Cognitive correlates of visual neural plasticity in schizophrenia

Neurophysiological indices of face processing in people with psychosis and their siblings: An event-related potential study

People with schizophrenia experience difficulties with social interactions. One contributor to these social deficits is dysfunction in processing facial features and facial emotional expressions. However, it is not known whether face processing deficits are evident in those with other psychotic disorders or in those genetically at-risk for psychosis (i.e., first-degree relatives of those with psychosis). We assessed event-related potentials (ERPs) during a facial and emotion processing task in 100 people with a diagnosis of schizophrenia or another psychotic condition (PSY), 32 of their siblings (SIB) and 45 healthy comparison participants (CTL). In separate blocks, participants identified the sex (male or female) or emotion (happy, angry, neutral) of faces. In a comparison condition, participants indicated whether buildings had one or two floors. ERPs were examined in two stages. First, we compared ERPs across the emotion, sex and building identification conditions. Second, we compared ERPs among the three different facial emotions. PSY exhibited significantly lower amplitudes over parietal-occipital regions between 111 and 151 ms when viewing faces but not buildings than CTL, consistent with a face-selective N170 ERP component deficit. The SIB group was intermediate for faces, but not significantly different than PSY or CTL. During emotion identification, all three groups showed increased N170 amplitudes to angry and happy versus neutral expressions, with no group differences. In follow up analyses, we examined differences between PSY with or without affective psychosis, and differences between those with schizophrenia versus other psychotic disorders; there were no significant differences in these analyses. Face processing deficits assessed with ERPs were observed in a group of diverse psychotic disorders, though deficits were not seen to be modulated by facial emotion expression. Additionally, N170 deficits are not evident in siblings of those with PSY.

Qualitatively Different Delay-Dependent Working Memory Distortions in People With Schizophrenia and Healthy Control Participants

BACKGROUND

Impairments in working memory (WM) have been well documented in people with schizophrenia (PSZ). However, these quantitative WM impairments can often be explained by nonspecific factors, such as impaired goal maintenance. Here, we used a spatial orientation delayed response task to explore a qualitative difference in WM dynamics between PSZ and healthy control participants (HCs). More specifically, we took advantage of the discovery that WM representations may drift either toward or away from previous trial targets (serial dependence). We tested the hypothesis that WM representations would drift toward the previous trial target in HCs but away from the previous trial target in PSZ.

METHODS

We assessed serial dependence in PSZ (n = 31) and HCs (n = 25) using orientation as the to-be-remembered feature and memory delays lasting from 0 to 8 seconds. Participants were asked to remember the orientation of a teardrop-shaped object and reproduce the orientation after a delay period of varying length.

RESULTS

Consistent with prior studies, we found that current trial memory representations were less precise in PSZ than in HCs. We also found that WM for the current trial orientation drifted toward the previous trial orientation in HCs (representational attraction) but drifted away from the previous trial orientation in PSZ (representational repulsion).

CONCLUSIONS

These results demonstrate a qualitative difference in WM dynamics between PSZ and HCs that cannot be easily explained by nuisance factors such as reduced effort. Most computational neuroscience models also fail to explain these results because they maintain information solely by means of sustained neural firing, which does not extend across trials. The results suggest a fundamental difference between PSZ and HCs in longer-term memory mechanisms that persist across trials, such as short-term potentiation and neuronal adaptation.

Peripheral nerve stimulation: A neuromodulation-based approach

Recent technological improvements have positioned us at the threshold of innovative discoveries that will assist in new perspectives and avenues of research. Increased attention has been directed towards peripheral nerve stimulation, particularly of the vagus, trigeminal, or greater occipital nerve, due to their unique pathway that engages neural circuits within networks involved in higher cognitive processes. Here, we question whether the effects of transcutaneous electrical stimulation are mediated by synergistic interactions of multiple neuromodulatory networks, considering this pathway is shared by more than one neuromodulatory system. By spotlighting this attractive transcutaneous pathway, this opinion piece aims to acknowledge the contributions of four vital neuromodulators and prompt researchers to consider them in future investigations or explanations.

