Can mismatch negativity be linked to synaptic processes? A glutamatergic approach to deviance detection

Mismatch Negativity as an Index of Auditory Short-Term Plasticity: Associations with Cortisol, Inflammation, and Gray Matter Volume in Youth at Clinical High Risk for Psychosis

Mismatch negativity (MMN) event-related potential (ERP) component reduction, indexing N-methyl-D-aspartate receptor (NMDAR)-dependent auditory echoic memory and short-term plasticity, is a well-established biomarker of schizophrenia that is sensitive to psychosis risk among individuals at clinical high-risk (CHR-P). Based on the NMDAR-hypofunction model of schizophrenia, NMDAR-dependent plasticity is predicted to contribute to aberrant neurodevelopmental processes involved in the pathogenesis of schizophrenia during late adolescence or young adulthood, including gray matter loss. Moreover, stress and inflammation disrupt plasticity. Therefore, using data collected during the 8-center North American Prodrome Longitudinal Study (NAPLS-2), we explored relationships between MMN amplitudes and salivary cortisol, gray matter volumes, and inflammatory cytokines. Participants included 303 CHR-P individuals with baseline electroencephalography (EEG) data recorded during an MMN paradigm as well as structural magnetic resonance imaging (MRI) and salivary cortisol, of which a subsample (n = 57) also completed blood draws. More deficient MMN amplitudes were associated with greater salivary cortisol and pro-inflammatory cytokine levels in future CHR-Converters, but not among those who did not convert to psychosis within the next two years. More deficient MMN amplitude was also associated with smaller total gray matter volume across participants regardless of future clinical outcomes, and with subcortical gray matter volumes among future CHR-Converters only. These findings are consistent with the theory that deficient NMDAR-dependent plasticity results in an overabundance of weak synapses that are subject to over-pruning during psychosis onset, contributing to gray matter loss. Further, MMN plasticity mechanisms may interact with stress, cortisol, and neuroinflammatory processes, representing a proximal influence of psychosis.

North American Prodrome Longitudinal Study (NAPLS 3): Methods and baseline description

The North American Prodrome Longitudinal Study (NAPLS) is a consortium of nine programs focusing on youth at clinical high risk (CHR) for psychosis. Funded by the National Institute of Mental Health (NIMH), the sites are located at Emory University, Harvard University, University of Calgary, University of California at Los Angeles, at San Diego, and at San Francisco, University of North Carolina Chapel Hill, Yale University, and Zucker Hillside Hospital. There have been two previous endeavors completed by this consortium, known as NAPLS-1 and NAPLS-2. This paper first offers an overview of the methodology of the third phase of the NAPLS consortium, the second five-year prospective study NAPLS-3, which aims to determine mechanisms of the development of psychosis. In addition, we report on the ascertainment and demographics of the 710 CHR participants in NAPLS-3.

Cortical Microcircuit Mechanisms of Mismatch Negativity and Its Underlying Subcomponents

