Transcranial direct-current stimulation of core language areas facilitates novel word acquisition.

Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique that can alter the state of the stimulated brain area and thereby affect neurocognitive processes and resulting behavioural performance. Previous studies have shown disparate results with respect to tDCS effects on language function, particularly with respect to language learning and word acquisition. To fill this gap, this study aimed at systematically addressing the effects of tDCS of core left-hemispheric language cortices on the brain mechanisms underpinning two main neurocognitive strategies of word learning: implicit inference-based Fast Mapping (FM) and direct instruction-based Explicit Encoding (EE). Prior to a word-learning session, 160 healthy participants were given 15 min of either anodal or cathodal tDCS of Wernicke's or Broca's areas, or a control sham (placebo) stimulation, using a between-group design. Each participant then learned 16 novel words (8 through FM and 8 through EE) in a contextual word-picture association session. Moreover, these words were learnt either perceptually via auditory exposure combined with a graphical image of the novel object, or in an articulatory mode, where the participants additionally had to overtly articulate the novel items. These learning conditions were fully counterbalanced across participants, stimuli and tDCS groups. Learning outcomes were tested at both lexical and semantic levels using two tasks: recognition and word-picture matching. EE and FM conditions produced similar outcomes, indicating comparable efficiency of the respective learning strategies. At the same time, articulatory learning produced generally better results than non-articulatory exposure, yielding higher recognition accuracies and shorter latencies in both tasks. Crucially, real tDCS led to global outcome improvements, demonstrated by faster (compared to sham) reactions, as well as some accuracy changes. There was also evidence of more specific tDCS effects: better word-recognition accuracy for EE vs. FM following cathodal stimulation as well as more expressed improvements in recognition accuracy and reaction times for anodal Broca's and cathodal Wernicke's stimulation, particularly for unarticulated FM items. These learning mode-specific effects support the notion of partially distinct brain mechanisms underpinning these two learning strategies. Overall, numerically largest improvements were observed for anodal Broca's tDCS, whereas the least expressed benefits of tDCS for learning were measured after anodal Wernicke stimulation. Finally, we did not find any inhibitory effects of either tDCS polarity in any of the comparisons. We conclude that tDCS of core language areas exerts a general facilitatory effect on new word acquisition with some limited specificity to learning protocols - the result that may be of potential applied value for future research aimed at ameliorating learning deficits and language disorders.

Morphosyntactic prediction in automatic neural processing of spoken language: EEG evidence.

Automatic parsing of syntactic information by the human brain is a well-established phenomenon, but its mechanisms remain poorly understood. Its best-known neurophysiological reflection is the so-called early left-anterior negativity (ELAN) component of event-related potentials (ERPs), with two alternative hypotheses for its origin: (1) error detection, or (2) morphosyntactic prediction/priming. To test these alternatives, we conducted two experiments using a non-attend passive design with visual distraction and recorded ERPs to spoken pronoun-verb phrases with/without agreement violations and to the same critical verbs presented in isolation without preceding pronouns. The results revealed an ELAN at ∼130-220 ms for pronoun-verb gender agreement violations, confirming a high degree of automaticity in early morphosyntactic parsing. Critically, the strongest ELAN was elicited by verbs outside phrasal context, which suggests that the typical ELAN pattern is underpinned by a reduction of ERP amplitudes for felicitous combinations, reflecting syntactic priming/predictability between related words/morphemes (potentially mediated by associative links formed during previous linguistic experience) rather than specialised error-detection processes.

Horizontal mapping of time-related words in first and second language.

The existence of a consistent horizontal spatial-conceptual mapping for words denoting time is a well-established phenomenon. For example, words related to the past or future (e.g., yesterday/tomorrow) facilitate respective leftward/rightward attentional shifts and responses, suggesting the visual-spatial grounding of temporal semantics, at least in the native language (L1). To examine whether similar horizontal bias also accompanies access to time-related words in a second language (L2), we tested 53 Russian-English (Experiment 1) and 48 German-English (Experiment 2) bilinguals, who classified randomly presented L1 and L2 time-related words as past- or future-related using left or right response keys. The predicted spatial congruency effect was registered in all tested languages and, furthermore, was positively associated with higher L2 proficiency in Experiment 2. Our findings (1) support the notion of horizontal spatial-conceptual mapping in diverse L1s, (2) demonstrate the existence of a similar spatial bias when processing temporal words in L2, and (3) show that the strength of time-space association in L2 may depend on individual L2 proficiency.

tDCS of right-hemispheric Wernicke's area homologue affects contextual learning of novel lexicon.