Qualitatively different delay-dependent working memory distortions in people with schizophrenia and healthy control subjects

Background

Impairments in working memory(WM) have been well-documented in people with schizophrenia(PSZ). However, these quantitative WM impairments can often be explained by nonspecific factors, such as impaired goal maintenance. Here, we used a spatial orientation delayed-response task to explore a qualitative difference in WM dynamics between PSZ and healthy control subjects(HCS). Specifically, we took advantage of the discovery that WM representations may drift either toward or away from previous-trial targets(serial dependence). We tested the hypothesis that WM representations drift toward the previous-trial target in HCS but away from the previous-trial target in PSZ.

Methods

We assessed serial dependence in PSZ(N=31) and HCS(N=25), using orientation as the to-be-remembered feature and memory delays from 0 to 8s. Participants were asked to remember the orientation of a teardrop-shaped object and reproduce the orientation after a varying delay period.

Results

Consistent with prior studies, we found that current-trial memory representations were less precise in PSZ than in HCS. We also found that WM for the current-trial orientation drifted toward the previous-trial orientation in HCS(representational attraction) but drifted away from the previous-trial orientation in PSZ(representational repulsion).

Conclusions

These results demonstrate a qualitative difference in WM dynamics between PSZ and HCS that cannot easily be explained by nuisance factors such as reduced effort. Most computational neuroscience models also fail to explain these results, because they maintain information solely by means of sustained neural firing, which does not extend across trials. The results suggest a fundamental difference between PSZ and HCS in longer-term memory mechanisms that persist across trials, such as short-term potentiation and neuronal adaptation.

Sensory tetanisation to induce long-term-potentiation-like plasticity: A review and reassessment of the approach

There is great interest in developing non-invasive approaches for studying cortical plasticity in humans. High-frequency presentation of auditory and visual stimuli, or sensory tetanisation, can induce long-term-potentiation-like (LTP-like) changes in cortical activity. However, contrasting effects across studies suggest that sensory tetanisation may be unreliable. We review these contrasting effects, conduct our own study of auditory and visual tetanisation, and perform meta-analyses to determine the average effect of sensory tetanisation across studies. We measured auditory-evoked amplitude changes in a group of younger (18-29 years of age) and older (55-83 years of age) adults following tetanisation to 1 and 4 kHz tone bursts and following a slow-presentation control. We also measured visual-evoked amplitude changes following tetanisation to horizontal and vertical sign gradients. Auditory and visual response amplitudes decreased following tetanisation, consistent with some studies but contrasting with others finding amplitude increases (i.e. LTP-like changes). Older adults exhibited more modest auditory-evoked amplitude decreases, but visual-evoked amplitude decreases like those of younger adults. Changes in response amplitude were not specific to tetanised stimuli. Importantly, slow presentation of auditory tone bursts produced response amplitude changes approximating those observed following tetanisation in younger adults. Meta-analyses of visual and auditory tetanisation studies found that the overall effect of sensory tetanisation was not significant across studies or study sites. The results suggest that sensory tetanisation may not produce reliable changes in cortical responses and more work is needed to determine the validity of sensory tetanisation as a method for inducing human cortical plasticity in vivo.

Long-Term Potentiation-Like Visual Synaptic Plasticity Is Negatively Associated With Self-Reported Symptoms of Depression and Stress in Healthy Adults

Long-term potentiation (LTP) is one of the most extensively studied forms of neuroplasticity and is considered the strongest candidate mechanism for memory and learning. The use of event-related potentials and sensory stimulation paradigms has allowed for the translation from animal studies to non-invasive studies of LTP-like synaptic plasticity in humans. Accumulating evidence suggests that synaptic plasticity as measured by stimulus-specific response modulation is reduced in neuropsychiatric disorders such as major depressive disorder (MDD), bipolar disorders and schizophrenia, suggesting that impaired synaptic plasticity plays a part in the underlying pathophysiology of these disorders. This is in line with the neuroplasticity hypothesis of depression, which postulate that deficits in neuroplasticity might be a common pathway underlying depressive disorders. The current study aims to replicate and confirm earlier reports that visual stimulus-specific response modulation is a viable probe into LTP-like synaptic plasticity in a large sample of healthy adults (n = 111). Further, this study explores whether impairments in LTP-like synaptic plasticity is associated with self-reported subclinical depressive symptoms and stress in a healthy population. Consistent with prior research, the current study replicated and confirmed reports demonstrating significant modulation of visual evoked potentials (VEP) following visual high-frequency stimulation. Current results further indicate that reduced LTP-like synaptic plasticity is associated with higher levels of self-reported symptoms of depression and perceived stress. This indicate that LTP-like plasticity is sensitive to sub-clinical levels of psychological distress, and might represent a vulnerability marker for the development of depressive symptoms.