In the neocortex, neuronal processing of sensory events is significantly influenced by context. For instance, responses in sensory cortices are suppressed to repetitive or redundant stimuli, a phenomenon termed “stimulus-specific adaptation” (SSA). However, in a context in which that same stimulus is novel, or deviates from expectations, neuronal responses are augmented. This augmentation is termed “deviance detection” (DD). This contextual modulation of neural responses is fundamental for how the brain efficiently processes the sensory world to guide immediate and future behaviors. Notably, context modulation is deficient in some neuropsychiatric disorders such as schizophrenia (SZ), as quantified by reduced “mismatch negativity” (MMN), an electroencephalography waveform reflecting a combination of SSA and DD in sensory cortex. Although the role of NMDA-receptor function and other neuromodulatory systems on MMN is established, the precise microcircuit mechanisms of MMN and its underlying components, SSA and DD, remain unknown. When coupled with animal models, the development of powerful precision neurotechnologies over the past decade carries significant promise for making new progress into understanding the neurobiology of MMN with previously unreachable spatial resolution. Currently, rodent models represent the best tool for mechanistic study due to the vast genetic tools available. While quantifying human-like MMN waveforms in rodents is not straightforward, the “oddball” paradigms used to study it in humans and its underlying subcomponents (SSA/DD) are highly translatable across species. Here we summarize efforts published so far, with a focus on cortically measured SSA and DD in animals to maintain relevance to the classically measured MMN, which has cortical origins. While mechanistic studies that measure and contrast both components are sparse, we synthesize a potential set of microcircuit mechanisms from the existing rodent, primate, and human literature. While MMN and its subcomponents likely reflect several mechanisms across multiple brain regions, understanding fundamental microcircuit mechanisms is an important step to understand MMN as a whole. We hypothesize that SSA reflects adaptations occurring at synapses along the sensory-thalamocortical pathways, while DD depends on both SSA inherited from afferent inputs and resulting disinhibition of non-adapted neurons arising from the distinct physiology and wiring properties of local interneuronal subpopulations and NMDA-receptor function.

The Predictive Processing Model of EMDR

Eye Movement Desensitization and Reprocessing Therapy (EMDR) is an effective treatment for Post-traumatic Stress Disorder (PTSD). The Adaptive Information Processing Model (AIP) guides the development and practice of EMDR. The AIP postulates inadequately processed memory as the foundation of PTSD pathology. Predictive Processing postulates that the primary function of the brain is prediction that serves to anticipate the next moment of experience in order to resist the dissipative force of entropy thus facilitating continued survival. Memory is the primary substrate of prediction, and is optimized by an ongoing process of precision weighted prediction error minimization that refines prediction by updating the memories on which it is based. The Predictive Processing model of EMDR postulates that EMDR facilitates the predictive processing of traumatic memory by overcoming the bias against exploration and evidence accumulation. The EMDR protocol brings the traumatic memory into an active state of re-experiencing. Defensive responding and/or low sensory precision preclude evidence accumulation to test the predictions of the traumatic memory in the present. Sets of therapist guided eye movements repeatedly challenge the bias against evidence accumulation and compel sensory sampling of the benign present. Eye movements reset the theta rhythm organizing the flow of information through the brain, facilitating the deployment of both overt and covert attention, and the mnemonic search for associations. Sampling of sensation does not support the predictions of the traumatic memory resulting in prediction error that the brain then attempts to minimize. The net result is a restoration of the integrity of the rhythmic deployment of attention, a recalibration of sensory precision, and the updating (reconsolidation) of the traumatic memory. Thus one prediction of the model is a decrease in Attention Bias Variability, a core dysfunction in PTSD, following successful treatment with EMDR.

Energy-information coupling during integrative cognitive processes

The physical conception of energy is a natural and general approach to describe functional interactions in the brain at different levels starting from studies of molecular interactions up to the integrative studies of cognitive neuroimaging. In this article, we describe the representation of the brain as a fluctuating energy field, which adaptively reflects the environment. Within this neuroenergetic conception, we indicate a physically solid approach to the problem of the link between brain function and information processing - transmission and integration of information between neuroglial populations is coupled with the corresponding energy increase used for information encoding. We develop the integrative neuroenergetic model of face recognition, in which the input to the model tries to modify the fluctuations of activity according to the free-energy minimization principle corresponding to metabolic efficiency. Once the spatial path of integration in neural activity is known, the processed information can be decoded by spatial differentiation. Energy-based feedback with activity rescaling does not influence the possibility of decoding the information. The model provides further evidence that the conception of energy facilitates at both the computational and conceptual levels the understanding of brain function and its relation to cognition.

