Random Noise Stimulation of the Cortex: Stochastic Resonance Enhances Central Mechanisms of Perception

Abstract

It has been demonstrated that detection thresholds decrease by adding random noise to the peripheral nervous system (Simonotto et al., 1996, Collins et al., 1996) in accordance to a stochastic resonance (SR) phenomenon. Psychophysical experiments in humans suggest that SR in the visual system can occur in the primary visual cortex (Aihara et al., 2008). In an EEG (electroencephalography) study in humans, it was shown that when a small amount of tactile random noise was applied to a tactile signal on the fingertip signal-to-noise ratio of the EEG signals decreased for the optimal level of peripheral noise, which is a characteristic of SR (Manjarrez et al., 2003). In previous work, signal and noise were always applied to the peripheral nervous system and not directly to the central nervous system. The question arises as to whether SR-like behavior occurs if noise is added to cortical areas. Here subjects performed a visual perception task while different levels of noise were added either to the peripheral or central nervous system. Peripheral noise was zero-mean Gaussian noise represented on the screen. Central noise was applied via transcranial random noise stimulation (tRNS, 100-640 Hz, intensity varying between 0 and 1.5 mA) with one electrode overlying the visual cortex. Our results indicate that a small amount of peripheral as well as central noise induces a SR effect for sub-threshold stimuli, i.e. the detection threshold is reduced for optimal noise levels. Increasing the noise beyond the optimal amount does not benefit perception.