Numerous studies have shown robust evidence of the right hemisphere's involvement in the language function, for instance in the processing of intonation, grammar, word meanings, metaphors, etc. However, its role in lexicon acquisition remains obscure. We applied transcranial direct current stimulation (tDCS) over the right-hemispheric homologue of Wernicke's area to assess its putative involvement in the processing of different types of novel semantics. After receiving 15 min of anodal, cathodal, or sham (placebo) tDCS, three groups of healthy participants learnt novel concrete and abstract words in the context of short stories. Learning outcomes were assessed using a battery of tests immediately after this contextual learning session and 24 h later. As a result, an inhibitory effect of cathodal tDCS and a facilitatory effect of anodal tDCS were found for abstract word acquisition only. We also found a significant drop in task performance on the second day of the assessment for both word types in all the stimulation groups, suggesting no significant influence of tDCS on the post-learning consolidation of new memory traces. The results suggest an involvement of Wernicke's right-hemispheric counterpart in initial encoding (but not consolidation) of abstract semantics, which may be explained either by the right hemispheres direct role in processing lexical semantics or by an indirect impact of tDCS on contralateral (left-hemispheric) cortical areas through cross-callosal connections.

Regionally specific cortical lateralization of abstract and concrete verb processing: Magnetic mismatch negativity study.

The neural underpinnings of processing concrete and abstract semantics remain poorly understood. Previous fMRI studies have shown that multimodal and amodal neural networks respond differentially to different semantic types; importantly, abstract semantics activates more left-lateralized networks, as opposed to more bilateral activity for concrete words. Due to the lack of temporal resolution, these fMRI results do not allow to easily separate language- and task-specific brain responses and to disentangle early processing stages from later post-comprehension phenomena. To tackle this, we used magnetoencephalography (MEG), a time-resolved neuroimaging technique, in combination with a task-free oddball mismatch negativity (MMN) paradigm, an established approach to tracking early automatic activation of word-specific memory traces in the brain. We recorded the magnetic MMN responses in 30 healthy adults to auditorily presented abstract and concrete action verbs to assess lateralization of word-specific lexico-semantic processing in a set of neocortical areas. We found that MMN responses to these stimuli showed different lateralization patterns of activity in the upper limb motor area (BA4) and parts of Broca's area (BA45/BA47) within ∼100-350 ms after the word disambiguation point. Importantly, the greater leftward response lateralization for abstract semantics was due to the lesser involvement of the right-hemispheric homologues, not increased left-hemispheric activity. These findings suggest differential region-specific involvement of bilateral sensorimotor systems already in the early automatic stages of processing abstract and concrete action semantics.

Keeping track of time: Horizontal spatial biases for hours, days, and months.

In many Western cultures, the processing of temporal words related to the past and to the future is associated with left and right space, respectively - a phenomenon known as the horizontal Mental Time Line (MTL). While this mapping is apparently quite ubiquitous, its regularity and consistency across different types of temporal concepts remain to be determined. Moreover, it is unclear whether such spatial mappings are an essential and early constituent of concept activation. In the present study, we used words denoting time units at different scales (hours of the day, days of the week, months of the year) associated with either left space (e.g., 9 a.m., Monday, February) or right space (e.g., 8 p.m., Saturday, November) as cues in a line bisection task. Fifty-seven healthy adults listened to temporal words and then moved a mouse cursor to the perceived midpoint of a horizontally presented line. We measured movement trajectories, initial line intersection coordinates, and final bisection response coordinates. We found movement trajectory displacements for left- vs. right-biasing hour and day cues. Initial line intersections were biased specifically by month cues, while final bisection responses were biased specifically by hour cues. Our findings offer general support to the notion of horizontal space-time associations and suggest further investigation of the exact chronometry and strength of this association across individual time units.

The impact of crossmodal predictions on the neural processing of aesthetic stimuli.

