Population receptive field estimates for motion-defined stimuli

The processing of motion changes throughout the visual hierarchy, from spatially restricted 9local motion9 in early visual cortex to more complex large-field 9global motion9 at later stages. Here we used functional magnetic resonance imaging (fMRI) to examine spatially selective responses in these areas related to the processing of random-dot stimuli defined by differences in motion. We used population receptive field (pRF) analyses to map retinotopic cortex using bar stimuli comprising coherently moving dots. In the first experiment, we used three separate background conditions: no background dots (dot-defined bar-only), dots moving coherently in the opposite direction to the bar (kinetic boundary) and dots moving incoherently in random directions (global motion). Clear retinotopic maps were obtained for the bar-only and kinetic-boundary conditions across visual areas V1-V3 and also in higher dorsal areas. For the global-motion condition, retinotopic maps were much weaker in early areas only and became clear only in higher areas, consistent with the emergence of global-motion processing in these areas. However, in a second experiment we found very similar results when the bar was low in visibility, with the clearest maps obtained in dorsal extrastriate cortex. This was the case both for a transparent-motion stimulus as well as a bar defined by a static low-level property (dot size) that should have driven responses particularly in V1. In fact, these extrastriate maps only manifested in participants who reported seeing this low-visibility stimulus. Our findings therefore indicate that dorsal extrastriate retinotopic maps may primarily be determined by general stimulus visibility (or salience) and suggests that claims about stimulus selectivity from pRF experiments must be interpreted with caution.