Influence of Musical Enculturation on Brain Responses to Metric Deviants

The ability to recognize metric accents is fundamental in both music and language perception. It has been suggested that music listeners prefer rhythms that follow simple binary meters, which are common in Western music. This means that listeners expect odd-numbered beats to be strong and even-numbered beats to be weak. In support of this, studies have shown that listeners exposed to Western music show stronger novelty and incongruity related P3 and irregularity detection related mismatch negativity (MMN) brain responses to attenuated odd- than attenuated even-numbered metric positions. Furthermore, behavioral evidence suggests that music listeners' preferences can be changed by long-term exposure to non-Western rhythms and meters, e.g., by listening to African or Balkan music. In our study, we investigated whether it might be possible to measure effects of music enculturation on neural responses to attenuated tones on specific metric positions. We compared the magnetic mismatch negativity (MMNm) to attenuated beats in a “Western group” of listeners (n = 12) mainly exposed to Western music and a “Bicultural group” of listeners (n = 13) exposed for at least 1 year to both Sub-Saharan African music in addition to Western music. We found that in the “Western group” the MMNm was higher in amplitude to deviant tones on odd compared to even metric positions, but not in the “Bicultural group.” In support of this finding, there was also a trend of the “Western group” to rate omitted beats as more surprising on odd than even metric positions, whereas the “Bicultural group” seemed to discriminate less between metric positions in terms of surprise ratings. Also, we observed that the overall latency of the MMNm was significantly shorter in the Bicultural group compared to the Western group. These effects were not biased by possible differences in rhythm perception ability or music training, measured with the Musical Ear Test (MET). Furthermore, source localization analyses suggest that auditory, inferior temporal, sensory-motor, superior frontal, and parahippocampal regions might be involved in eliciting the MMNm to the metric deviants. These findings suggest that effects of music enculturation can be measured on MMNm responses to attenuated tones on specific metric positions.

Binge Drinking and the Young Brain: A Mini Review of the Neurobiological Underpinnings of Alcohol-Induced Blackout

Binge drinking has significant effects on memory, particularly with regards to the transfer of information to long-term storage. Partial or complete blocking of memory formation is known as blackout. Youth represents a critical period in brain development that is particularly vulnerable to alcohol misuse. Animal models show that the adolescent brain is more vulnerable to the acute and chronic effects of alcohol compared with the adult brain. This mini-review addresses the neurobiological underpinnings of binge drinking and associated memory loss (blackout) in the adolescent and young adult period. Although the extent to which there are pre-existing versus alcohol-induced neurobiological changes remains unclear, it is likely that repetitive binge drinking in youth has detrimental effects on cognitive and social functioning. Given its role in learning and memory, the hippocampus is a critical region with neuroimaging research showing notable changes in this structure associated with alcohol misuse in young people. There is a great need for earlier identification of biological markers associated with alcohol-related brain damage. As a means to assess in vivo neurochemistry, magnetic resonance spectroscopy (MRS) has emerged as a particularly promising technique since changes in neurometabolites often precede gross structural changes. Thus, the current paper addresses how MRS biomarkers of neurotransmission (glutamate, GABA) and oxidative stress (indexed by depleted glutathione) in the hippocampal region of young binge drinkers may underlie propensity for blackouts and other memory impairments. MRS biomarkers may have particular utility in determining the acute versus longer-term effects of binge drinking in young people.

Role of N-Methyl-D-Aspartate Receptors in Action-Based Predictive Coding Deficits in Schizophrenia

BACKGROUND

Recent theoretical models of schizophrenia posit that dysfunction of the neural mechanisms subserving predictive coding contributes to symptoms and cognitive deficits, and this dysfunction is further posited to result from N-methyl-D-aspartate glutamate receptor (NMDAR) hypofunction. Previously, by examining auditory cortical responses to self-generated speech sounds, we demonstrated that predictive coding during vocalization is disrupted in schizophrenia. To test the hypothesized contribution of NMDAR hypofunction to this disruption, we examined the effects of the NMDAR antagonist, ketamine, on predictive coding during vocalization in healthy volunteers and compared them with the effects of schizophrenia.