Neuroaesthetic research has focused on neural predictive processes involved in the encounter with art stimuli or the related evaluative judgements, and it has been mainly conducted unimodally. Here, with electroencephalography, magnetoencephalography and an affective priming protocol, we investigated whether and how the neural responses to non-representational aesthetic stimuli are top-down modulated by affective representational (i.e. semantically meaningful) predictions between audition and vision. Also, the neural chronometry of affect processing of these aesthetic stimuli was investigated. We hypothesized that the early affective components of crossmodal aesthetic responses are dependent on the affective and representational predictions formed in another sensory modality resulting in differentiated brain responses, and that audition and vision indicate different processing latencies for affect. The target stimuli were aesthetic visual patterns and musical chords, and they were preceded by a prime from the opposing sensory modality. We found that early auditory-cortex responses to chords were more affected by valence than the corresponding visual-cortex ones. Furthermore, the assessments of visual targets were more facilitated by affective congruency of crossmodal primes than the acoustic targets. These results indicate, first, that the brain uses early affective information for predictively guiding aesthetic responses; second, that an affective transfer of information takes place crossmodally, mainly from audition to vision, impacting the aesthetic assessment. This article is part of the theme issue 'Art, aesthetics and predictive processing: theoretical and empirical perspectives'.

Breakdown of category-specific word representations in a brain-constrained neurocomputational model of semantic dementia.

The neurobiological nature of semantic knowledge, i.e., the encoding and storage of conceptual information in the human brain, remains a poorly understood and hotly debated subject. Clinical data on semantic deficits and neuroimaging evidence from healthy individuals have suggested multiple cortical regions to be involved in the processing of meaning. These include semantic hubs (most notably, anterior temporal lobe, ATL) that take part in semantic processing in general as well as sensorimotor areas that process specific aspects/categories according to their modality. Biologically inspired neurocomputational models can help elucidate the exact roles of these regions in the functioning of the semantic system and, importantly, in its breakdown in neurological deficits. We used a neuroanatomically constrained computational model of frontotemporal cortices implicated in word acquisition and processing, and adapted it to simulate and explain the effects of semantic dementia (SD) on word processing abilities. SD is a devastating, yet insufficiently understood progressive neurodegenerative disease, characterised by semantic knowledge deterioration that is hypothesised to be specifically related to neural damage in the ATL. The behaviour of our brain-based model is in full accordance with clinical data-namely, word comprehension performance decreases as SD lesions in ATL progress, whereas word repetition abilities remain less affected. Furthermore, our model makes predictions about lesion- and category-specific effects of SD: our simulation results indicate that word processing should be more impaired for object- than for action-related words, and that degradation of white matter should produce more severe consequences than the same proportion of grey matter decay. In sum, the present results provide a neuromechanistic explanatory account of cortical-level language impairments observed during the onset and progress of semantic dementia.

Brain Signatures of Embodied Semantics and Language: A Consensus Paper.

According to embodied theories (including embodied, embedded, extended, enacted, situated, and grounded approaches to cognition), language representation is intrinsically linked to our interactions with the world around us, which is reflected in specific brain signatures during language processing and learning. Moving on from the original rivalry of embodied vs. amodal theories, this consensus paper addresses a series of carefully selected questions that aim at determining when and how rather than whether motor and perceptual processes are involved in language processes. We cover a wide range of research areas, from the neurophysiological signatures of embodied semantics, e.g., event-related potentials and fields as well as neural oscillations, to semantic processing and semantic priming effects on concrete and abstract words, to first and second language learning and, finally, the use of virtual reality for examining embodied semantics. Our common aim is to better understand the role of motor and perceptual processes in language representation as indexed by language comprehension and learning. We come to the consensus that, based on seminal research conducted in the field, future directions now call for enhancing the external validity of findings by acknowledging the multimodality, multidimensionality, flexibility and idiosyncrasy of embodied and situated language and semantic processes.

Conversational pragmatics: memory reporting strategies in different social contexts.

Previous studies in conversational pragmatics have showed that the information people share with others heavily depends on the confidence they have in the correctness of a candidate answer. At the same time, different social contexts prompt different incentive structures, which set a higher or lower confidence criterion to determine which potential answer to report. In this study, we investigated how the different incentive structures of several types of social contexts and how different levels of knowledge affect the amount of information we are willing to share. Participants answered easy, intermediate, and difficult general-knowledge questions and decided whether they would report or withhold their selected answer in different social contexts: formal vs. informal, that could be either constrained (a context that promotes providing only responses we are certain about) or loose (with an incentive structure that maximizes providing any type of answer). Overall, our results confirmed that social contexts are associated with different incentive structures which affects memory reporting strategies. We also found that the difficulty of the questions is an important factor in conversational pragmatics. Our results highlight the relevance of studying different incentive structures of social contexts to understand the underlying processes of conversational pragmatics, and stress the importance of considering metamemory theories of memory reporting.

