Facilitation of Inferior Frontal Cortex by Transcranial Direct Current Stimulation Induces Perceptual Learning of Severely Degraded Speech

Abstract

Perceptual learning requires the generalization of categorical perceptual sensitivity from trained to untrained items. For degraded speech, perceptual learning modulates activation in a left-lateralized network, including inferior frontal gyrus (IFG) and inferior parietal cortex (IPC). Here we demonstrate that facilitatory anodal transcranial direct current stimulation (tDCS anodal ) can induce perceptual learning in healthy humans. In a sham-controlled, parallel design study, 36 volunteers were allocated to the three following intervention groups: tDCS anodal over left IFG, IPC, or sham. Participants decided on the match between an acoustically degraded and an undegraded written word by forced same-different choice. Acoustic degradation varied in four noise-vocoding levels (2, 3, 4, and 6 bands). Participants were trained to discriminate between minimal (/Tisch/-FISCH) and identical word pairs (/Tisch/-TISCH) over a period of 3 d, and tDCS anodal was applied during the first 20 min of training. Perceptual sensitivity (dЈ) for trained word pairs, and an equal number of untrained word pairs, was tested before and after training. Increases in dЈ indicate perceptual learning for untrained word pairs, and a combination of item-specific and perceptual learning for trained word pairs. Most notably for the lowest intelligibility level, perceptual learning occurred only when tDCS anodal was applied over left IFG. For trained pairs, improved dЈ was seen on all intelligibility levels regardless of tDCS intervention. Over left IPC, tDCS anodal did not modulate learning but instead introduced a response bias during training. Volunteers were more likely to respond "same," potentially indicating enhanced perceptual fusion of degraded auditory with undegraded written input. Our results supply first evidence that neural facilitation of higher-order language areas can induce perceptual learning of severely degraded speech.

Listening under difficult conditions: An activation likelihood estimation meta‐analysis

“…This was unexpected considering that most studies were fairly similar in methodology and stimuli. This is also surprising given the evidence from transcranial direct current stimulation (tDCS) studies suggesting that stimulation of the left IFG can improve identification of degraded speech (Sehm et al, ). The lack of a common pattern of activation in prefrontal cortex suggests that activation within the prefrontal cortex is widely distributed during tasks involving spectrally degraded stimuli.…”

Reprint of: Transcranial direct current stimulation (tDCS) – Application in neuropsychology

“…In contrast, learning of verbal material was impaired by cathodal tDCS of the left dorsolateral prefrontal cortex (Elmer et al, 2009;Hammer et al, 2011). tDCS might also be suited to disentangle the role of involved cortical areas in different components of perceptual learning, because anodal tDCS over the left inferior frontal gyrus improved perceptual learning in a degraded speech paradigm, whereas stimulation over the inferior parietal cortex introduced a response bias (Sehm et al, 2013). …”

Boosting learning efficacy with non-invasive brain stimulation in intact and brain-damaged humans

“…in chronic CB patients, Huxlin et al, 2009), and 2) persistence of visual learning and/or recovered abilities after training ends. Non-invasive brain stimulation coupled with perceptual training has emerged as a potentially promising solution for both of these limitations in healthy adults (Ammann et al, 2016;Cappelletti et al, 2013;Chesters et al, 2017;Falcone et al, 2012;Fertonani et al, 2011;Mulquiney et al, 2011;Sehm et al, 2013;Snowball et al, 2013;Zoefel & Davis, 2017).…”