Neural correlates of phi and reverse-phi motion aftereffects: Temporal dynamics of ON-OFF pathway interactions

Sensory processing is dynamically shaped by neural adaptation, allowing the brain to adjust to recent sensory input. Phi and reverse-phi motion provide a framework for probing adaptive processes and ON-OFF pathway interactions in visual motion processing. We recorded 64-channel EEG while 30 participants completed a rapid motion-adaptation paradigm. Participants adapted to contrast-preserving phi motion or contrast-reversing reverse-phi motion for a short (188 msec) or long (752 msec) duration and then judged the direction of an ambiguous counterphase-flickering test grating. Behaviourally, adaptation duration primarily determined perceptual bias: short adaptation produced a modest same-direction, priming-like bias, whereas long adaptation produced an opposite-direction motion aftereffect. Test-locked event-related potentials (ERPs) revealed a robust Duration × Adaptation Type interaction over early posterior scalp regions (128-160 msec), primarily expressed in the N1 range. This interaction reflected greater differentiation between phi and reverse-phi adaptation after long than short adaptation. Exploratory whole-scalp topographic analyses indicated that the interaction involved changes in scalp-response configuration rather than uniform amplitude modulation. Adapter-locked analyses further showed that phi and reverse-phi motion elicited distinguishable ERP responses during the initial adaptation period and separable multivariate EEG patterns from approximately 50 msec after adapter onset, indicating early neural divergence between the two motion types. Together, these findings provide electrophysiological evidence that rapid adaptation to phi and reverse-phi motion leaves distinct neural traces that differentially influence subsequent processing of an ambiguous motion stimulus, advancing our understanding of rapid sensory adaptation and ON-OFF pathway interactions in human motion perception.