Transcranial magnetic stimulation of the left middle frontal gyrus modulates the information people communicate in different social contexts.

Neocortical structures of the left frontal lobe, middle frontal gyrus (MFG) in particular, have been suggested to be linked to the processing of punishing and unpleasant outcomes in decision tasks. To assess the role of left MFG (lMFG) in communicative decisions, we used repetitive transcranial magnetic stimulation (rTMS) to inhibit its function during communicational exchanges under two types of social contexts: formal and informal. Three groups of participants received an offline 1-Hz inhibitory rTMS of lMFG, right MFG as an active control site, or lMFG sham/placebo TMS as a passive control condition. Participants' task included answering difficult general-knowledge questions, rating their confidence in their answers' correctness, and, finally, deciding if they would report or withhold these answers in formal and informal social contexts. There were significantly more reported than withheld answers in the informal context in all groups. The formal context showed no differences between reported and withheld answers in both control conditions, while, crucially, real rTMS of lMFG produced a different pattern, with more withheld than reported answers. Thus, lMFG inhibition seems to result in more rational decisions made only in formal communication contexts, where there is a perception of a certain pressure or possible negative outcomes. In informal social contexts and in the absence of negative consequences the pattern of answers did not change, regardless of the reporting strategy or the TMS protocol used. These results suggest selective context-dependent involvement of the lMFG in decision-making processes during communicational exchanges taking place under social pressure.

Speech comprehension across time, space, frequency, and age: MEG-MVPA classification of intertrial phase coherence.

Language is a key part of human cognition, essential for our well-being at all stages of our lives. Whereas many neurocognitive abilities decline with age, for language the picture is much less clear, and how exactly speech comprehension changes with ageing is still unknown. To investigate this, we employed magnetoencephalography (MEG) and recorded neuromagnetic brain responses to auditory linguistic stimuli in healthy participants of younger and older age using a passive task-free paradigm and a range of different linguistic stimulus contrasts, which enabled us to assess neural processing of spoken language at multiple levels (lexical, semantic, morphosyntactic). Using machine learning-based classification algorithms to scrutinise intertrial phase coherence of MEG responses in cortical source space, we found that patterns of oscillatory neural activity diverged between younger and older participants across several frequency bands (alpha, beta, gamma) for all tested linguistic information types. The results suggest multiple age-related changes in the brain's neurolinguistic circuits, which may be due to both healthy ageing in general and compensatory processes in particular.

Altered evoked responses for motor-related words in children with upper limb motor impairments.

OBJECTIVE: Obstetric brachial plexus palsy (OBPP) and amyoplasia, the classical type of arthrogryposis multiplex congenita, manifest themselves as highly limited mobility of the upper limb. At the same time, according to the embodiment cognition theories, the motor impairments might lead to the alteration of cognitive functions in OBPP/amyoplasia patients. In the current study, we examined whether OBPP/amyoplasia children exhibit altered processing of motor-related verbs. METHODS: We conducted a case-control study using clinical population and control children. Oddball series were used to elicit mismatch negativity (MMN) EEG responses. The series consisted of limb-related verbs (deviant stimuli) and matched pseudowords (standard stimuli). 27 patients and 32 control children were included in the analysis. RESULTS: We showed that MMN waveforms differed between OBPP/amyoplasia children and their control peers in the frontal and temporal electrodes when the stimuli contained hand-related verbs. In particular, the MMN peak latency in the OBPP/amyoplasia children was significantly delayed as compared with the healthy controls. At the same time, neither series with leg-related verbs nor series of pseudowords resulted in statistically significant differences. CONCLUSIONS: Our findings suggest altered processing of hand-related verbs in OBPP/amyoplasia children with hand-related disabilities. SIGNIFICANCE: Our results contribute to the growing evidence in support of the theory of embodied cognition, which proposes that various domains of cognition are shaped by bodily interactions with the environment.