Towards biologically constrained attractor models of schizophrenia

Alterations in neuromodulation or synaptic transmission in biophysical attractor network models, as proposed by the dominant dopaminergic and glutamatergic theories of schizophrenia, successfully mimic working memory (WM) deficits in people with schizophrenia (PSZ). Yet, multiple, often opposing alterations in memory circuits can lead to the same behavioral patterns in these network models. Here, we critically revise the computational and experimental literature that links NMDAR hypofunction to WM precision loss in PSZ. We show in network simulations that currently available experimental evidence cannot set apart competing biophysical accounts. Critical points to resolve are the effects of increases vs. decreases in E/I ratio (e.g. through NMDAR blockade) on firing rate tuning and shared noise modulations and possible concomitant deficits in short-term plasticity. We argue that these concerted experimental and computational efforts will lead to a better understanding of the neurobiology underlying cognitive deficits in PSZ.

Visual cortical plasticity and the risk for psychosis: An interim analysis of the North American Prodrome Longitudinal Study

BACKGROUND

Adolescence/early adulthood coincides with accelerated pruning of cortical synapses and the onset of schizophrenia. Cortical gray matter reduction and dysconnectivity in schizophrenia are hypothesized to result from impaired synaptic plasticity mechanisms, including long-term potentiation (LTP), since deficient LTP may result in too many weak synapses that are then subject to over-pruning. Deficient plasticity has already been observed in schizophrenia. Here, we assessed whether such deficits are present in the psychosis risk syndrome (PRS), particularly those who subsequently convert to full psychosis.

METHODS

An interim analysis was performed on a sub-sample from the NAPLS-3 study, including 46 healthy controls (HC) and 246 PRS participants. All participants performed an LTP-like visual cortical plasticity paradigm involving assessment of visual evoked potentials (VEPs) elicited by vertical and horizontal line gratings before and after high frequency ("tetanizing") visual stimulation with one of the gratings to induce "input-specific" neuroplasticity (i.e., VEP changes specific to the tetanized stimulus). Non-parametric, cluster-based permutation testing was used to identify electrodes and timepoints that demonstrated input-specific plasticity effects.

RESULTS

Input-specific pre-post VEP changes (i.e., increased negative voltage) were found in a single spatio-temporal cluster covering multiple occipital electrodes in a 126-223 ms time window. This plasticity effect was deficient in PRS individuals who subsequently converted to psychosis, relative to PRS non-converters and HC.

CONCLUSIONS

Input-specific LTP-like visual plasticity can be measured from VEPs in adolescents and young adults. Interim analyses suggest that deficient visual cortical plasticity is evident in those PRS individuals at greatest risk for transition to psychosis.

Reduced visual cortical plasticity in autism spectrum disorder

There is increasing evidence implicating altered NMDA-receptor function in autism spectrum disorder (ASD). To investigate potential alterations in NMDA-dependent cortical plasticity in ASD, we examined the effect of visual high-frequency stimulation (HFS) on changes in plasticity in the visual cortex, measured by persistent changes in visual evoked potentials (VEPs), in individuals with ASD (n = 16) and neurotypical controls (NT; n = 15). VEPs were elicited by a checkerboard circle (0.83 Hz, 2-min blocks) at baseline and at 2, 4, and 20 min following exposure to HFS (8.87 Hz, 2 min), previously shown to induce LTP-like changes in the visual cortex. Difference waves were created by subtracting VEPs measured at baseline from each Post-HFS measure, and group differences assessed. We found that HFS resulted in reduced short-term potentiation of VEPs in ASD compared to NT participants. Thus, whilst ASD participants showed significant potentiation of the VEP immediately after HFS, this enhancement was not maintained, and only persisted into the second post-HFS assessment block in NT participants. Notably, ASD individuals who self-reported being more sensitive to visual stimuli showed greater shorter-term potentiation following visual HFS. Critically, there were no group differences in degree of neural entrainment to the visual HFS, or in attentional vigilance and task performance. These findings suggests that visual cortical plasticity is atypical in ASD, results consistent with reported altered NMDA receptor function in ASD.

