The perceptual and cortical consequences of adaptation to smooth pursuit: An MEG study of the extra-retinal motion aftereffect

Abstract

Adaptation to smooth pursuit produces a compelling motion aftereffect, such that a stationary test stimulus is perceived to move in the opposite direction to that of the adapting eye movement. It has been proposed that the illusory motion results from adaptation of pursuit-sensitive neurons in MT+ that receive extra-retinal eye movement signals. We sought to expand on these findings by examining the cortical substrates of this ‘extra-retinal’ motion aftereffect (tERMAE) using MEG, by asking whether oscillatory changes in cortical areas thought to subserve pursuit correlates with the subjective percept of the illusion. Participants (n = 17) adapted to a moving target dot executing a repetitive sawtooth wave (six 2 s upwards sweeps at 12°/s) in the absence of any peripheral retinal motion. This was followed by a 12 s test phase (stationary fixation point), during which time participants reported the duration and direction of any illusory motion with a button press when the illusion had decayed. Results showed that 15/17 participants reported tERMAE on more than 50% of trials. Analysis of MEG data for these 15 participants revealed sustained alpha and beta suppression in FEF following eye movement adaption, with the suppression persisting for the duration of tERMAE. MT+ showed sustained alpha and beta decreases during pursuit-adaptation but only transient suppression during tERMAE. This post-adaptation suppression almost immediately returned to baseline levels. Parietal regions also showed similar spectral characteristics to MT+. We speculate that oscillatory changes in FEF could reflect the oculomotor control of fixation which in this case is likely to require the suppression of post-adaptation afternystagmus, as well as any tendency to track the illusory motion. As for the absence of any sustained MT+ responses during the illusion, it is possible that adaptation of motion-processing cortex does not necessarily manifest itself as a macroscopic oscillatory change.