METHODS

In two separate studies, the N1 component of the event-related potential elicited by speech sounds during vocalization (talk) and passive playback (listen) were compared to assess the degree of N1 suppression during vocalization, a putative measure of auditory predictive coding. In the crossover study, 31 healthy volunteers completed two randomly ordered test days, a saline day and a ketamine day. Event-related potentials during the talk/listen task were obtained before infusion and during infusion on both days, and N1 amplitudes were compared across days. In the case-control study, N1 amplitudes from 34 schizophrenia patients and 33 healthy control volunteers were compared.

RESULTS

N1 suppression to self-produced vocalizations was significantly and similarly diminished by ketamine (Cohen's d = 1.14) and schizophrenia (Cohen's d = .85).

CONCLUSIONS

Disruption of NMDARs causes dysfunction in predictive coding during vocalization in a manner similar to the dysfunction observed in schizophrenia patients, consistent with the theorized contribution of NMDAR hypofunction to predictive coding deficits in schizophrenia.

Hippocampal glutamate is increased and associated with risky drinking in young adults with major depression

BACKGROUND

Risky drinking in young people is harmful, highly prevalent and often complicated by comorbid mental health problems that compound alcohol-induced impairment. The hippocampus and the glutamate system have been implicated in the pathophysiology of alcoholism and depression. This study aimed to determine whether risky drinking is associated with glutamate levels recorded within the hippocampus of young adults with major depression.

METHODS

Sixty-three young persons with major depression (22.1±3.1 years; 65% female) and 38 healthy controls were recruited. Participants completed the alcohol use disorder identification test and underwent proton magnetic resonance spectroscopy to measure in vivo glutamate levels within the hippocampus following a period of at least 48h of abstinence.

RESULTS

Young adults with depression had significantly increased hippocampal glutamate levels and a positive association between the level of alcohol use and glutamate. Regression analysis revealed that higher levels of hippocampal glutamate were predicted by having increased levels of risky drinking and depression.

LIMITATIONS

Small sample sizes for testing diagnosis by risky drinking interaction and use of creatine ratios rather than the absolute concentrations of glutamate.

DISCUSSION

The hippocampus is a critical region; given its role in learning and memory as well as mood regulation, and the neurochemical changes observed in this study may precede structural changes, which are commonly observed in both depression and alcohol misuse. These findings suggest that young adults with major depression who engage in risky drinking may be at increased risk of glutamate excitotoxicity.

Alcohol use in bipolar disorder: A neurobiological model to help predict susceptibility, select treatments and attenuate cortical insult

In a series of neurophysiological and neuroimaging studies we investigated the neurobiology related to alcohol use in young people with bipolar disorder. Impairments were identified across frontal and temporal representations of event-related potential and proton magnetic resonance spectroscopy markers; mismatch negativity and in vivo glutathione, respectively. We propose these findings reflect impairments in the N-methyl-D-aspartate receptor and antioxidant capacity. This review seeks to place these findings within the broader literature in the context of two propositions: 1. Pathophysiological impairments in N-methyl-D-aspartate receptor functioning in bipolar disorder contribute to susceptibility toward developing alcohol problems. 2. Alcohol aggravates bipolar disorder neuroprogression via oxidative stress. A neurobiological model that incorporates these propositions is presented, with a focus on the potential for N-methyl-D-aspartate receptor antagonism and glutathione augmentation as potential adjunctive pharmacotherapies to treat the comorbidity. While this review highlights the importance of alcohol monitoring and reduction strategies in the treatment of bipolar disorder, the clinical impact of the proposed model remains limited by the lack of controlled trials of novel pharmacological interventions.

The N-methyl-D-aspartate receptor as a neurobiological intersection between bipolar disorder and alcohol use: a longitudinal mismatch negativity study

BACKGROUND

Comorbid risky alcohol use in bipolar disorder (BD) is recognized for its high prevalence and clinical relevance, though understanding of its neurobiological underpinning is limited. The N-methyl-D-aspartate (NMDA) receptor has recognized alterations in BD and is a major site of ethanol's effects in the brain. The present study aimed to examine the NMDA receptor system in adolescents and young adults with BD by evaluating the longitudinal changes in a robust marker of NMDA function, mismatch negativity (MMN), in relation to changes in alcohol use patterns.

