  Stochastic Resonance (SR) describes a phenomenon where an additive noise
(stochastic carrier-wave) enhances the signal transmission in a nonlinear
system. In the nervous system, nonlinear properties are present from the level
of single ion channels all the way to perception and appear to support the
emergence of SR. For example, SR has been repeatedly demonstrated for visual
detection tasks, also by adding noise directly to cortical areas via
transcranial random noise stimulation (tRNS). When dealing with nonlinear
physical systems, it has been suggested that resonance can be induced not only
by adding stochastic signals (i.e., noise) but also by adding a large class of
signals that are not stochastic in nature which cause "deterministic amplitude
resonance" (DAR). Here we mathematically show that high-frequency,
deterministic, periodic signals can yield resonance-like effects with linear
transfer and infinite signal-to-noise ratio at the output. We tested this
prediction empirically and investigated whether non-random, high-frequency,
transcranial alternating current stimulation applied to visual cortex could
induce resonance-like effects and enhance performance of a visual detection
task. We demonstrated in 28 participants that applying 80 Hz triangular-waves
or sine-waves with tACS reduced visual contrast detection threshold for optimal
brain stimulation intensities. The influence of tACS on contrast sensitivity
was equally effective to tRNS-induced modulation, demonstrating that both tACS
and tRNS can reduce contrast detection thresholds. Our findings suggest that a
resonance-like mechanism can also emerge when deterministic electrical
waveforms are applied via tACS.