Inflectional zero morphology - Linguistic myth or neurocognitive reality?

Knowledge of language, its structure and grammar are an essential part of our education and daily activities. Despite the importance of language in our lives, linguistic theories that explain how the language system operates are often disconnected from our knowledge of the brain's neurocognitive mechanisms underpinning the linguistic function. This is reflected, for example, in the inclusion of abstract and often controversial elements into theories of language. Here, we discuss the case of the so-called null constituent and its smallest and the most controversial variant - the zero morpheme, a hypothetical morphosyntactic device that has no overt physical (phonological or orthographic) expression. Focusing on the putative inflectional zero morpheme, we discuss the theoretical origins and pitfalls of this approach and advocate the important role for neurobiological research that could try to elucidate the neurocognitive reality of such constructs in linguistic communication.

Oscillatory beta/alpha band modulations: A potential biomarker of functional language and motor recovery in chronic stroke?

Stroke remains one of the leading causes of various disabilities, including debilitating motor and language impairments. Though various treatments exist, post-stroke impairments frequently become chronic, dramatically reducing daily life quality, and requiring specific rehabilitation. A critical goal of chronic stroke rehabilitation is to induce, usually through behavioral training, experience-dependent plasticity processes in order to promote functional recovery. However, the efficiency of such interventions is typically modest, and very little is known regarding the neural dynamics underpinning recovery processes and possible biomarkers of their efficiency. Some studies have emphasized specific alterations of excitatory-inhibitory balance within distributed neural networks as an important recovery correlate. Neural processes sensitive to these alterations, such as task-dependent oscillatory activity in beta as well as alpha bands, may be candidate biomarkers of chronic stroke functional recovery. In this review, we discuss the results of studies on motor and language recovery with a focus on oscillatory processes centered around the beta band and their modulations during functional recovery in chronic stroke. The discussion is based on a framework where task-dependent modulations of beta and alpha oscillatory activity, generated by the deep cortical excitatory-inhibitory microcircuits, serve as a neural mechanism of domain-general top-down control processes. We discuss the findings, their limitations, and possible directions for future research.

Repetitive transcranial magnetic stimulation modulates action naming over the left but not right inferior frontal gyrus.

fMRI language mapping studies report right-hemispheric contribution to language in healthy individuals. However, it remains unclear whether these right-hemispheric patterns of activity are critical for language, which is highly relevant for clinical preoperative language mapping. The available findings are controversial. In this study, we first measured individual patterns of language lateralization with an fMRI language localizer in healthy participants with different handedness (N = 31). Then, the same participants received rTMS over the individual coordinates of peak fMRI-based activation in the left and right inferior frontal gyri. During rTMS, participants performed a picture naming task. It included both objects and actions to test whether naming of nouns and verbs would be equally modulated by rTMS. Stimulation of the left inferior frontal gyrus resulted in accuracy facilitation of verb production regardless of individual language lateralization. No modulation of object naming was found at any stimulation site in terms of accuracy nor reaction time. This study causally confirmed the critical contribution of the left, but not the right hemisphere to verb production regardless of the language lateralization patterns observed with fMRI. Also, the results stress that action rather than object naming is the task of choice for mapping language in the frontal lobe.

Neurophysiological correlates of automatic integration of voice and gender information during grammatical processing.

During verbal communication, interlocutors rely on both linguistic (e.g., words, syntax) and extralinguistic (e.g., voice quality) information. The neural mechanisms of extralinguistic information processing are particularly poorly understood. To address this, we used EEG and recorded event-related brain potentials while participants listened to Russian pronoun-verb phrases presented in either male or female voice. Crucially, we manipulated congruency between the grammatical gender signaled by the verbs' ending and the speakers' apparent gender. To focus on putative automatic integration of extralinguistic information into syntactic processing and avoid confounds arising from secondary top-down processes, we used passive non-attend auditory presentation with visual distraction and no stimulus-related task. Most expressed neural responses were found at both early (150 ms, ELAN-like) and late (400 ms, N400-like) phrase processing stages. Crucially, both of these brain responses exhibited sensitivity to extralinguistic information and were significantly enhanced for phrases whose voice and grammatical gender were incongruent, similar to what is known for ERPs effects related to overt grammatical violations. Our data suggest a high degree of automaticity in processing extralinguistic information during spoken language comprehension which indicates existence of a rapid automatic syntactic integration mechanism sensitive to both linguistic and extralinguistic information.

