Article Figures & Data
Figures
- Figure 1.
Experimental setup. A, The 2-AFC visual detection task. Subjects fixated on the center of the screen and a visual stimulus was randomly presented either in the first (shown here) or second interval. After the second interval, participants had to indicate which interval contained the stimulus. In the main experiment, the stimulus contrast was fixed to yield either 60% detection accuracy (subthreshold group) or 80% detection accuracy (suprathreshold group). B, Representation of the three different experiments. Note that in the tRNS–noise and tRNS–control experiments, no noise was presented on the screen. C, Representative data of individual participants. The participants in the visual–noise (left) and tRNS–noise (middle) experiments show a peak in their detection performance when noise was added to a subthreshold stimulus (orange line), but not to a suprathreshold stimulus (blue line).
- Figure 2.
Example of a visual target stimulus with five different noise levels. In the visual–noise experiment, different levels of noise (multiples of individual noise detection thresholds) were added to the target stimulus.
- Figure 3.
Results when noise is added to the visual stimulus (visual–noise experiment). A, Detection accuracy relative to the zero noise condition is shown for each noise level in the subthreshold and suprathreshold stimulus groups. B, Distribution of the individual optimal noise levels (defined as the noise intensity that yields maximum accuracy detection). C, A comparison between the average maximum improvement (i.e., the percentage difference between maximum accuracy and zero noise accuracy) showed a significant difference between the subthreshold and suprathreshold groups. *p < 0.05. D, We found a significant correlation between baseline performance and maximal improvement, indicating that participants with low detection accuracy in the zero noise condition benefited most from adding optimal noise. Error bars indicate SEs.
- Figure 4.
Results when noise is added centrally to the visual cortex (tRNS–noise Experiment). A, Detection accuracy relative to the zero noise condition is shown for each noise level in the subthreshold and suprathreshold stimulus groups. B, Distribution of the individual optimal noise levels. C, The average maximum improvement was significantly different between the subthreshold and suprathreshold groups. *p < 0.05. D, A significant correlation was present between baseline performance and maximal improvement, indicating that participants with low detection accuracy in the zero noise condition benefited most from adding optimal noise. Conventions are the same as in Figure 3.
- Figure 5.
Results of the tRNS–control experiment. A, Detection accuracy relative to the zero noise condition is shown for each noise level in the subthreshold and suprathreshold stimulus groups. B, Distribution of the individual optimal noise levels. C, The average maximum improvement was not significantly different between the subthreshold and suprathreshold groups. D, The correlation between baseline performance and maximal improvement was not significant. Conventions are the same as in Figure 3.
Tables
- Table 1.
Modeled electric field strength for different transcranial electrical current stimulation intensities and frequencies
Frequency (Hz) Intensity (mA) 0.5 mA (V/m) 0.75 mA (V/m) 1 mA (V/m) 1.5 mA (V/m) 100 Hz 0.7 1.6 2.2 3.2 150 Hz 0.8 1.5 2 3.1 200 Hz 1.1 1.5 2.2 3.1 250 Hz 1 1.6 2.2 3 300 Hz 1 1.6 2.1 3 350 HZ 1 1.5 2.1 3 400 Hz 1 1.5 2.1 3 450 Hz 1 1.5 2.2 3 500 Hz 1 1.5 2.2 3 The modeling shows that that all stimulation frequencies were transmitted to the brain and that the current applied is strong enough to reach the cortex (scaling quasi-linearly with tRNS intensity).
- Table 2.
Overview of effect sizes and obtained statistical power for the influence of noise on detection performance in the different experiments
Experiment Effect sizes (Cohen's d) Obtained power Initial analysis Control analysis Initial analysis Control analysis Visual–noise 1.15 0.94 0.85 0.68 tRNS–noise 0.81 0.77 0.65 0.59 tRNS–control 0.05 0.24 0.05 0.08 The effect sizes are based on the maximum improvement data for each experiment. This was done for the initial and control analyses.
- Table 3.
Overview of the maximum improvement and optimal noise levels for the different experiments and different analysis
Experiment Initial analysis Control analysis Optimal noise level mode Maximum improvement (%, mean ± SE) Optimal noise level mode Maximum improvement (%, mean ± SE) Subthreshold Visual–noise 60% of noise threshold 15.67 ± 2.69 60% of noise threshold 12.04 ± 2.83 tRNS–noise 1 mA 14.38 ± 2.41 1 mA 11.07 ± 2.27 tRNS–control 0 mA 5 ± 2.01 0 mA 2.45 ± 1.65 Suprathreshold Visual–noise 0% of noise threshold 5.53 ± 1.78 30% of noise threshold 3.71 ± 1.55 tRNS–noise 0 mA 7.34 ± 1.47 0 mA 4.87 ± 1.18 tRNS–control 1.5 mA 4.44 ± 1.54 0 mA 1.48 ± 0.89 The results of our additional analysis confirmed the results of our optimal noise level analysis even though the performance improvement observed for the optimal noise level is, as expected, smaller in our additional analysis.
