07-24 – Influences of Neural Oscillation Phase on Perception of the Tilt Illusion

Rhythmic influences on neural processing have been shown at multiple levels from synapse to
behaviour and have inspired many theories of the role oscillations may play in neural computation. Despite this, however, the results of studies testing an association between oscillation phase and human perception are inconsistent. Several null results have emerged in studies of phasic influences on visual discrimination, prompting questions about whether oscillatory influences on neural processing have any impact on perceptual outcomes. In recent theoretical work, we proposed reasons why oscillation analyses may be insensitive when associated with higher-level visual functions, potentially producing spurious null results. Here, we followed up on that work, testing for an association between oscillation phase and visual discrimination in a low-level visual effect, the direct tilt illusion, in which the perceived orientation of a central grating is biased away from the angle of an oriented surround. We titrated a central grating to each participant’s perceived-vertical angle in the presence of an oriented surround of ±30°. We then had them make forced choice reports of whether the central ‘vertical’ stimulus was rotated clockwise or counter-clockwise on each trial while we measured their brain activity with EEG. We replicated the direct tilt illusion in both
surround conditions. Furthermore, we found that the likelihood of participants making an ‘illusion-consistent’ (i.e., repulsive) response fluctuated with the power and phase of pre-stimulus neural oscillations in the 8-14 Hz ‘alpha’ range, consistent with an influence of these oscillations on the strength of the illusion on single trials. These results explain one source of across-trials variance in perceptual reports of the direct tilt illusion, and confirm that neural oscillations influence visual discrimination, consistent with their broader role in neural communication and coordination.