Stochastic resonance at early visual cortex during figure orientation discrimination using transcranial magnetic stimulation.

Visual noise usually reduces the visibility of stimuli. However, very low contrast or subliminal visual noise can sometimes enhance the visibility of low-contrast stimuli. It has been suggested that this enhancement occurs at the visual cortex. The aims of this study are to clarify the role of the early visual cortex (V1/V2) in the enhancement effect and to clarify the relationship of the SR characteristics among different experiments. Noise was added directly to the visual cortex by using transcranial magnetic stimulation (TMS) with randomly varying intensity. The location on the scalp and the timing (stimulus onset asynchrony, SOA) of TMS were specifically adjusted to target the early visual cortex. Contrast thresholds for figure orientation discrimination were measured as a function of TMS noise intensity. With increasing TMS noise intensity the contrast threshold for figure discrimination first decreased (enhancement) and then increased (impairment). These effects were clearly dependent both on scalp location and timing (SOA). The optimum SOA was around 60 ms, while the optimum location varied across participants. Outside the optimum location and SOA values, no TMS effects were found. The enhancement effect can be accounted for by the stochastic resonance (SR) theory based on a threshold device. In addition, we reveal similarity in characteristics of the SR phenomenon between different experiments.