Neurophysiological basis of the N400 deflection, from Mismatch Negativity to Semantic Prediction Potentials and late positive components

Biomarker and neuropsychological correlates of the N400 event-related potential in Alzheimer's disease

OBJECTIVE

The current study sought to characterize the relationship of the N400 (N4) effect event-related potential to Alzheimer's disease (AD) biomarkers and broader cognition in older adults on the late-life cognitive continuum.

METHOD

Participants who were cognitively intact (n = 43), or had amnestic mild cognitive impairment (MCI; n = 19), or mild AD (n = 12), completed a word-pair judgement task during concurrent EEG recording to elicit the N400. The Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) and biomarker data (PET-imaged beta-amyloid (aβ) deposition, apolipoprotein-E ε4 (APOE4) allele status, hippocampal volumes) were collected as part of a larger study.

RESULTS

The AD group had slower response times and poorer accuracy on the word-pair judgement task than the intact group. The N4 effect was smaller and occurred later in AD relative to intact participants. MCI participants' values were intermediate. N4 effect amplitudes were not associated with RBANS scores but were positively associated with aβ deposition. Conversely, poorer performance across most RBANS Indexes and the Total score was associated with longer N4 latencies. There was also a negative association between hippocampal volumes and the N4 latency and a positive association between aβ deposition and latency. Finally, the latency of the N4 independently predicted variance in RBANS Total scores, above and beyond aβ deposition, hippocampal volumes, and APOE4 allele status.

CONCLUSIONS

These findings support the relevance of the N4 effect in individuals along the late-life cognitive continuum, and motivate future studies into its potential as a longitudinal predictor in AD.

Subcortical Aphasia: An Update

PURPOSE OF REVIEW

This review aims to rediscuss the leading theories concerning the role of basal ganglia and the thalamus in the genesis of aphasic symptoms in the absence of gross anatomical lesions in cortical language areas as assessed by conventional neuroimaging studies.

RECENT FINDINGS

New concepts in language processing and modern neuroimaging techniques have enabled some progress in resolving the impasse between the current dominant theories: (a) direct and specific linguistic processing and (b) subcortical structures as processing relays in domain-general functions. Of particular interest are studies of connectivity based on functional magnetic resonance imaging (MRI) and tractography that highlight the impact of white matter pathway lesions on aphasia development and recovery. Connectivity studies have put into evidence the central role of the arcuate fasciculus (AF), inferior frontal occipital fasciculus (IFOF), and uncinate fasciculus (UF) in the genesis of aphasia. Regarding the thalamus, its involvement in lexical-semantic processing through modulation of the frontal cortex is becoming increasingly apparent.

Somatostatin and the pathophysiology of Alzheimer's disease

Among the central features of Alzheimer's disease (AD) progression are altered levels of the neuropeptide somatostatin (SST), and the colocalisation of SST-positive interneurons (SST-INs) with amyloid-β plaques, leading to cell death. In this theoretical review, I propose a molecular model for the pathogenesis of AD based on SST-IN hypofunction and hyperactivity. Namely, hypofunctional and hyperactive SST-INs struggle to control hyperactivity in medial regions in early stages, leading to axonal Aβ production through excessive presynaptic GABAB inhibition, GABAB1a/APP complex downregulation and internalisation. Concomitantly, excessive SST-14 release accumulates near SST-INs in the form of amyloids, which bind to Aβ to form toxic mixed oligomers. This leads to differential SST-IN death through excitotoxicity, further disinhibition, SST deficits, and increased Aβ release, fibrillation and plaque formation. Aβ plaques, hyperactive networks and SST-IN distributions thereby tightly overlap in the brain. Conversely, chronic stimulation of postsynaptic SST2/4 on gulutamatergic neurons by hyperactive SST-INs promotes intense Mitogen-Activated Protein Kinase (MAPK) p38 activity, leading to somatodendritic p-tau staining and apoptosis/neurodegeneration - in agreement with a near complete overlap between p38 and neurofibrillary tangles. This model is suitable to explain some of the principal risk factors and markers of AD progression, including mitochondrial dysfunction, APOE4 genotype, sex-dependent vulnerability, overactive glial cells, dystrophic neurites, synaptic/spine losses, inter alia. Finally, the model can also shed light on qualitative aspects of AD neuropsychology, especially within the domains of spatial and declarative (episodic, semantic) memory, under an overlying pattern of contextual indiscrimination, ensemble instability, interference and generalisation.

