Assessment of Transcranial Direct Current Stimulation (tDCS) on MMN-Indexed Auditory Sensory Processing
Creators: name: Impey, Danielle
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 2016
Subjects: direct current stimulation
event-related potentials
mismatch negativity (MMN)
NMDA receptors
auditory sensory memory
schizophrenia

description: Transcranial direct current stimulation (tDCS) is a non-invasive form of brain stimulation which uses a very weak constant current to temporarily excite or inhibit activity in the brain area of interest via electrodes placed on the scalp, depending on the polarity and strength of the current. Presently, tDCS is being used as a tool to investigate frontal cognition in healthy controls and to improve symptoms in neurological and psychiatric patients. Relatively little research has been conducted with respect to tDCS and the auditory cortex (AC). The primary aim of this thesis was to elucidate the effects of tDCS on auditory sensory discrimination, assessed with the mismatch negativity (MMN) event-related potential (ERP). In the first pilot study, healthy participants were assessed in a randomized, double-blind, sham-controlled design, in which participants received anodal tDCS over the primary AC (2 mA for 20 minutes) in one session and ‘sham’ stimulation (i.e. no stimulation) in the other. Pitch MMN was found to be enhanced after receiving anodal tDCS, with the effects being evidenced in individuals with relatively low (vs. high) baseline amplitudes. No significant effects were seen with sham stimulation. A second study examined the separate and interacting effects of anodal and cathodal tDCS on MMN measures. MMN was assessed pre- and post-tDCS (2 mA, 20 minutes) in 2 separate sessions, one involving sham stimulation, followed by anodal stimulation, and one involving cathodal stimulation, followed by anodal stimulation. Only anodal tDCS over the AC increased pitch MMN in baseline-stratified groups, and while cathodal tDCS decreased MMN, subsequent anodal stimulation did not significantly alter MMNs. As evidence has shown that tDCS lasting effects may be dependent on N-methyl-D-aspartate (NMDA) receptor activity, a pharmacological study investigated the use of dextromethorphan (DMO), an NMDA antagonist, to assess possible modulation of tDCS’ effects on both MMN and working memory (WM) performance. The study involved four test sessions that compared pre- and post-anodal tDCS over the AC and sham stimulation with both DMO (50 mL) and placebo administration. MMN amplitude increases were only seen with anodal tDCS with placebo administration, not with sham stimulation, nor with DMO administration. In the sham condition, DMO decreased MMN amplitudes. Anodal tDCS improved WM performance in the active drug condition. Findings from this study contribute to the understanding of underlying neurobiological mechanisms mediating tDCS-sensory and memory improvements. As cognitive impairment has been proposed to be the core feature of schizophrenia disorder (Sz) and MMN is a putative biomarker of Sz, a pilot study was conducted to assess the effects of pre- and post-tDCS on MMN measures in 12 Sz patients, as well as WM performance. Temporal, frontal and sham tDCS were applied in separate sessions. Results demonstrated a trend for pitch MMNs to increase with anodal temporal tDCS, which was significant in a subgroup of Sz individuals with auditory hallucinations, who had low MMNs at baseline. Anodal frontal tDCS significantly increased WM performance, which was found to positively correlate with MMN-tDCS effects. The findings contribute to our understanding of tDCS effects for MMN-indexed sensory discrimination and WM performance in healthy participants and individuals with Sz disorder and may have implications for treatment of sensory processing deficits in neuropsychiatric illness.