The Effect of Alpha tACS on the Temporal Resolution of Visual Perception

Abstract

We experience the world around us as a smooth and continuous flow. However, there is growing evidence that the stream of sensory inputs is not elaborated in an analog way but is instead organized in discrete or quasi-discrete temporal processing windows. These discrete windows are suggested to depend on rhythmic neural activity in the alpha (and theta) frequency bands, which in turn reflect changes in neural activity within, and coupling between, cortical areas. In the present study, we investigated a possible causal link between oscillatory brain activity in the alpha range (8-12 Hz) and the temporal resolution of visual perception, which determines whether sequential stimuli are perceived as distinct entities or combined into a single representation. To this aim, we employed a two-flash fusion task while participants received focal transcranial alternating current stimulation (tACS) in extra-striate visual regions including V5/MT of the right hemisphere. Our findings show that 10-Hz tACS, as opposed to a placebo (sham tACS), reduces the temporal resolution of perception, inducing participants to integrate the two stimuli into a unique percept more often. This pattern was observed only in the contralateral visual hemifield, providing further support for a specific effect of alpha tACS. The present findings corroborate the idea of a causal link between temporal windows of integration/segregation and oscillatory alpha activity in V5/MT and extra-striate visual regions. They also stimulate future research on possible ways to shape the temporal resolution of human vision in an individualized manner.

Keywords: EEG alpha activity; attention; neural oscillations; perception; transcranial AC stimulation.

FIGURE 1

(A) Illustration of a trial example and of the two-flash fusion task employed in the present study. (B) tACS/EEG montage. (C) tACS voltage distribution on the cortical surface; as it can be seen, the maximum voltage was delivered on the right extra-striate visual areas including V5/MT, in agreement with previous studies stimulating in PO8 (stimulation electrode was placed in PO8) (Zito et al., 2015).

FIGURE 2

Effect of tACS on temporal integration/segregation performance. Average two-flash response rate as a function of inter-stimulus interval (ISI) and relative psychometric functions for the (A) left/contralateral and the (D) right/ipsilateral hemifield trials obtained while participants received 10-Hz, 18-Hz, or sham tACS. Two-flash response rate (averaged across ISIs) as a function of tACS conditions for the (B) left/contralateral and the (E) right/ipsilateral hemifield trials, with dots representing individual observations and bars showing group average. Slope values of the psychometric curve (logistic) obtained from the individual data as a function of tACS conditions for the (C) left/contralateral and the (F) right/ipsilateral hemifield trials, with dots representing individual observations and bars showing group average. ^∗ Indicates p-values < 0.05. Error bars represent the SEM.