Abstract
Contrast sensitivity to low-contrast luminance-modulated target gratings is higher when presented at the leading edge of an in-phase abutting inducer, rather than at the trailing edge (N.W. Roach, P.V. McGraw, A. Johnston, 2011). This phase-contingent facilitation effect has been linked to spatial summation (D. H. Arnold, M. Marinovic, D. Whitney, 2014). In the previous paradigms, eyes were always fixated on a steady fixation. Here, we investigated this in the presence of smooth pursuit eye movement. In different conditions, the target (a luminance-modulated sinusoidal grating with a spatial frequency of 1 cpd, drifting at a speed of 5 deg/sec) was presented inside a rectangular window either 1deg above or below of a static fixation spot or moved horizontally (11 deg/sec) across the screen together with the fixation. Abutting the target was a high-contrast inducer grating drifting at the same speed. Inducers were presented in both sides of the fixation. In a 2-AFC task, subjects reported whether the target appeared alongside the above or below inducer. The results of our control condition, where eyes were fixated on a steady fixation, were similar to the findings of Arnold et al such that the thresholds in the 0 deg-phase-difference condition between the target and the inducer were as low as in the no-inducer baseline, whereas they were significantly higher in the 180 deg-phase-difference condition. Moreover, there was a trend of phase modulation being higher for the targets in the leading position than those in the trailing position. In the main experiment, where participants were performing a smooth pursuit eye movement while performing the task, the phase modulation was significant only when the target was on the leading position with respect to both eye movement and inducer motion. These results indicate that underlying the effects of phase modulation and eye movement may be different mechanisms.