Investigating the neural basis of auditory false perceptions in a signal detection paradigm using transcranial direct current stimulation

Transcranial direct current stimulation (tDCS) is a non invasive brain
stimulation technique, used to increase or decrease cortical excitability via a
weak electrical current, applied through electrodes placed on the scalp. Many
studies report effects on neurophysiological or cognitive measures following
stimulation to specific cortical regions; however, tDCS studies have often used
small sample sizes, and concerns have been raised about the replicability of
these effects. For example, the reported findings surrounding effects on
auditory and language-based tasks have been variable, and larger scale studies
are needed. Considering that tDCS applied to cortical regions associated with
language has been tested as a treatment option for some psychiatric disorders
(for example, left superior temporal stimulation for auditory hallucinations in
schizophrenia), it is crucial to fully test and replicate effects on processing
of auditory stimuli. The present study will replicate and extend a previous
study into the effect of tDCS applied to the left superior temporal gyrus on
basic auditory signal detection, as well as testing whether any effect is
specific to speech-based stimuli.

The study will investigate auditory signal detection for voices or tones during
and after anodal tDCS to the left STG, compared to stimulation of the left mPFC
and sham stimulation. The experimental hypotheses are as follows:
1) There will be a main effect of stimulation condition; specifically, that
anodal stimulation to the left STG will lead to a reduced response bias on
the signal detection task, compared to the other conditions.
2) We will also test for an interaction between stimulation condition and
stimulus type; that is, to test for a difference in the effect of stimulation
on response bias (*) to voice and tone stimuli. If any effect of stimulation is
specific to voice stimuli, this would imply the effects are due to stimulation
of language specific areas of cortex. In contrast, if there is an effect of
stimulation on all types of auditory signal detection, this would imply a more
general effect of stimulation on auditory stimuli.

This is a cross-over study. Stimulation condition (anodal stimulation to
superior temporal/medial prefrontal/sham) will be a within subjects variable.
All participants will complete all three stimulation conditions in separate
experimental sessions. The order in which participants complete the three
stimulation conditions will be counterbalanced so that all six possible orders
are equally represented in the sample. The sham condition acts as a control
condition, assessing task performance when no stimulation is applied, despite
electrodes being placed on the scalp. The unrelated area condition acts as an
*active* control condition, assessing task performance when an area other than
STG is stimulated, to exclude general effects on performance from receiving
tDCS, regardless of the region stimulated.

Participants will come in 3 sessions, each lasting aprr. 50 minutes. They
receive tDCS during the task. Before they will be screened if it is safe for
them to participate.

If tDCS is applied in compliance of the valid safety protocols it is considered
as a safe method of brainstimulation, without short or long term damage. It is
possible that during the stimulation the person experiences an itching feeling
on the head, and possible afterwards a light headache.