Modelling thalamocortical circuitry shows that visually induced LTP changes laminar connectivity in human visual cortex

Neuroplasticity is essential to learning and memory in the brain; it has therefore also been implicated in numerous neurological and psychiatric disorders, making measuring the state of neuroplasticity of foremost importance to clinical neuroscience. Long-term potentiation (LTP) is a key mechanism of neuroplasticity and has been studied extensively, and invasively in non-human animals. Translation to human application largely relies on the validation of non-invasive measures of LTP. The current study presents a generative thalamocortical computational model of visual cortex for investigating and replicating interlaminar connectivity changes using non-invasive EEG recording of humans. The model is combined with a commonly used visual sensory LTP paradigm and fit to the empirical EEG data using dynamic causal modelling. The thalamocortical model demonstrated remarkable accuracy recapitulating post-tetanus changes seen in invasive research, including increased excitatory connectivity from thalamus to layer IV and from layer IV to II/III, established major sites of LTP in visual cortex. These findings provide justification for the implementation of the presented thalamocortical model for ERP research, including to provide increased detail on the nature of changes that underlie LTP induced in visual cortex. Future applications include translating rodent findings to non-invasive research in humans concerning deficits to LTP that may underlie neurological and psychiatric disease.

The brain’s ability to learn and form memories is governed by neuroplasticity. One of the major mechanisms of neuroplasticity is long-term potentiation (LTP). To study LTP in detail necessitates implanting electrodes in the brain of non-human animals. However, to translate this knowledge to humans requires a non-invasive method. Neural mass models use mathematical equations to describe the brain’s neural architecture and function over time. Fitting these models to real data, using methods such as dynamic causal modelling (DCM), helps to elucidate the connectivity and major channel changes that could have plausibly caused the observed effects in electroencephalography data recorded non-invasively from the scalp.

The current study presents a thalamocortical model of the neural architecture of the visual system combined with a thalamic compartment. The model is able to represent the basic transfer of visual information to the cortex, mediated by major receptor types. We combined the thalamocortical model with a visual processing task that uses black and white grating images to induce and measure LTP in visual cortex. We hypothesised that the changes in the model would be consistent with what is seen in animal invasive recordings.

The model demonstrated remarkable accuracy in recapitulating changes to neural architecture consistent with the induction of LTP in visual cortex. Additionally, the result demonstrated specificity to the visual input that induced LTP. Future applications include translating animal findings that are beginning to determine how disordered LTP may underlie neurological and psychiatric disease (for example depression, schizophrenia, autism, and dementia).

Transcranial direct current stimulation induces long-term potentiation-like plasticity in the human visual cortex

Transcranial direct current stimulation (tDCS) is increasingly used as a form of noninvasive brain stimulation to treat psychiatric disorders; however, its mechanism of action remains unclear. Prolonged visual stimulation (PVS) can enhance evoked EEG potentials (visually evoked potentials, VEPs) and has been proposed as a tool to examine long-term potentiation (LTP) in humans. The objective of the current study was to induce and analyze VEP plasticity and examine whether tDCS could either modulate or mimic plasticity changes induced by PVS. Thirty-eight healthy participants received tDCS, PVS, either treatment combined or neither treatment, with stimulation sessions being separated by one week. One session consisted of a baseline VEP measurement, one stimulation block, and six test VEP measurements. For PVS, a checkerboard reversal pattern was presented, and for tDCS, a constant current of 1 mA was applied via each bioccipital anodal target electrode for 10 min (Fig. S1). Both stimulation types decreased amplitudes of C1 compared to no stimulation (F = 10.1; p = 0.002) and led to a significantly smaller increase (PVS) or even decrease (tDCS) in N1 compared to no stimulation (F = 4.7; p = 0.034). While all stimulation types increased P1 amplitudes, the linear mixed effects model did not detect a significant difference between active stimulation and no stimulation. Combined stimulation induced sustained plastic modulation of C1 and N1 but with a smaller effect size than what would be expected for an additive effect. The results demonstrate that tDCS can directly induce LTP-like plasticity in the human cortex and suggest a mechanism of action of tDCS relying on the restoration of dysregulated synaptic plasticity in psychiatric disorders such as depression and schizophrenia.

