Neural changes after training with transcranial direct current stimulation to increase speech fluency in adults who stutter

In a randomised controlled trial, we showed that a five-day intervention combining anodal transcranial direct current stimulation over the left inferior frontal cortex with temporary speech fluency enhancing techniques reduces stuttering. Speech fluency was unchanged by the fluency training alone, as predicted. Here, we report the neural changes associated with the intervention, measured using functional MRI during sentence reading before the training and one-week later. We obtained imaging data in 25 adult men who stutter (median age = 32 y, inter-quartile range = 11) at the pre-intervention baseline and again one-week post-intervention. A control group of 15 adult men who do not stutter (median age = 30 y, inter-quartile range = 10) and did not complete the intervention were scanned on one occasion. In a whole-brain analysis of perceptibly fluent sentence reading, we compared the change in task-evoked neural activity in the sub-group of men who stutter who had received active stimulation during the intervention (N=13) with those who had sham stimulation (N=12). We hypothesised that the combination of anodal stimulation over the left inferior frontal cortex and fluency-enhancing training would result in lasting change to the brain networks supporting fluent speech production. An additional region-of-interest analysis explored effects on basal ganglia nuclei, which are thought to have a key role in the casual mechanism of stuttering, and which we hypothesised would be engaged by the behavioural approach used during training (choral and metronome-timed speaking). One week after the intervention, the group who had received active transcranial stimulation showed increased activity in speech-related brain regions, relative to the group who had received sham stimulation. Cortically, these changes were evident in left inferior frontal cortex (pars opercularis and orbitalis), anterior insula, anterior superior temporal gyrus, anterior cingulate cortex, and supplementary motor area. Subcortically, activation increased in the caudate nuclei and putamen bilaterally, and in right globus pallidus and thalamus. Together these regions form cortico-striatal-thalamo-cortical loops involved in the planning and initiation and control of speech movements.Our findings reveal that the mechanism of action of the tDCS intervention involved increasing activity across the network involved in the production of fluent speech, indicating that tDCS can be used to promote neural plasticity to strengthen networks supporting natural fluency. This study advances the potential of using non-invasive brain stimulation to improve therapy efficacy for those people who stutter who choose to work on increasing fluency.

Disrupting inferior frontal cortex activity alters affect decoding efficiency from clear but not from ambiguous affective speech

The evaluation of socio-affective sound information is accomplished by the primate neural auditory cortex in collaboration with limbic and inferior frontal brain nodes. For the latter, activity in inferior frontal cortex (IFC) is often observed during classification of voice sounds, especially if they carry affective information. Partly opposing views have been proposed, with IFC either coding cognitive processing challenges in case of sensory ambiguity or representing categorical object and affect information for clear vocalizations. Here, we presented clear and ambiguous affective speech to two groups of human participants during neuroimaging, while in one group we inhibited right IFC activity with transcranial magnetic stimulation (TMS) prior to brain scanning. Inhibition of IFC activity led to partly faster affective decisions, more accurate choice probabilities and reduced auditory cortical activity for clear affective speech, while fronto-limbic connectivity increased for clear vocalizations. This indicates that IFC inhibition might lead to a more intuitive and efficient processing of affect information in voices. Contrarily, normal IFC activity might represent a more deliberate form of affective sound processing (i.e., enforcing cognitive analysis) that flags categorial sound decisions with precaution (i.e., representation of categorial uncertainty). This would point to an intermediate functional property of the IFC between previously assumed mechanisms.

Pitch and Duration Mismatch Negativity Are Associated with Distinct Auditory Cortex and Inferior Frontal Cortex Volumes in the First-Episode Schizophrenia Spectrum