The 40-Hz auditory steady-state response in bipolar disorder: A meta-analysis.

OBJECTIVE: Bipolar disorder is characterized by aberrant neurophysiological responses as measured with electroencephalography (EEG) and magnetoencephalography (MEG), including the 40-Hz auditory steady-state response (ASSR). 40-Hz ASSR deficits are also found in patients with schizophrenia and may represent a transdiagnostic biomarker of neuronal circuit dysfunction. In this systematic review and meta-analysis, we summarize and evaluate the evidence for 40-Hz ASSR deficits in patients with bipolar disorder. METHODS: We identified studies from PubMed, EMBASE, and SCOPUS. We assessed the risk of bias, calculated Hedges' g meta-level effect sizes, and investigated small-study effects using funnel plots and Egger regression. RESULTS: Seven studies, comprising 396 patients with bipolar disorder and 404 healthy controls, were included in the meta-analysis. Studies displayed methodological heterogeneity and an overall high risk of bias. Patients with bipolar disorder showed consistent reductions in 40-Hz ASSR evoked power (Hedges' g = -0.49; 95% confidence intervals [-0.67, -0.31]) and inter-trial phase coherence (ITPC) (Hedges' g = -0.43; 95 %CI [-0.58, -0.29]) compared with healthy controls. CONCLUSIONS: Our meta-analysis provides evidence that 40-Hz ASSRs are reduced in patients with bipolar disorder compared with healthy controls. SIGNIFICANCE: Future large-scale studies are warranted to link 40-Hz ASSR deficits to clinical features and developmental trajectories.

Look and ye shall hear: Selective auditory attention modulates the audiovisual correspondence effect.

One of the unresolved questions in multisensory research is that of automaticity of consistent associations between sensory features from different modalities (e.g. high visual locations associated with high sound pitch). We addressed this issue by examining a possible role of selective attention in the audiovisual correspondence effect. We orthogonally manipulated loudness and pitch, directing participants' attention to the auditory modality only and using pitch and loudness identification tasks. Visual stimuli in high, low or central spatial locations appeared simultaneously with the sounds. If the correspondence effect is automatic, it should not be affected by task changes. The results, however, demonstrated a cross-modal pitch-verticality correspondence effect only when participants' attention was directed to pitch, but not to loudness identification task; moreover, the effect was present only in the upper location. The findings underscore the involvement of selective attention in cross-modal associations and support a top-down account of audiovisual correspondence effects.

Explicit encoding vs. fast mapping of novel spoken words: Electrophysiological and behavioural evidence of diverging mechanisms.

It has been claimed that two major neurocognitive mechanisms - instruction-based explicit encoding (EE) and inference-driven fast mapping (FM) may be involved in rapid acquisition of novel words, but their exact neural underpinnings remain poorly understood. To address this, we trained 36 adult participants with 20 novel spoken words in an audio-visual task, carefully balanced between the EE and FM conditions for physical, psycholinguistic and pragmatic properties as well as the overall task setup. To assess the neural dynamics associated with novel word acquisition, we recorded event-related potentials (ERPs) elicited by these words before and after training, and analysed their relationship with the behavioural learning outcomes, measured in a semantic matching task. Both learning regimes led to successful acquisition, which was somewhat more efficient for EE than FM, as indicated by higher accuracy in the behavioural task. We also found that, whereas words learnt via both EE and FM protocols elicited most pronounced ERP peaks at ∼196 and ∼280 ms, these two phases of activity diverged with respect to the learning type. Multiple linear regression and correlation analyses indicated that the learning-induced amplitude dynamics in the earlier peak was significantly related to behavioural performance for FM-learned items, which may possibly be explained by FM's stronger reliance on early automatic mechanisms of word processing. Performance on EE words was, in turn, significantly linked to the amplitude of the second peak only, potentially due to the involvement of later, top-down controlled processes in this type of word acquisition. Grand-average ERP-based source analysis indicated a left-lateralised activity in the anterior-temporal lobe for FM learning, and a bilateral activation for EE. The results confirm the existence of partially diverging neurocognitive systems for word acquisition and suggest that the configuration of newly established word memory circuits depends on the mode of their acquisition.