Abstract
The perception of optic flow relies on the integration of many local pieces of information to generate a global percept of motion direction. This processing of motion requires mechanisms that can integrate information over space and time. Here we investigate the temporal integration of motion direction across early visual areas using EEG source-imaging and participants who each had fMRI-mapped retinotopic and functionally defined visual areas. These individually defined regions of interest allowed us to average data across participants in a way that respects individual differences in sulcal anatomy and electrode placement.
We used a random-dot motion stimulus with individual dots updating at 30 Hz that alternated between globally coherent motion and incoherent motion at 1 Hz. During the incoherent interval, each dot had a random direction on each update, but during the coherent interval we systematically varied the duration any single dot spent on a consistent trajectory before being extinguished and replaced with a dot in a new random location. The local information provided by the individual dots was tagged with 30 Hz while the global alternation from coherent flow to incoherent boil was tagged at 1 Hz. These tags enable us to distinguish responses associated with global and local information.
We find that activity in dorsal visual areas increased monotonically with longer dot trajectories. While activity from human MT complex differentiates coherent from incoherent global flow, its response does not grow with increasing consistency of trajectory. In addition we find a signature of nonlinear coupling between local information updates and the presence of coherent motion. Specifically, in dorsal visual areas the response at the frequency tagged by the local dot refresh rate increases during the coherent motion interval. These results suggest that dorsal visual areas such as V3A are responsible for the integration of motion information over extended trajectories.