METHODS

Forty-six BD patients (aged 16-30) were recruited at baseline and 59% (n = 27) returned for follow-up 17.9 +/- 7.3 months later. At both time-points a two-tone, passive, duration-deviant MMN paradigm was conducted and alcohol measures were collected. Pearson's correlations were performed between changes in MMN amplitudes and changes in alcohol use. Multiple regression was used to assess whether MMN amplitudes at baseline could predict alcohol use at follow-up.

RESULTS

Reduction in risky drinking patterns was associated with increased temporal MMN and decreased fronto-central MMN. Larger temporal MMN at baseline was a significant predictor of greater alcohol use at follow-up.

CONCLUSIONS

Results suggest risky alcohol use in BD may further compound pre-existing NMDA receptor abnormalities and, importantly, reducing alcohol use early in stages of illness is associated with changes in MMN. This highlights the importance of monitoring alcohol use from first presentation. In addition, preliminary results present an exciting potential for utility of MMN as a neurobiological marker used to determine risk for alcohol misuse in BD.

Detection of cognitive impairment in patients with obstructive sleep apnea hypopnea syndrome using mismatch negativity

In this experiment, 97 patients with obstructive sleep apnea hypopnea syndrome were divided into three groups (mild, moderate, severe) according to minimum oxygen saturation, and 35 healthy subjects were examined as controls. Cognitive function was determined using the mismatch negativity paradigm and the Montreal Cognitive Assessment. The results revealed that as the disease worsened, the mismatch negativity latency was gradually extended, and the amplitude gradually declined in patients with obstructive sleep apnea hypopnea syndrome. Importantly, mismatch negativity latency in severe patients with a persistent time of minimum oxygen saturation < 60 seconds was significantly shorter than that with a persistent time of minimum oxygen saturation > 60 seconds. Correlation analysis revealed a negative correlation between minimum oxygen saturation latency and Montreal Cognitive Assessment scores. These findings indicate that intermittent night-time hypoxemia affects mismatch negativity waveforms and Montreal Cognitive Assessment scores. As indicators for detecting the cognitive functional status of obstructive sleep apnea hypopnea syndrome patients, the sensitivity of mismatch negativity is 82.93%, the specificity is 73.33%, the accuracy rate is 81.52%, the positive predictive value is 85.00%, the negative predictive value is 70.21%, the positive likelihood ratio is 3, and the negative likelihood ratio is 0.23. These results indicate that mismatch negativity can be used as an effective tool for diagnosis of cognitive dysfunction in obstructive sleep apnea hypopnea syndrome patients.

PET-imaging of brain plasticity after cochlear implantation

In this article, we review the PET neuroimaging literature, which indicates peculiarities of brain networks involved in speech restoration after cochlear implantation. We consider data on implanted patients during stimulation as well as during resting state, which indicates basic long-term reorganisation of brain functional architecture. On the basis of our analysis of neuroimaging literature and considering our own studies, we indicate that auditory recovery in deaf patients after cochlear implantation partly relies on visual cues. The brain develops mechanisms of audio-visual integration as a strategy to achieve high levels of speech recognition. It turns out that this neuroimaging evidence is in line with behavioural findings of better audiovisual integration in these patients. Thus, strong visually and audio-visually based rehabilitation during the first months after cochlear implantation would significantly improve and fasten the functional recovery of speech intelligibility and other auditory functions in these patients. We provide perspectives for further neuroimaging studies in cochlear implanted patients, which would help understand brain organisation to restore auditory cognitive processing in the implanted patients and would potentially suggest novel approaches for their rehabilitation. This article is part of a Special Issue entitled <Lasker Award>.

