Perception and neural processing of radial motion

As we navigate the environment, images flow over our retinae and produce complex moving patterns including contraction and expansion. These in turn serve to inform us about our movement in the world, as well as about the positions and relative motions of the objects around us. The processes underlying the perception of radial motion were explored using psychophysics, eye tracking and functional magnetic resonance imaging (fMRI). The initial experiments studied how adaptation to radial motion affects discrimination using partially-coherent dot stimuli. Adaptation increased absolute detection thresholds in the adapted direction, but had no effect on the discrimination of higher pedestals. The change in sensitivity is consistent with a divisive inhibition mechanism, as well as with one based on proportion estimates. An orientation discrimination experiment adds support for the latter. Directed attention to one component of a motion-balanced transparently-moving stimulus impairs sensitivity to a level comparable to that following adaptation, although there were large inter-subject differences. A novel approach using relative velocity adjustments of transparently-moving dot fields was used to investigate the reference frame of motion adaptation, which is shown to be mainly retinotopic and diminished by gaze modulations. Finally, fMRI was used to probe the neural substrates of radial motion perception, including retinotopic mapping, localisation of MT/MST, and selectivity for contraction and expansion within the MT+ complex