Abstract
ABSTRACTTranscranial electric stimulation (tES), comprising transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), involves applying weak electrical current to the scalp, which can be used to modulate membrane potentials and thereby modify neural activity. Critically, behavioural or perceptual consequences of this modulation provide evidence for a causal role of neural activity in the stimulated brain region for the observed outcome. We present tES as a tool for the investigation of which neural responses are necessary for successful speech perception and comprehension. We summarise existing studies, along with challenges that need to be overcome, potential solutions, and future directions. We conclude that, although standardised stimulation parameters still need to be established, tES is a promising tool for revealing the neural basis of speech processing. Future research can use this method to explore the causal role of brain regions and neural processes for the perception and comprehension of speech.
Highlights
In imaging research on speech perception and comprehension, the independent variable is commonly the experimental stimuli that participants hear and the tasks that they perform during data collection
Depending on the imaging measure used, we can conclude that speech perception or comprehension is associated with activity in certain brain regions (Rodd, Davis, & Johnsrude, 2004; Silbert, Honey, Simony, Poeppel, & Hasson, 2014) or leads to neural oscillations entrained to connected speech in multi-speaker scenarios (Zion Golumbic et al, 2013; Zoefel & VanRullen, 2015c)
These methods do not provide evidence that these neural correlates of speech processing are necessary – or causal – for successful speech perception and comprehension: only if brain activity is manipulated or controlled – as an independent variable – are we able to deduce causal relationships between neural responses and the observed processing outcome
Summary
In imaging research on speech perception and comprehension, the independent variable is commonly the experimental stimuli that participants hear and the tasks that they perform during data collection. It was shown that the direction of this modulation of motor potentials seems to depend on the direction of current flow: an anodal electrode above the target site led to an increased motor potential, whereas a cathodal stimulation led to a decreased motor potential (Nitsche & Paulus, 2000; Priori, 2003; Priori et al, 1998) These results have been widely cited as suggesting that anodal tDCS is “excitatory” and cathodal tDCS is “inhibitory” based on their effect on neuronal excitability (e.g. Nitsche & Paulus, 2000). As we allude to in our review of the existing literature below and summarise in Figure 1(C) and Tables 1–3, studies applying tES for the investigation of speech processing differ widely in key parameters such as (1) stimulation intensity or duration, (2) electrode location, (3) stimulation protocol, and (4) task. Summary of studies using tES assumed to target PAC for the investigation of speech and non-speech auditory perception (non-speech studies are marked with an asterisk)
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