Neuroenergetics at the brain-mind interface: a conceptual approach

Modern neuroimaging techniques, such as PET and fMRI, attracted specialists in cognitive processing to the problems of brain energy and its transformations in relation to information processing. Neuroenergetics has experienced explosive progress during the last decade, complex biochemical and biophysical models of energy turnover in the brain necessitate the search of the general principles behind them, which could be linked to the cognitive view of the brain. In our conceptual descriptive generalization, we consider how the basic thermodynamical reasoning can be used to better understand brain energy. We suggest how thermodynamical principles can be applied to the existing data and theories to obtain the holistic framework of energetic processes in the brain coupled with information processing. This novel and purely descriptive framework permits the integration of approaches of different disciplines to cognitive processing: psychology, physics, physiology, mathematics, molecular biology, biochemistry, etc. Thus, the proposed general principled approach would be helpful for specialists from different fields of cognition.

Risky alcohol use predicts temporal mismatch negativity impairments in young people with bipolar disorder

Alcohol misuse in bipolar disorder (BD) has a negative impact on illness progression. The NMDA/glutamatergic system is implicated in BD pathophysiology and is critically involved in the effects of alcohol on the brain. Mismatch negativity (MMN) is purported to reflect NMDA receptor output, providing a measure for investigating this association. Forty-two patients and 34 controls (16-30 years) were split into low and high-risk drinkers (based on the Alcohol Use Disorders Identification Test) and underwent a two-tone passive auditory oddball, duration deviant MMN paradigm. Multiple regression models revealed risky drinking and BD diagnosis were predictors of impaired temporal MMN. Potentially reflecting an additive effect of alcohol on a perturbed NMDA/glutamatergic system in BD, these findings highlight alcohol as both a modifiable risk factor of neurobiological impairments and as a potential confounder in MMN studies. Given the increasing use of glutamatergic agents for BD treatment, this finding is important clinically.

Brain prediction of auditory emphasis by facial expressions during audiovisual continuous speech

The visual cues involved in auditory speech processing are not restricted to information from lip movements but also include head or chin gestures and facial expressions such as eyebrow movements. The fact that visual gestures precede the auditory signal implicates that visual information may influence the auditory activity. As visual stimuli are very close in time to the auditory information for audiovisual syllables, the cortical response to them usually overlaps with that for the auditory stimulation; the neural dynamics underlying the visual facilitation for continuous speech therefore remain unclear. In this study, we used a three-word phrase to study continuous speech processing. We presented video clips with even (without emphasis) phrases as the frequent stimuli and with one word visually emphasized by the speaker as the non-frequent stimuli. Negativity in the resulting ERPs was detected after the start of the emphasizing articulatory movements but before the auditory stimulus, a finding that was confirmed by the statistical comparisons of the audiovisual and visual stimulation. No such negativity was present in the control visual-only condition. The propagation of this negativity was observed between the visual and fronto-temporal electrodes. Thus, in continuous speech, the visual modality evokes predictive coding for the auditory speech, which is analysed by the cerebral cortex in the context of the phrase even before the arrival of the corresponding auditory signal.

Mismatch negativity/P3a complex in young people with psychiatric disorders: a cluster analysis

BACKGROUND

We have recently shown that the event-related potential biomarkers, mismatch negativity (MMN) and P3a, are similarly impaired in young patients with schizophrenia- and affective-spectrum psychoses as well as those with bipolar disorder. A data driven approach may help to further elucidate novel patterns of MMN/P3a amplitudes that characterise distinct subgroups in patients with emerging psychiatric disorders.

METHODS

Eighty seven outpatients (16 to 30 years) were assessed: 19 diagnosed with a depressive disorder; 26 with a bipolar disorder; and 42 with a psychotic disorder. The MMN/P3a complex was elicited using a two-tone passive auditory oddball paradigm with duration deviant tones. Hierarchical cluster analysis utilising frontal, central and temporal neurophysiological variables was conducted.