Do visual and auditory stimulus-specific response modulation reflect different mechanisms of neocortical plasticity?

Stimulus response modulation (SRM) of sensory evoked potentials represents a promising method as a non-invasive index of long-term potentiation (LTP)-like synaptic plasticity in the human sensory cortices. As of today, however, no consensus exists regarding which experimental parameters elicit the most robust SRM response. The aim of the current study was twofold; firstly, we aimed to replicate former studies demonstrating visual SRM in healthy adults. Second, we integrated visual and auditory stimuli within the same SRM recording session to assay potential cross-modal associations. Such an association between modalities would strengthen the assumption that the SRM effect reflects common mechanisms underlying synaptic plasticity rather than reflecting modality-specific phenomena. A replication of previous findings showing robust potentiation of the visual evoked potential was evident, supporting the majority of previous work using similar paradigms, lending further support to the notion that high-frequent visual stimulation is a viable probe into LTP-like synaptic plasticity in the human visual cortex. The auditory evoked potentials (AEPs) did not, however, fully replicate previous work, though a significant increase of temporally later AEP components was found. In contrast to our hypothesis, there were no significant within-subject cross-modality correlations between the visual and auditory SRM. This lack of significant association might suggest that auditory and visual SRM depend on different mechanisms, and that further SRM studies on non-invasive LTP-like synaptic plasticity should focus on optimizing paradigms within the visual modality.

The role of Hebbian learning in human perception: a methodological and theoretical review of the human Visual Long-Term Potentiation paradigm

Long-term potentiation (LTP) is one of the most widely studied forms of neural plasticity, and is thought to be the principle mechanism underlying long-term memory and learning in the brain. Sensory paradigms utilising electroencephalography (EEG) and sensory stimulation to induce LTP have allowed translation from rodent and primate invasive research to non-invasive human investigations. This review focusses on visual sensory LTP induced using repetitive visual stimulation, resulting in changes in the visually evoked response recorded at the scalp with EEG. Across 15 years of use and replication in humans several major paradigm variants for eliciting visual LTP have emerged. The application of different paradigms, and the broad implementation of visual LTP across different populations combines to provide a rich and sensitive account of Hebbian LTP, and potentially non-Hebbian plasticity mechanisms. This review will conclude with a discussion of how these findings have advanced existing theories of perceptual learning by positioning Hebbian learning both alongside and within other major theories such as Predictive Coding and The Free Energy Principle.

Impaired Potentiation of Theta Oscillations During a Visual Cortical Plasticity Paradigm in Individuals With Schizophrenia

Long-term potentiation (LTP) is a form of experience-dependent synaptic plasticity mediated by glutamatergic transmission at N-methyl-D-aspartate receptors (NMDARs). Impaired neuroplasticity has been implicated in the pathophysiology of schizophrenia, possibly due to underlying NMDAR hypofunction. Analogous to the high frequency electrical stimulation used to induce LTP in vitro and in vivo in animal models, repeated high frequency presentation of a visual stimulus in humans in vivo has been shown to induce enduring LTP-like neuroplastic changes in electroencephalography (EEG)-based visual evoked potentials (VEPs) elicited by the stimulus. Using this LTP-like visual plasticity paradigm, we previously showed that visual high-frequency stimulation (VHFS) induced sustained changes in VEP amplitudes in healthy controls, but not in patients with schizophrenia. Here, we extend this prior work by re-analyzing the EEG data underlying the VEPs, focusing on neuroplastic changes in stimulus-evoked EEG oscillatory activity following VHFS. EEG data were recorded from 19 patients with schizophrenia and 21 healthy controls during the visual plasticity paradigm. Event-related EEG oscillations (total power, intertrial phase coherence; ITC) elicited by a standard black and white checkerboard stimulus (~0.83 Hz, several 2-min blocks) were assessed before and after exposure to VHFS with the same stimulus (~8.9 Hz, 2 min). A cluster-based permutation testing approach was applied to time-frequency data to examine LTP-like plasticity effects following VHFS. VHFS enhanced theta band total power and ITC in healthy controls but not in patients with schizophrenia. The magnitude and phase synchrony of theta oscillations in response to a visual stimulus were enhanced for at least 22 min following VHFS, a frequency domain manifestation of LTP-like visual cortical plasticity. These theta oscillation changes are deficient in patients with schizophrenia, consistent with hypothesized NMDA receptor dysfunction.

