The effects of smooth pursuit eye movements and abutting pattern motion on luminance contrast sensitivity

Didem Alaşhan

Spatiotemporal context alters the visibility. The detectability of a low-contrast luminance-modulated sinusoidal target abutting a high-contrast drifting grating is impaired when the two stimuli are out-of-phase, suppression being strongest at the leading than at the trailing edge of motion. This effect was attributed to the predictive signals (Roach, 2011) or a spatial summation process and inhibitory motion deblurring occurring at the trailing edge (Arnold, 2014). In the previous studies, however, eyes were steady and the grating envelopes were stationary. It was shown that smooth pursuit eye movements influence the luminance sensitivity in a directionally selective manner. To gain a better understanding of the phase dependent modulation of contrast sensitivity, I conducted a set of experiments, where the contextual modulation was investigated in the presence of smooth pursuit to examine the effects of pursuit velocity, directional congruence between the pursuit trajectory and the drifting gratings, and the contrast-dependency. Results indicated that the phase-dependent modulation occurs both at the leading and at the trailing edge under the fixation, although in different magnitudes contradicting the predictive model. During pursuit, the size of modulation at the leading edge depends both on the pursuit velocity and the directional congruency. Additionally, the magnitude of the modulation is contrast-dependent only at the leading edge. These findings are consistent with neither the predictive nor the spatial summation account but rather suggest different underlying mechanisms at the leading and trailing edges, which may be modulated by feedback connections from higher-order sensorimotor areas.