Transcranial magnetic stimulation (TMS) of early visual cortex reveals a window of integration of substantial duration

Abstract

How the brain achieves integration of temporally dispersed information is one of the enigmas in the neurosciences. By combining feature fusion with transcranial magnetic stimulation (TMS), we show that individual visual features are independently stored for several hundred milliseconds before condensed in a conscious percept. We presented two verniers with opposite offset directions in rapid succession at the same retinotopic location. Because of the short durations (30ms each) only one fused vernier is perceived. The perceived offset of the fused vernier is a combination of the offsets of the two verniers which themselves are consciously not accessible. First, we balanced performance at approximately 50%, so that neither the first nor the second vernier dominated the fused percept. We then applied TMS at different times after the onset of the first vernier over the occipital cortex to interfere with visual processing. When TMS was applied from 45ms to 120ms after the onset of the first vernier, the second vernier dominated. For later onset asynchronies of up to 370ms, the first vernier dominated. Hence, even though the individual verniers are not consciously accessible, they still can be manipulated by TMS for a period of more than 300ms. Our results demonstrate that the individual verniers are stored independently for several hundred milliseconds before they are condensed into a conscious percept. Thus, the brain collects and integrates feature information across a broad time window before consciousness.

Footnotes

 This work was supported by the SNSF (Swiss National Science Foundation).