Evaluating visual neuroplasticity with EEG in schizophrenia outpatients

Deficient neuroplasticity has been implicated in schizophrenia and can be examined with non-invasive methods in humans. High frequency visual stimulation (HFS) induces neuroplastic changes in visual evoked potential (VEP) components, similar to the tetanizing electrical stimulation that induces synaptic long-term potentiation (LTP). While visual HFS paradigms have been used in schizophrenia, the use of a single visual stimulus has precluded demonstration of whether the plasticity effects are specific to the stimulus presented during HFS (i.e., input specific). Additionally, test-retest reliability of VEP plasticity effects, an important consideration for applications of HFS paradigms in schizophrenia clinical trials, remains unknown. Accordingly, we administered a visual HFS paradigm to 38 schizophrenia patients and 27 healthy controls at baseline and two-weeks later. VEPs were elicited by horizontal and vertical line gratings before and after HFS; only one orientation was tetanized with HFS. Using a mass univariate permutation approach, we identified an input-specific cluster across groups that was broadly distributed over parietal-occipital areas between 108 and 183 ms. However, the groups did not differ in terms of the strength of plasticity effect. The test-retest reliability of the input-specific plasticity effect was modest over two weeks, suggesting that this HFS paradigm requires further development before it could be used to track plasticity change in clinical trials. Moreover, while the current HFS paradigm induced significant input-specific neuroplasticity, it did not replicate prior studies showing deficient neuroplasticity in schizophrenia. Accordingly, demonstration of deficient visual LTP-like neuroplasticity in schizophrenia may depend on paradigm parameters that remain to be fully elucidated.

Evidence for intact stimulus-specific neural adaptation for visual objects in schizophrenia and bipolar disorder: An ERP study

People with schizophrenia (SZ) or bipolar disorder (BD) experience dysfunction in visual processing. Dysfunctional neural tuning, in which neurons and neuronal populations are selectively activated by specific features of visual stimuli, may contribute to these deficits. Few studies have examined this possibility and there are inconsistent findings of tuning deficits in the literature. We utilized an event-related potential (ERP) paradigm to examine neural adaptation for visual objects, a measure of neural tuning whereby neurons respond less strongly to the repeated presentation of the same stimulus. Seventy-seven SZ, 53 BD, and 49 healthy comparison participants (HC) were examined. In three separate conditions, pictures of objects were presented repeatedly: the same object (SS), different objects from the same category (e.g., two different vases; SD), or different objects from different categories (e.g., a barrel and a clock, DD). Mass-univariate cluster-based permutation analyses identified electrodes and time-windows in which there were significant differences between the SS vs. DD and the SD vs. DD conditions. Mean ERP amplitudes were extracted from these clusters and analyzed for group differences. Results revealed a significant condition difference over parieto-occipital electrodes for the SS-DD comparison between 109–164 ms and for the SD-DD comparison between 78–203 ms, with larger amplitudes in the DD compared to either SS or SD condition. However, there were no significant differences in the pattern of results between groups. Thus, while we found neural adaptation effects using this ERP paradigm, we did not find evidence of group differences. Our results suggest that people with SZ or BD may not exhibit deficits in neural tuning for processing of visual objects using this EEG task with rapidly presented stimuli. However, the results are inconsistent with other studies using different methodologies (e.g., fMRI, behavioral tasks) that have found tuning deficits in people with schizophrenia.

