Is attention involved in the smooth pursuit system?

Abstract

When observers visually track a moving target, they use smooth pursuit eye movements to stabilize it and thereby maintain clear vision. Most objects we pursue in the natural world extend beyond the fovea, however the predominant pursuit stimulus in the laboratory is a small, foveal spot. Since larger, natural objects also stimulate peripheral retina, the pursuit system could benefit from the additional motion information that peripheral stimulation provides, yet little previous work has explored this. In previous studies that have studied pursuit with peripheral motion, the frequency of saccadic intrusions was decreased, and improvements were observed in pursuit initiation and direction discrimination. There is also evidence that the benefit afforded by peripheral motion allows observers to release attention from the foveal target. Specifically, more accurate identification of a brief change in a peripheral feature of the pursuit target occurs with peripheral motion present. In the thesis, I investigate potential benefits of peripheral motion to feature identification for the saccadic system, and the sources and distribution of attentional resources during pursuit. A saccade task was imposed on the pursuit task by flashing a random one of four small dots close to the pursuit target during pursuit, and asking the observer to saccade to that dot. As expected, adding the peripheral motion decreased saccade latency, implying that the additional motion released the fixation mechanism from the foveal spot. I then asked if saccade latency could be reduced further by turning off the pursuit target just before the saccade target was flashed. Latency decreased both with and without the peripheral motion, indicating that the fixation mechanism is not all-or-none but can be released in steps. To obtain direct evidence for the claim that the peripheral motion was releasing attention, I changed the color of the foveal pursuit stimulus briefly to attract attention to it. This manipulation eliminated the benefit to saccade latency that the peripheral motion normally confers, supporting this claim. Finally, I sought to determine whether the released attention was specific to the visuomotor system or if it came from a general pool. First, the luminance of the saccade target was increased to eliminate the benefit of the peripheral motion and then observers performed a memory task to load general attention. The benefit of peripheral motion reappeared with the memory task, supporting the hypothesis that foveal pursuit usurps attentional resources from a general pool.