Results of Experiment 2.

<p>The orientation tuning curve. The data show a significant main effect of congruency and a significant interaction between congruency and tactile orientations. The star indicates a significant difference between congruent and incongruent conditions. The curves only differ significantly when the haptic orientation is 45°, the point at which it was perfectly aligned with the visual grating (also at 45°). Error bars showing standard error of the mean.</p

Results of Experiment 3.

<p>The spatial frequency tuning curve. The data show a significant main effect of congruency. Error bars showing standard error of the mean.</p

Visual stimuli.

<p>The visual target stimulus was a low contrast grating added to a background of random visual noise of 20% contrast. The grating could be presented counterclockwise or clockwise by 45° relative to vertical. In this figure, the visual target is shown at suprathreshold contrast, although in the experiment the contrast was much lower and was adjusted over trials to find the contrast detection threshold. In the non-target interval, only the noise was present, and the order of target and null intervals was randomized over trials.</p

Experimental setup.

<p>Subjects explored the haptic grating with the index finger of their dominant hand while looking into a mirror which imaged a visual grating in a collocated position and with the same spatial frequency. Subjects could not see their hand through the mirror.</p

Experimental procedure.

<p>The visual target stimulus, 45° clockwise or counterclockwise (here clockwise) could be presented in the first or second interval. In this figure, the stimulus is shown in the first interval at suprathreshold contrast. After the green fixation cross disappeared, participants started to explore the haptic grating with the index finger of their dominant hand. Two intervals were presented on the screen, separated by a blank screen for 40 ms. After the second interval disappeared, subjects had to indicate in which interval the visual grating was presented. The grating's orientation alternated randomly between +/−45°. After the response a red fixation-cross appeared, indicating that the participant was not allowed to touch the grating. The motor made two random turns before the next trial started.</p

Stochastic resonance enhances the rate of evidence accumulation during combined brain stimulation and perceptual decision-making

<div><p>Perceptual decision-making relies on the gradual accumulation of noisy sensory evidence. It is often assumed that such decisions are degraded by adding noise to a stimulus, or to the neural systems involved in the decision making process itself. But it has been suggested that adding an optimal amount of noise can, under appropriate conditions, enhance the quality of subthreshold signals in nonlinear systems, a phenomenon known as <i>stochastic resonance</i>. Here we asked whether perceptual decisions made by human observers obey these stochastic resonance principles, by adding noise directly to the visual cortex using transcranial random noise stimulation (tRNS) while participants judged the direction of coherent motion in random-dot kinematograms presented at the fovea. We found that adding tRNS bilaterally to visual cortex enhanced decision-making when stimuli were just below perceptual threshold, but not when they were well below or above threshold. We modelled the data under a drift diffusion framework, and showed that bilateral tRNS selectively increased the drift rate parameter, which indexes the rate of evidence accumulation. Our study is the first to provide causal evidence that perceptual decision-making is susceptible to a stochastic resonance effect induced by tRNS, and to show that this effect arises from selective enhancement of the rate of evidence accumulation for sub-threshold sensory events.</p></div

Effects of transcranial random noise stimulation (tRNS) on perceptual decision-making in the dot-motion discrimination task for unilateral stimulation of the left visual cortex (A) and right visual cortex (B).

<p>The left panels show performance for each motion coherence level as a function of tRNS intensity. The right panels show the drift rate derived from modelling of the data shown in the corresponding plots to the left.</p

Effects of transcranial random noise stimulation (tRNS) on perceptual decision-making in the dot-motion discrimination task for bilateral stimulation.

<p>The left panel shows performance for each motion coherence level as a function of tRNS intensity. The right panel shows the drift rate derived from modelling of the data shown in the corresponding plot to the left. *p_corrected < 0.05.</p

Electrode pad montages and modelled electrical field strength (normE) for each of the three tRNS experiments.

<p><b>A</b>. Bilateral visual cortex stimulation (Experiment 1). <b>B</b>. Left visual cortex stimulation (Experiment 2). <b>C</b>. Right visual cortex stimulation (Experiment 3).</p

Stochastic resonance enhances the rate of evidence accumulation during combined brain stimulation and perceptual decision-making - Fig 2

<p><b>A</b>: Schematic of the drift diffusion modelling (DDM) framework used to model perceptual decision-making in the dot motion task. In the model, evidence is accumulated over time until a response boundary is crossed. <i>t</i> is the non-decision time, which includes the time taken to execute a motor response. <i>v</i> is the drift rate, which reflects the rate at which sensory evidence is accumulated. This parameter is taken as an index of the quality of sensory information. <i>a</i> represents the boundary separation (<i>correct</i> at the top, <i>incorrect</i> at the bottom), indicating how much information is needed to make a decision. <b>B</b>: Schematic of the random dot-motion task in which participants judged whether signal dots moved on average to the left or right. Task difficulty was titrated by altering the proportion of coherently moving dots (shown with arrows attached, for purposes of illustration) amongst randomly moving dots. In this example the coherent motion is rightward, but in the experiment the dots were equally likely to move toward the left or right. The circles surrounding the dot stimuli are shown here for illustration only, and were not present in the actual displays.</p