Investigation of feature integration and feature separation by TMS

How the brain integrates visual information across time into coherent percepts is an open question. Here, we investigated this integration using a feature fusion paradigm. In feature fusion two stimuli are presented in immediate succession. The stimuli are not perceived individually but as one fused stimulus. For example, a red and a green disc presented in rapid succession are seen as a yellow disc. It has been shown that feature fusion can be modulated by transcranial magnet stimulation (TMS) over occipital cortex for a surprisingly long duration of 400ms, suggesting that neural representations interact for this duration. If fusion were always to take place, fusion would lead to considerable smear. Hence, the question arises under which conditions stimuli fuse. Our current results suggest that stimulus transients prevent fusion because transients signal the presence of different objects. TMS cannot modulate this process. Suppressing the transients with a mask leads to the fusion of the stimuli which can be modulated by occipital TMS. Our results suggest that long lasting feature integration in occipital cortex occurs only when feature belong'' to one object, but not when the very same features presented at the very same spatio-temporal location "belonging'' to different objects."