Probing sensory plasticity with rapid forms of motion adaptation
Perception is shaped by both immediate pattern of sensory inputs and previous experience with the external environment. Visual adaptation, a temporary change in perception following exposure to a stimulus, has been widely employed to understand how previous sensory experience on different timescales shapes perception. Visual motion adaptation is a powerful investigative tool to understand sensory plasticity and neural adaptation. However, the neural mechanisms underlying adaptation induced changes by visual motion are still subject to debate. In the present thesis, spatiotemporal dynamics, neural substrates, and functional role of sensory plasticity in the human visual system was examined using rapid forms of motion adaptation paradigm combined with EEG. Specifically, how motion adaption-induced short-term sensory plasticity is reflected at the neural level and parallel with perceptual performance were explored. Participants were adapted to directional drifting gratings for either short (640 ms in Experiment 1; 188 ms in Experiments 2 and 3) or long (6.4 s in Experiment 1; 752 ms in Experiments 2 and 3) durations and used a counter-phase flickering (with constant polarity in experiments 1 and 2; polarity inverting within every step in Experiment 3) grating as a test pattern. Sinusoidal gratings of phi motion were employed in Experiment 1 whereas; square wave gratings were used for phi and reverse-phi adaptations in Experiments 2 and 3 to examine how ON and OFF pathways operate in the visual processing stream. Based on the EEG analyses in Experiment 1, the scalp sites relevant to motion adaptation were identified. Experiment 1 showed that both adapting durations led to significant motion aftereffects and EEG results showed that long adaptation produced stronger aftereffects than the short adaptation condition within 64-112 ms time range over occipital and parieto-occipital sites. Taken together, these findings provide important electrophysiological evidence that motion aftereffects reflect changes in cortical areas mediating low- and mid-level visual motion processing. They also suggest that adaptation is an active process that involves neural mechanisms operating at different time scales. In Experiments 2 and 3, the short-term adaptation induced changes over these identified scalp sites were further examined based on Experiment 1. Given that the phi and reverse-phi motion mainly engage within (ON or OFF) and across (ON and OFF) pathway mechanisms, the comparisons of adaptation induced changes across these motion types provided further insights into the nature of corresponding mechanisms over visual cortex. The behavioral and EEG findings pointed to efficient convergence of information provided by these pathways and some distinct characteristics of across pathway mechanisms.