RESULTS

Three clusters were determined: the 'globally impaired' cluster (n = 53) displayed reduced frontal and temporal MMN as well as reduced central P3a amplitudes; the 'largest frontal MMN' cluster (n = 17) were distinguished by increased frontal MMN amplitudes and the 'largest temporal MMN' cluster (n = 17) was characterised by increases in temporal MMN only. Notably, 55% of those in the globally impaired cluster were diagnosed with schizophrenia-spectrum disorder, whereas the three patient subgroups were equally represented in the remaining two clusters. The three cluster-groups did not differ in their current symptomatology; however, the globally impaired cluster was the most neuropsychologically impaired, compared with controls.

CONCLUSIONS

These findings suggest that in emerging psychiatric disorders there are distinct MMN/P3a profiles of patient subgroups independent of current symptomatology. Schizophrenia-spectrum patients tended to show the most global impairments in this neurophysiological complex. Two other subgroups of patients were found to have neurophysiological profiles suggestive of quite different neurobiological (and hence, treatment) implications.

Alcohol use and mismatch negativity in young patients with psychotic disorder

Mismatch negativity (MMN) is a neurophysiological indicator of the brain's ability to extract relevant information from an irrelevant background. MMN has been described as a reliable biomarker of schizophrenia and more recently it has found to be impaired in the early stages of psychosis. In addition, drugs (including alcohol) that block glutamate's N-methyl-D-aspartate receptor have been shown to reduce MMN. This study aims to determine whether risky alcohol consumption in young patients with psychotic disorder further impacts or changes their MMN response. Patients with high-alcohol use were found to show reduced temporal MMN amplitudes compared with patients with low-alcohol use and controls. In contrast, early psychosis patients with low-alcohol use showed reduced fronto-central MMN amplitudes compared with controls; whereas patients with high-alcohol use showed an intermediate response at these sites. Correlational analysis revealed distinct patterns of association between MMN and alcohol use in patients with early psychosis compared with controls. This study shows that early psychosis outpatients who engaged in risky drinking have decreased temporal MMN amplitudes, compared with their peers. This may reflect an additive effect of N-methyl-D-aspartate receptor hypofunction and high-alcohol consumption.

Dynamic causal modelling of precision and synaptic gain in visual perception - an EEG study

Estimating the precision or uncertainty associated with sensory signals is an important part of perception. Based on a previous computational model, we tested the hypothesis that increasing visual contrast increased the precision encoded in early visual areas by the gain or excitability of superficial pyramidal cells. This hypothesis was investigated using electroencephalography and dynamic causal modelling (DCM); a biologically constrained modelling of the cortical processes underlying EEG activity. Source localisation identified the electromagnetic sources of visually evoked responses and DCM was used to characterise the coupling among these sources. Bayesian model selection was used to select the most likely connectivity pattern and contrast-dependent changes in connectivity. As predicted, the model with the highest evidence entailed increased superficial pyramidal cell gain in higher-contrast trials. As predicted theoretically, contrast-dependent increases were reduced at higher levels of the hierarchy. These results demonstrate that increased signal-to-noise ratio in sensory signals produce (or are represented by) increased superficial pyramidal cell gain, and that synaptic parameters encoding statistical properties like sensory precision can be quantified using EEG and dynamic causal modelling.

Cross-diagnostic comparison of duration mismatch negativity and P3a in bipolar disorder and schizophrenia

OBJECTIVES

Bipolar disorder and schizophrenia share common pathophysiological processes and may have similar perceptual abnormalities. Mismatch negativity (MMN) and P3a - event-related potentials associated with auditory preattentional processing - have been extensively studied in schizophrenia, but rarely in bipolar disorder. Furthermore, MMN and P3a have not been examined between diagnostic subgroups of patients with bipolar disorder. We evaluated MMN and P3a in patients with bipolar disorder compared to patients with schizophrenia and healthy controls.

