Phonological facilitation in picture naming: When and where? A tDCS study

Neural Correlates of Semantic Interference and Phonological Facilitation in Picture Naming: A Systematic Review and Coordinate-Based Meta-analysis

Semantic interference (SI) and phonological facilitation (PF) effects occur when multiple representations are co-activated simultaneously in complex naming paradigms, manipulating the context in which word production is set. Although the behavioral consequences of these psycholinguistic effects are well-known, the involved brain structures are still controversial. This paper aims to provide a systematic review and a coordinate-based meta-analysis of the available functional neuroimaging studies investigating SI and PF in picture naming paradigms. The included studies were fMRI experiments on healthy subjects, employing paradigms in which co-activations of representations were obtained by manipulating the naming context using semantically or phonologically related items. We examined the principal methodological aspects of the included studies, emphasizing the existing commonalities and discrepancies across single investigations. We then performed an exploratory coordinate-based meta-analysis of the reported activation peaks of neural response related to SI and PF. Our results consolidated previous findings regarding the involvement of the left inferior frontal gyrus and the left middle temporal gyrus in SI and brought out the role of bilateral inferior parietal regions in PF.

The role of left temporo-parietal and inferior frontal cortex in comprehending syntactically complex sentences: A brain stimulation study

BACKGROUND AND OBJECTIVES

Syntactic competence relies on a left-lateralized network converging on hubs in inferior-frontal and posterior-temporal cortices. We address the question whether anodal transcranial direct current stimulation (a-tDCS) over these hubs can modulate comprehension of sentences, whose syntactic complexity systematically varied along the factors embedding depths and canonicity. Semantic content and length of the sentences were kept identical and forced choice picture matching was required after the full sentence had been presented.

METHODS

We used a single-blind, within-subject, sham-controlled design, applying a-tDCS targeting left posterior tempo-parietal (TP) and left inferior frontal cortex (FC). Stimulation sites were determined by individual neuro-navigation. 20 participants were included of whom 19 entered the analysis. Results were analysed using (generalized) mixed models. In a pilot-experiment in another group of 20 participants we validated the manipulation of syntactic complexity by the two factors embedding depth and argument-order.

RESULTS

Reaction times increased and accuracy decreased with higher embedding depth and non-canonical argument order in both experiments. Notably a-tDCS over TP enhanced sentence-to-picture matching, while FC-stimulation showed no consistent effect. Moreover, the analysis disclosed a session effect, indicating improvements of task performance especially regarding speed.

CONCLUSIONS

We conclude that the posterior 'hub' of the neuronal network affording syntactic analysis represents a 'bottleneck', likely due to working-memory capacity and the challenges of mapping semantic to syntactic information allowing for role assignment. While this does not challenge the role of left inferior-frontal cortex for syntax processing and novel-grammar learning, the application of highly established syntactic rules during sentence comprehension may be considered optimized, thus not augmentable by a-tDCS in the uncompromised network.

SIGNIFICANCE STATEMENT

Anodal transcranial direct current stimulation (a-tDCS) over left temporo-parietal cortex enhances comprehension of complex sentences in uncompromised young speakers. Since a-tDCS over left frontal cortex did not elicit any change, the 'bottleneck' for the understanding of complex sentences seems to be the posterior, temporo-parietal rather than the anterior inferior-frontal 'hub' of language processing. Regarding the attested role of inferior-frontal cortex in syntax processing, we suggest that its function is optimized in competent young speakers, preventing further enhancement by (facilitatory) tDCS. Results shed light on the functional anatomy of syntax processing during sentence comprehension; moreover, they open perspectives for research in the lesioned language network of people with syntactic deficits due to aphasia.

Transcranial Direct Current Stimulation over the Right Inferior Parietal Cortex Reduces Transposition Errors in a Syllabic Reordering Task

Evidence derived from functional imaging and brain-lesion studies has shown a strong left lateralization for language, and a complementary right hemisphere dominance for visuospatial abilities. Nevertheless, the symmetrical functional division of the two hemispheres gives no reason for the complexity of the cognitive operations involved in carrying out a linguistic task. In fact, a growing number of neuroimaging and neurostimulation studies suggest a possible right hemisphere involvement in language processing. The objective of this work was to verify the contribution of the left and right parietal areas in a phonological task. We applied anodal transcranial direct current stimulation (tDCS) to the right or left inferior parietal lobe, during a syllabic reordering task. After having learnt a combination of images of real objects and trisyllabic pseudowords with a simple consonant–vowel (CV) syllabic structure (e.g., tu-ru-cu), participants were shown the same images paired to two different pseudowords: one correct but with transposed syllables, and one alternative, never before seen. The participant’s task was to orally produce the chosen pseudoword, after having rearranged the order of its syllables. Two types of error were considered: transposition (correct pseudoword but incorrectly reordered) and identity (incorrect pseudoword). The results showed that right anodal stimulation significantly reduced the number of transposition errors, whereas left anodal stimulation significantly reduced the number of identity errors. These results suggested that both left and right inferior parietal areas were differentially involved in a syllabic reordering task, and, crucially, they demonstrated that visuospatial processes served by the right inferior parietal area could be competent for establishing the correct syllabic order within a word.