Top-down modulation of brain responses in spelling error recognition

The task being undertaken can influence orthographic, phonological and semantic processes. In linguistic research, two tasks are most often used: a task requiring a decision in relation to the presented word and a passive reading task which does not require a decision regarding the presented word. The results of studies using these different tasks are not always consistent. This study aimed to explore brain responses associated with the process of recognition of spelling errors, as well as the influence of the task on this process. Event-related potentials (ERPs) were recorded in 40 adults during an orthographic decision task to determine correctly spelled words and words written with errors that did not change the phonology and during the passive reading. During spelling recognition, the early stages up to 100 ms after the stimulus were automatic and did not depend on the requirements of the task. The amplitude of the N1 component (90-160 ms) was greater in the orthographic decision task, but did not depend on the correct spelling of the word. Late word recognition after 350-500 ms was task dependent, but spelling effects were similar across the two tasks: misspelled words evoked an increase in the amplitude of the N400 component related to lexical and semantic processing regardless of the task. In addition, the orthographic decision task modulated spelling effects, this was reflected in an increase in the amplitude of the P2 component (180-260 ms) for correctly spelled words compared with misspelled words. Thus, our results show that spelling recognition involves general lexico-semantic processes independent of the task. Simultaneously, the orthographic decision task modulates the spelling-specific processes necessary to quickly detect conflicts between orthographic and phonological representations of words in memory.

The neural hierarchy of consciousness

The chief undertaking in the studies of consciousness is that of unravelling "the minimal set of neural processes that are together sufficient for the conscious experience of a particular content - the neural correlates of consciousness". To this day, this crusade remains at an impasse, with a clash of two main theories: consciousness may arise either in a graded and cortically-localised fashion, or in an all-or-none and widespread one. In spite of the long-lasting theoretical debates, neurophysiological theories of consciousness have been mostly dissociated from them. Herein, a theoretical review will be put forth with the aim to change that. In its first half, we will cover the hard available evidence on the neurophysiology of consciousness, whereas in its second half we will weave a series of considerations on both theories and substantiate a novel take on conscious awareness: the levels of processing approach, partitioning the conscious architecture into lower- and higher-order, graded and nonlinear.

Electrophysiological Evidence of Enhanced Auditory Retrieval in Musically Trained Children

Abstract. Musical practice enhances auditory processing in children as related to pitch perception or tonal discrimination. The purpose of this study was to examine whether these benefits also occur in auditory working memory by influencing its neural substrates. Two groups of children aged between 7 and 11 years old were compared using an auditory retrieval task with three conditions: frequency retrieval, duration retrieval, and control. Musician children had weekly private violin or cello lessons for at least 14 months, whereas non-musician children had no musical training. Results showed that musicians’ scores on the Gordon’s Primary Measure of Music Audiation test were significantly higher than non-musicians’ scores in the rhythm and tone conditions. On memory tasks, musicians outperformed non-musicians in frequency retrieval but not in duration retrieval. Differences in retrieval performance were associated with a larger P200-like waveform over frontal sites in musicians and a larger N400-like waveform over centro-parietal sites in non-musicians. A source current density analysis revealed differences in frontal activities between musicians and non-musicians, suggesting that musical training influenced the neural mechanisms supporting auditory retrieval in children. These results are in agreement with previous studies that showed a better auditory memory in musicians. Furthermore, they suggest that in children, the effect of musical training can be strong enough to positively influence higher-order auditory memory processes such as active retrieval, as well as their neural correlates.

Semantic priming and neurobiology in schizophrenia: A theoretical review

In this theoretical review we bridge the cognitive and neurobiological sciences to shed light on the neurocognitive foundations of the semantic priming effect in schizophrenia. We review and theoretically evaluate the neurotransmitter systems (dopaminergic, GABAergic and glutamatergic) and neurobiological underpinnings of behavioural and electrophysiological (N400) semantic priming in the pathology, and the main hypotheses on their geneses: a disinhibition of the semantic spread of activation, a disorganised semantic storage or noisy lexical-semantic associations, a psychomotor artefact, an artefact of relatedness proportions, or an inability to mobilise contextual information. We further assess the literature on the endophenotype of Formal Thought Disorder from multiple standpoints, ranging from neurophysiology to cognition: considerations are weaved on neuronal (PV basket cell, SST, VIP) and receptor deficits (DRD1, NMDA), neurotransmitter imbalances (dopamine), cortical and dopaminergic lateralisation, inter alia. In conclusion, we put forth novel postulates on the underlying causes of controlled hypopriming, automatic hyperpriming, N400 reversals (larger amplitudes for close associations), indirect versus direct hyperpriming, and the endophenotype of lexical-semantic disturbances in schizophrenia.