Transcranial alternating current stimulation in the theta band but not in the delta band modulates the comprehension of naturalistic speech in noise Abstract   Auditory cortical activity entrains to speech rhythms and has been proposed as a mechanism for online speech processing. In particular, neural activity in the theta frequency band (4xe2x80x938xc2xa0xe2x80x8bHz) tracks the onset of syllables which may aid the parsing of a speech stream. Similarly, cortical activity in the delta band (1xe2x80x934xc2xa0xe2x80x8bHz) entrains to the onset of words in natural speech and has been found to encode both syntactic as well as semantic information. Such neural entrainment to speech rhythms is not merely an epiphenomenon of other neural processes, but plays a functional role in speech processing: modulating the neural entrainment through transcranial alternating current stimulation influences the speechrelated neural activity and modulates the comprehension of degraded speech. However, the distinct functional contributions of the delta and of the thetaband entrainment to the modulation of speech comprehension have not yet been investigated. Here we use transcranial alternating current stimulation with waveforms derived from the speech envelope and filtered in the delta and theta frequency bands to alter cortical entrainment in both bands separately. We find that transcranial alternating current stimulation in the theta band but not in the delta band impacts speech comprehension. Moreover, we find that transcranial alternating current stimulation with the thetaband portion of the speech envelope can improve speechinnoise comprehension beyond sham stimulation. Our results show a distinct contribution of the theta but not of the deltaband stimulation to the modulation of speech comprehension. In addition, our findings open up a potential avenue of enhancing the comprehension of speech in noise.

Transcranial direct current stimulation (tDCS) elicits stimulusspecific enhancement of cortical plasticity. Abstract   Background  Deficits in plasticity underlie many severe psychiatric disorders. Transcranial direct current stimulation (tDCS) is a promising method for modulating plasticity. However, given its nonfocal nature, there are open questions as to how targeting and outcome specificity can best be achieved.    Objective  Understanding how tDCS interacts with concurrent brain activity is necessary for the rational advancement of tDCS. In the present study, we use an eventrelated potential (ERP) paradigm to assess the stimulusspecific effects of tDCS on cortical plasticity.    Methods  22 healthy volunteers underwent a blinded, shamcontrolled plasticity paradigm in a crossover design. High frequency presentation of auditory stimuli was used to induce potentiation in specific components of the ERP. We investigated whether anodal tDCS targeting the auditory cortex would modulate plasticity induction across time. Two pure tones were used as stimuli, only one of the tones, the target tone, was used for plasticity induction. Plasticity was quantified as change in the mean amplitude of the N100 component relative to baseline.    Results  TDCS significantly modulated plasticity in the target tone compared to sham (pxe2x80xafxe2x80xaf0.02) but had no effect on the control tone (pxe2x80xafxe2x80xaf0.73). This effect was time dependent, with tDCS effects no longer apparent 30xe2x80xafmin after stimulation.    Conclusions  Our results indicate that tDCS can modulate cortical plasticity in the auditory cortex in an activitydependent manner. These findings bolster the idea that tDCS can be an effective tool to target and modulate plasticity both for research and therapeutic purposes.