Background Pitch and duration mismatch negativity (pMMN/dMMN) are related to left Heschl’s gyrus gray matter volumes in first-episode schizophrenia (FESz). Previous methods were unable to delineate functional subregions within and outside Heschl’s gyrus. The Human Connectome Project multimodal parcellation (HCP-MMP) atlas overcomes this limitation by parcellating these functional subregions. Further, MMN has generators in inferior frontal cortex, and therefore, may be associated with inferior frontal cortex pathology. With the novel use of the HCP-MMP to precisely parcellate auditory and inferior frontal cortex, we investigated relationships between gray matter and pMMN and dMMN in FESz. Methods pMMN and dMMN were measured at Fz from 27 FESz and 27 matched healthy controls. T1-weighted MRI scans were acquired. The HCP-MMP atlas was applied to individuals, and gray matter volumes were calculated for bilateral auditory and inferior frontal cortex parcels and correlated with MMN. FDR correction was used for multiple comparisons. Results In FESz only, pMMN was negatively correlated with left medial belt in auditory cortex and area 47L in inferior frontal cortex. Duration MMN negatively correlated with the following auditory parcels: left medial belt, lateral belt, parabelt, TA2, and right A5. Further, dMMN was associated with left area 47L, right area 44, and right area 47L in inferior frontal cortex. Conclusions The novel approach revealed overlapping and distinct gray matter associations for pMMN and dMMN in auditory and inferior frontal cortex in FESz. Thus, pMMN and dMMN may serve as biomarkers of underlying pathological deficits in both similar and slightly different cortical areas.

Cued Naming and the left inferior frontal cortex

Clinical studies have shown that naming can be behaviorally facilitated by priming, e.g., phonemic cues reduce anomia. Rehabilitation of language is argued to rely upon the same processes of priming in healthy speakers. Here we show, in healthy older adults, the immediate facilitatory behavioral and neural priming elicited by phonemic cues presented during an fMRI experiment of overt naming; thus, bridging the gap between lesion and neuroimaging studies. Four types of auditory cues were presented concurrently with an object picture (e.g., cat): (i) word (i.e., the target name (/kat/), (ii) initial phoneme segment (e.g., /ka/), (iii) final phoneme segment (/at/), or (iv) acoustic (noise) control cue. Naming was significantly faster with word, initial and final phonemic cues compared to noise; and word and initial cues compared to final cues, with no difference between word and initial cues. A neural priming effect – a significant decrease in neural activity – was observed in the left inferior frontal cortex (LIFC, pars triangularis, BA45) and the anterior insula bilaterally consistent with theories of primed articulatory encoding and post-lexical selection. The reverse contrast revealed increased activation in left posterior dorsal supramarginal gyrus for word cues that, we argue, may reflect integration of semantic and phonology processing during word rather than phonemic conditions. Taken together, these data from unimpaired speakers identified nodes within the naming network affected by phonemic cues. Activity within these regions may act as a possible biomarker to index anomic individuals’ responsiveness to phonemically cued anomia treatment.

Modulating the left inferior frontal cortex by task domain, cognitive challenge and tDCS

ABSTRACTThe left inferior frontal cortex (LIFC) is a key region for spoken language processing, but its neurocognitive architecture remains controversial. Here we assess the domain-generality vs. domain-specificity of the LIFC from behavioural, functional neuroimaging and neuromodulation data. Using concurrent fMRI and transcranial direct current stimulation (tDCS) delivered to the LIFC, we investigated how brain activity and behavioural performance are modulated by task domain (naming vs. non-naming), cognitive challenge (low vs. high), and tDCS (anodal vs. sham). The data revealed: (1) co-existence of neural signatures both common and distinct across tasks within the LIFC; (2) domain-preferential effects of task (naming); (3) significant tDCS modulations of activity in a LIFC sub-region selectively during high-challenge naming. The presence of both domain-specific and domain-general signals, and the existence of a gradient of activation where naming relied more on sub-regions within the LIFC, may help reconcile both perspectives on spoken language processing.

Distinct neural networks for the volitional control of vocal and manual actions in the monkey homologue of Broca's area

The ventrolateral frontal lobe (Broca's area) of the human brain is crucial in speech production. In macaques, neurons in the ventrolateral prefrontal cortex, the suggested monkey homologue of Broca's area, signal the volitional initiation of vocalizations. We explored whether this brain area became specialized for vocal initiation during primate evolution and trained macaques to alternate between a vocal and manual action in response to arbitrary cues. During task performance, single neurons recorded from the ventrolateral prefrontal cortex and the rostroventral premotor cortex of the inferior frontal cortex predominantly signaled the impending vocal or, to a lesser extent, manual action, but not both. Neuronal activity was specific for volitional action plans and differed during spontaneous movement preparations. This implies that the primate inferior frontal cortex controls the initiation of volitional utterances via a dedicated network of vocal selective neurons that might have been exploited during the evolution of Broca’s area.