Brain Networks Modulation in Young and Old Subjects During Transcranial Direct Current Stimulation Applied on Prefrontal and Parietal Cortex

Evidence indicates that the transcranial direct current stimulation (tDCS) has the potential to transiently modulate cognitive function, including age-related changes in brain performance. Only a small number of studies have explored the interaction between the stimulation sites on the scalp, task performance, and brain network connectivity within the frame of physiological aging. We aimed to evaluate the spread of brain activation in both young and older adults in response to anodal tDCS applied to two different scalp stimulation sites: Prefrontal cortex (PFC) and posterior parietal cortex (PPC). EEG data were recorded during tDCS stimulation and evaluated using the Small World (SW) index as a graph theory metric. Before and after tDCS, participants performed a behavioral task; a performance accuracy index was computed and correlated with the SW index. Results showed that the SW index increased during tDCS of the PPC compared to the PFC at higher EEG frequencies only in young participants. tDCS at the PPC site did not exert significant effects on the performance, while tDCS at the PFC site appeared to influence task reaction times in the same direction in both young and older participants. In conclusion, studies using tDCS to modulate functional connectivity and influence behavior can help identify suitable protocols for the aging brain.

Transcranial direct current stimulation modulates implicit attitudes towards food in eating disorders

OBJECTIVE

Neuromodulation of regions involved in food processing is increasingly used in studies on eating behaviors, but results are controversial. We assessed the effects of anodal transcranial direct current stimulation (a-tDCS) on food and body implicit preferences in patients with eating disorders (EDs).

METHOD

Thirty-six ED patients and 36 healthy females completed three sessions with a-tDCS applied to the medial-prefrontal cortex (mPFC), the right extrastriate body area (rEBA) or in sham mode. Each participant then completed three Implicit Association Tests (IATs) on tasty/tasteless food, underweight/overweight body images, flowers versus insects as control. Differences in latency between incongruent and congruent blocks were calculated (D score).

RESULTS

The tDCS by group interaction was significant for the IAT-food D score, with patients showing weaker preference for tasty food than controls in sham, but not a-tDCS sessions. In particular, rEBA stimulation significantly increased patients' D score compared to sham. Moreover, a-tDCS over mPFC and rEBA selectively increased patients' reaction times in the incongruent blocks of the IAT-food.

DISCUSSION

A-tDCS on frontal and occipito-temporal cortices modulated food preferences in ED patients. The effect was specific for food images and selective in patients, but not in healthy participants. These findings suggest that neuromodulation of these regions could affect implicit food attitudes.

Putting focus on transcranial direct current stimulation in language production studies

Previous language production studies targeting the inferior frontal and superior temporal gyrus using anodal tDCS have provided mixed results. Part of this heterogeneity may be explained by limited target region focality of conventionally used electrode montages. We examined the focality of conventionally and alternative electrode montages. Electrical field distributions of anodal tDCS targeting IFG and pSTG were simulated in conventional setups (anodal electrode over left IFG/pSTG, reference electrode over right supraorbital region) and an alternative electrode montage in four different brains. Conventional montages showed maximum field strengths outside of the target regions. Results from alternative electrode montages showed that focality of tDCS could be improved by adjustments in electrode placement. Heterogeneity of findings of language production studies deploying conventional montages may in part be explained by diffuse electrical field distributions. Alternative montages may improve focality and provide more unequivocal results.

Effects of repetitive transcranial magnetic stimulation (rTMS) on attribution of movement to ambiguous stimuli and EEG mu suppression

Recent research suggests that attributing human movement to ambiguous and static Rorschach stimuli (M responses) is associated with EEG mu suppression, and that disrupting the left inferior gyrus (LIFG; a putative area implicated in mirroring activity) decreases the tendency to see human movement when exposed to the Rorschach ambiguous stimuli. The current study aimed to test whether disrupting the LIFG via repetitive transcranial stimulation (rTMS) would decrease both the number of human movement attributions and EEG mu suppression. Each participant was exposed to the Rorschach stimuli twice, i.e., during a baseline condition (without rTMS but with EEG recording) and soon after rTMS (TMS condition with EEG recording). Experimental group (N = 15) was stimulated over the LIFG, while the control group (N = 13) was stimulated over the Vertex. As expected, disrupting the LIFG but not Vertex, decreased the number of M attributions provided by the participants exposed to the Rorschach stimuli, with a significant interaction effect. Unexpectedly, however, rTMS did not significantly influence EEG mu suppression.

Primum non nocere or primum facere meliorem? Hacking the brain in the 21st century

Abstract Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that modulates cortical excitability. It is devoid of serious adverse events and exerts variable effects on cognition, with several research findings suggesting that it can improve memory, verbal and mathematical skills. Because tDCS devices are low-cost, portable and relatively easy to assemble, they have become available outside of the medical setting and used for non-medical (“cosmetic”) purposes by laypersons. In this sense, tDCS has become a popular technique aiming to improve cognition and the achievement of a better performance not only at work, but also in other fields such as sports, leisure activities (video games) and even the military. In spite of these unforeseen developments, there has been a general paralysis of the medical and regulatory agencies to develop guidelines for the use of tDCS for cosmetic purposes. Several challenges are present, most importantly, how to restrict tDCS use outside of the medical setting in face of variable and sometimes conflicting results from scientific research. This article aims to describe the popular use of tDCS, in light of the pillars of neuroethics, a branch of bioethics relative to brain research. Between two possible but extreme solutions – total release or total restriction of tDCS – it is paramount to develop a spectrum of alternatives, which may vary over time and in different cultural backgrounds.