Optimized auditory transcranial alternating current stimulation improves individual auditory temporal resolution.

Background: Temporal resolution of cortical, auditory processing mechanisms is suggested to be linked to peak frequency of neuronal gamma oscillations in auditory cortex areas (individual gamma frequency, IGF): Individuals with higher IGF tend to have better temporal resolution.
Hypothesis: Modulating ongoing gamma activity with transcranial alternating current stimulation (tACS) is expected to improve performance in gap detection (GD) tasks (shorter GD thresholds) if the frequency is higher and to decrease GD performance (longer GD thresholds) if the frequency is lower than IGF.
Methods: For 26 healthy participants the IGF and temporal resolution were identified using an auditory steady state response (ASSR) paradigm and a between-channel GD task. Finite element modelling was used to generate an optimized tACS electrode montage (one channel per hemisphere: FC5-TP7/P7 and FC6-TP8/P8). Afterwards, GD thresholds were examined during tACS (tACS frequency group A: above IGF, tACS frequency group B: below IGF). Relative changes of GD thresholds were compared between groups. Additionally, effects of tACS on oscillatory activity were investigated comparing relative changes of ASSR amplitudes before and after stimulation.
Results: Performance of group-A-participants improved significantly during tACS in comparison to performance of group-B-participants. Significant relative changes of ASSR amplitudes were found in both groups.
Conclusion: The possibility to improve gap detection with individualized stimulation protocols for tACS further supports the link between oscillatory activity and temporal resolution, whereby the improvement of temporal resolution is particularly relevant for the clinical aspect of auditory tACS.
(Copyright © 2017 Elsevier Inc. All rights reserved.)