Altered short-term neural plasticity related to schizotypal traits: Evidence from visual adaptation

Abnormalities in synaptic plasticity are argued to underlie the neural dysconnectivity observed in schizophrenia. One way to measure synaptic plasticity is through sensory adaptation, whereby sensory neurons exhibit reduced sensitivity after sustained stimulus exposure. Evidence for decreased adaptation in individuals with schizophrenia is currently inconclusive, possibly due to heterogeneity in clinical and medication status across samples. Here we circumvent these confounds by examining whether altered adaptation is represented sub-clinically in the general population. To test this we used three paradigms from visual perception research that provide a precise and non-invasive index of adaptation in the visual system. Two paradigms involve a class of illusory percepts termed visual aftereffects. The third relies on a visual phenomenon termed binocular rivalry, where incompatible stimuli are presented to the two eyes and observers alternate between perceiving exclusively one stimulus or a combination of the two (i.e. mixed perception). We analyzed the strength and dynamics of visual adaptation in these paradigms, in relation to schizotypy. Our results showed that increased schizotypal traits were related to reduced orientation, but not luminance, aftereffect strength (Exp. 1). Further, increased schizotypy was related to a greater proportion of mixed perception during binocular rivalry (Exp. 1 and 2). Given that visual adaption is well understood at cellular and computational levels, our data suggest that short-term plasticity in the visual system can provide important information about the disease mechanisms of schizophrenia.

Motor cortical plasticity in schizophrenia: A meta-analysis of Transcranial Magnetic Stimulation - Electromyography studies

BACKGROUND

Several lines of investigations converge upon aberrant synaptic plasticity as a potential pathophysiological characteristic of schizophrenia. In vivo experiments using neuromodulatory perturbation techniques like Transcranial Magnetic and Direct Current Stimulation (TMS & tDCS) have been increasingly used to measure 'motor cortical plasticity' in schizophrenia. A systematic quantification of cortical plasticity and its moderators in schizophrenia is however lacking.

METHOD

The PubMed/MEDLINE database was searched for studies up to December 31st, 2017 that examined case-control experiments comparing neuromodulation following single-session of TMS or tDCS. The primary outcome was the standardized mean difference for differential changes in motor evoked potential (MEP) amplitudes measured with single-pulse TMS (MEP Δ) between patients and healthy subjects following TMS or tDCS. After examining heterogeneity, meta-analyses were performed using fixed effects models.

RESULTS

A total of 16 datasets comparing cortical plasticity (MEP Δ) between 189 schizophrenia patients and 187 healthy controls were included in the meta-analysis. Patients demonstrated diminished MEP Δ with effect sizes (Cohen's d) ranging from 0.66 (LTP-like plasticity) to 0.68 (LTD-like plasticity). Heterosynaptic plasticity studies demonstrated a greater effect size (0.79) compared to homosynaptic plasticity studies (0.62), though not significant (P = 0.43). Clinical, perturbation protocol- and measurement-related factors, and study quality did not significantly moderate the aberrant plasticity demonstrated in schizophrenia.

CONCLUSIONS

Schizophrenia patients demonstrate diminished LTP- and LTD-like motor cortical plasticity, which is not influenced by the various clinical and experimental protocol related confounders. These consistent findings should encourage the use of perturbation-based biomarkers to characterize illness trajectories and treatment response.

N-methyl-d-aspartate-type glutamate receptor modulators and related medications for the enhancement of auditory system plasticity in schizophrenia

Deficits in N-methyl-d-aspartate-type (NMDAR) function contribute to cognitive deficits in schizophrenia, particularly dysfunction in neuroplasticity, defined as reduced learning during training on exercises that place implicit, increasing demands on early sensory (auditory and visual) information processing. Auditory mismatch negativity (MMN) can be both a target engagement biomarker for the NMDAR and a proxy measure of neurophysiological plasticity. This review covers the evidence for using NMDAR modulator and related compounds for enhancement of cognition, with a particular focus on early auditory processing/plasticity. Compounds covered include glycine site agonists, glycine and system A-type transporter inhibitors, d-amino acid oxidase inhibitors, memantine and nicotinic alpha-7 acetylcholine receptor agonists. As opposed to daily treatment studies focusing on schizophrenia in general, intermittent, non-daily treatment combining NMDAR modulators with neuroplasticity-based paradigms, using MMN as target-engagement biomarkers show promise as treatments to both remediate plasticity deficits and overall functional deficits.

