Neural limits of temporal feature binding in human visual cortex

Humans are able to track the change of color and orientation (CO) of an object at a stunning rate. However, spatially separating those features slows this rate by an order of magnitude - a temporal limit of feature binding that is little understood. Here, we investigate the underlying mechanisms using high-temporal resolution cortical source localization of the magnetic field response elicited by CO-pairs alternating at different frequencies in color and orientation. We show that (1) binding requires a rapid recurrent modulation (~120 ms) of activity in V1/V2, whose latency is independent of the alternation rate. (2) Performance drops to chance when the alternation rate becomes too high to accommodate the recurrent process within a single alternation. (3) CO binding itself is mediated by color-based attention, which gradually biases re-occurring CO combinations containing the attended color. These observations together demonstrate that CO-binding across space depends on rapid recurrent processing in early visual cortex, with its temporal limit being set by the time this process takes to be completed. Binding ultimately results from a slower and gradual attentional bias of the recurrent process instantiating a form of stroboscopic stationarity in the stream of alternations for unambiguous discrimination of the correct feature-combination.