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Carmel A. Levitan, Paul R. MacNeilage, Martin S. Banks, Clifton M. Schor; Do pursuit eye movements improve discrimination of object speed?. Journal of Vision 2005;5(8):439. doi: 10.1167/5.8.439.
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The speed of an object in the world is the sum of speed on the retina and speed of the eye: S = R + E. Thus, the precision of speed estimation depends on the precision of retinal and extra-retinal signals and how their precisions vary with speed. People usually pursue an object when estimating its speed in everyday tasks; they do not choose to hold the eye steady. Compared to the non-pursuit strategy, pursuing the object minimizes retinal speed while increasing eye speed. How does this affect the precision of object-speed estimates? To examine this, we had observers judge the relative speeds of Gabor patches in a 2-IFC task in two conditions: 1) non-pursuit in which they held the eye steady as the stimuli moved across the retina, and 2) pursuit in which they pursued the stimuli. To make the stimulus information equivalent during pursuit and non-pursuit, we could not present a fixation aid. Thus the stimuli were pairs of Gabor patches moving rigidly together, with one patch above the place where the observer fixated and one below. Before each stimulus presentation, a lead dot appeared moving at a speed close to (but not indicative of) the speed of the upcoming target. The dot then disappeared and the Gabor patches appeared. In the pursuit condition, observers pursued the lead dot, so their eyes were already moving when the target appeared. In the non-pursuit condition, observers kept their eyes stationary for the whole trial, including the presentation of the lead dot. Observers were trained under these conditions to make smooth pursuits in the pursuit condition and to hold the eye steady in the non-pursuit condition. Eye movements were measured to determine accuracy on each trial. Speed-discrimination thresholds were lower in the pursuit than in the non-pursuit condition for most observers, but the differences were small. The difference depended on target speed, which means that the noises in the retinal and extra-retinal signals are affected differently by speed.
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