position of a visually tracked moving target when they heard an acoustic signal. The signal was sounded near the beginning or near the end of the motion. The distance between the perceived positions was shorter than the distance between the corresponding physical positions of the target. The 'shortening' became more pronounced with higher tracking velocity. In another condition the observers estimated the length of the motion path between two successive sound signals, one presented near the beginning and one near the end of the motion. The length of path travelled was underestimated, the effect being stronger with higher tracking velocity. In the second experiment this effect of velocity on the underestimation of distance was shown to exist only during ocular pursuit and not during steady fixation. The hypothesis that localization and estimation of distance during ocular pursuit share a common mechanism is discussed.