METHODS

MMN and P3a were assessed in 52 bipolar disorder patients, 30 schizophrenia patients, and 27 healthy control subjects during a duration-deviant auditory oddball paradigm.

RESULTS

Significant MMN and P3a amplitude reductions were present in patients with bipolar disorder and schizophrenia relative to controls. The MMN reduction was more prominent in patients with schizophrenia than bipolar disorder, at a trend level. P3a did not differ significantly between patient groups. There were no MMN or P3a differences between patients with bipolar I (n = 34) and bipolar II (n = 18) disorder. Patients with bipolar I disorder failed to show lateralized MMN, in contrast to the other groups. No MMN or P3a differences were found between patients with bipolar disorder taking (n = 12) and not taking (n = 40) lithium, as well as between those taking (n = 30) and not taking (n = 22) antipsychotic medications.

CONCLUSIONS

Patients with bipolar disorder showed deficits in preattentive auditory processing, including MMN deficits that are less severe and P3a deficits that are slightly more pronounced, than those seen in schizophrenia.

Auditory hallucinations: expectation-perception model

In this paper, we aimed to present a hypothesis that would explain the mechanism of auditory hallucinations, one of the main symptoms of schizophrenia. We propose that auditory hallucinations arise from abnormalities in the predictive coding which underlies normal perception, specifically, from the absence or attenuation of prediction error. The suggested deficiencies in processing prediction error could arise from (1) abnormal modulation of thalamus by prefrontal cortex, (2) absence or impaired transmission of external input, (3) dysfunction of the auditory and association cortex, (4) neurotransmitter dysfunction and abnormal connectivity, and (5) hyperactivity activity in auditory cortex and broad prior probability. If there is no prediction error, the initially vague prior probability develops into an explicit percept in the absence of external input, as a result of a recursive pathological exchange between auditory and prefrontal cortex. Unlike existing explanations of auditory hallucinations, we propose concrete mechanisms which underlie the imbalance between perceptual expectation and external input. Impaired processing of prediction error is reflected in reduced mismatch negativity and increased tendency to report non-existing meaningful language stimuli in white noise, shown by those suffering from auditory hallucinations. We believe that the expectation-perception model of auditory hallucinations offers a comprehensive explanation of the underpinnings of auditory hallucinations in both patients and those not diagnosed with mental illness. Therefore, our hypothesis has the potential to fill the gaps in the existing knowledge about this distressing phenomenon and contribute to improved effectiveness of treatments, targeting specific mechanisms.

Schizophrenia and language--shall we look for a deficit of deviance detection?

In this article, we consider the view on schizophrenia that asserts this disease originates from a deficit in the hemispheric specialization for language. We suggest that a deficit in the hemispheric specialization for language may be a consequence of the other recently shown neurophysiological deficit of schizophrenia, namely deviance detection. We hypothesise that a deficit of deviance detection related to the dysfunction of NMDA receptors in schizophrenia leads to the abnormal interaction between the parallel and sequential streams of speech processing in the brain. This hypothesis opens perspectives for genetic, molecular and pharmacological studies of the deficit of deviance detection in schizophrenia, as reflected by event-related potentials and neuroimaging during speech processing.

Does brain activity at rest reflect adaptive strategies? Evidence from speech processing after cochlear implantation

In functional neuroimaging studies, task-related activity refers to the signal difference between the stimulation and rest conditions. We asked whether long-term changes in the sensory environment may affect brain activity at rest. To answer this question, we compared regional cerebral blood flow between a group of normally hearing controls and a group of cochlear-implanted (CI) deaf patients. Here we present evidence that long-term alteration of auditory experience, such as profound deafness followed by partial auditory recuperation through cochlear implantation, leads to functional cortical reorganizations at rest. Without any visual or auditory stimulation, CI subjects showed changes of cerebral blood flow in the visual, auditory cortex, Broca area, and in the posterior temporal cortex with an increment of activity in these areas from the time of activation of the implant to less than a year after the implantation.