Auditory System Target Engagement During Plasticity-Based Interventions in Schizophrenia: A Focus on Modulation of N-Methyl-D-Aspartate-Type Glutamate Receptor Function

Cognitive deficits are predictive of long-term social and occupational functional deficits in schizophrenia but are currently without gold-standard treatments. In particular, augmentation of auditory cortical neuroplasticity may represent a rate-limiting first step before addressing higher-order cognitive deficits. We review the rationale for N-methyl-d-aspartate-type glutamate receptor (NMDAR) modulators as treatments for auditory plasticity deficits in schizophrenia, along with potential serum and electroencephalographic target engagement biomarkers for NMDAR function. Several recently published NMDAR-modulating treatment studies are covered, involving D-serine, memantine, and transcranial direct current stimulation. While all three interventions appear to modulate auditory plasticity, direct agonists (D-serine) appear to have the largest and most consistent effects on plasticity, at least acutely. We hypothesize that there may be synergistic effects of combining procognitive NMDAR-modulating approaches with auditory cortical neuroplasticity cognitive training interventions. Future studies should assess biomarkers for target engagement and patient stratification, along with head-to-head studies comparing putative interventions and potential long-term versus acute effects.

Omega-3 fatty acid supplementation may prevent loss of gray matter thickness in the left parieto-occipital cortex in first episode schizophrenia: A secondary outcome analysis of the OFFER randomized controlled study

The aim of the study was to assess changes in cortical thickness related to the use of n-3 polyunsaturated fatty acids (PUFA) as add-on therapy in patients with first episode schizophrenia. A double-blind randomized controlled study was conducted using a 26-week intervention composed of concentrated fish oil containing 2.2g/d of eicosapentaenoic (EPA) and docosahexaenoic acid (DHA) or placebo (olive oil). Participants underwent MRI scanning twice to assess changes in cortical thickness: at the beginning and at the end of intervention. Data of suitable quality was obtained from 29 participants. The T1-weighted images for each participant were analyzed using FreeSurfer methodology for longitudinal pipeline. Significant differences in cortical thickness loss were observed between the groups in the parieto-occipital regions of Brodmann areas 7 and 19 of the left hemisphere, dysfunctions in which may be involved in schizophrenia symptomatology. The results of the study support the previous observations carried out in older individuals and patients with mild cognitive impairment, indicating that n-3 PUFA may have neuroprotective properties, especially at early stages of neurodegenerative diseases, such as schizophrenia. If replicated, the results of the present study may encourage clinicians to consider n-3 PUFA as a promising addition to antipsychotics for long-term treatment of schizophrenia.

Enhancing Neuroplasticity to Augment Cognitive Remediation in Schizophrenia

There is a burgeoning need for innovative treatment strategies to improve the cognitive deficits in schizophrenia. Cognitive remediation (CR) is effective at the group level, but the variability in treatment response is large. Given that CR may depend on intact neuroplasticity to produce cognitive gains, it is reasonable to combine it with strategies that harness patients' neuroplastic potential. In this review, we discuss two non-pharmacological approaches that can enhance neuroplasticity and possibly augment the effects of CR in schizophrenia: physical exercise and transcranial direct current stimulation (tDCS). Substantial body of evidence supports the beneficial effect of physical exercise on cognition, and a handful of studies in schizophrenia have shown that physical exercise in conjunction with CR has a larger impact on cognition than CR alone. Physical exercise is thought to stimulate neuroplasticity through the regulation of central growth factors, and current evidence points to brain-derived neurotrophic factor as the potential underlying mechanism through which physical exercise might enhance the effectiveness of CR. tDCS has emerged as a potential tool for cognitive enhancement and seems to affect the cellular mechanisms involved in long-term potentiation (LTP). A few reports have demonstrated the feasibility of integrating tDCS with CR in schizophrenia, but there are insufficient data to determine if this multimodal approach leads to incremental performance gain in patients. Larger randomized controlled trials are necessary to understand the mechanisms of the combined tDCS-CR intervention. Future research should take advantage of new developments in neuroplasticity paradigms to examine the effects of these interventions on LTP.