Spatial frequency-specific potentiation of human visual-evoked potentials

Decoding electroencephalographic responses to visual stimuli compatible with electrical stimulation

Electrical stimulation of the visual nervous system could improve the quality of life of patients affected by acquired blindness by restoring some visual sensations, but requires careful optimization of stimulation parameters to produce useful perceptions. Neural correlates of elicited perceptions could be used for fast automatic optimization, with electroencephalography as a natural choice as it can be acquired non-invasively. Nonetheless, its low signal-to-noise ratio may hinder discrimination of similar visual patterns, preventing its use in the optimization of electrical stimulation. Our work investigates for the first time the discriminability of the electroencephalographic responses to visual stimuli compatible with electrical stimulation, employing a newly acquired dataset whose stimuli encompass the concurrent variation of several features, while neuroscience research tends to study the neural correlates of single visual features. We then performed above-chance single-trial decoding of multiple features of our newly crafted visual stimuli using relatively simple machine learning algorithms. A decoding scheme employing the information from multiple stimulus presentations was implemented, substantially improving our decoding performance, suggesting that such methods should be used systematically in future applications. The significance of the present work relies in the determination of which visual features can be decoded from electroencephalographic responses to electrical stimulation-compatible stimuli and at which granularity they can be discriminated. Our methods pave the way to using electroencephalographic correlates to optimize electrical stimulation parameters, thus increasing the effectiveness of current visual neuroprostheses.

Rapidly repeated visual stimulation induces long-term potentiation of VEPs and increased content of membrane AMPA and NMDA receptors in the V1 cortex of cats

Studies report that rapidly repeated sensory stimulation can evoke LTP-like improvement of neural response in the sensory cortex. Whether this neural response potentiation is similar to the classic LTP induced by presynaptic electrical stimulation remains unclear. This study examined the effects of repeated high-frequency (9 Hz) versus low-frequency (1 Hz) visual stimulation on visually-evoked field potentials (VEPs) and the membrane protein content of AMPA / NMDA receptors in the primary visual cortex (V1) of cats. The results showed that repeated high-frequency visual stimulation (HFS) caused a long-term improvement in peak-to-peak amplitude of V1-cortical VEPs in response to visual stimuli at HFS-stimulated orientation (SO: 90°) and non-stimulated orientation (NSO: 180°), but the effect exhibited variations depending on stimulus orientation: the amplitude increase of VEPs in response to visual stimuli at SO was larger, reached a maximum earlier and lasted longer than at NSO. By contrast, repeated low-frequency visual stimulation (LFS) had not significantly affected the amplitude of V1-cortical VEPs in response to visual stimuli at both SO and NSO. Furthermore, the membrane protein content of the key subunit GluA1 of AMPA receptors and main subunit NR1 of AMPA receptors in V1 cortex was significantly increased after HFS but not LFS when compared with that of control cats. Taken together, these results indicate that HFS can induce LTP-like improvement of VEPs and an increase in membrane protein of AMPA and NMDA receptors in the V1 cortex of cats, which is similar to but less specific to stimulus orientation than the classic LTP.

Modulation of long-term potentiation following microdoses of LSD captured by thalamo-cortical modelling in a randomised, controlled trial

BACKGROUND

Microdosing psychedelics is a phenomenon with claimed cognitive benefits that are relatively untested clinically. Pre-clinically, psychedelics have demonstrated enhancing effects on neuroplasticity, which cannot be measured directly in humans, but may be indexed by non-invasive electroencephalography (EEG) paradigms. This study used a visual long-term potentiation (LTP) EEG paradigm to test the effects of microdosed lysergic acid diethylamide (LSD) on neural plasticity, both acutely while on the drug and cumulatively after microdosing every third day for six weeks. Healthy adult males (n = 80) completed the visual LTP paradigm at baseline, 2.5 h following a dose of 10 µg of LSD or inactive placebo, and 6 weeks later after taking 14 repeated microdoses. Visually induced LTP was used as indirect index of neural plasticity. Surface level event-related potential (ERPs) based analyses are presented alongside dynamic causal modelling of the source localised data using a generative thalamocortical model (TCM) of visual cortex to elucidate underlying synaptic circuitry.

RESULTS

Event-related potential (ERP) analyses of N1b and P2 components did not show evidence of changes in visually induced LTP by LSD either acutely or after 6 weeks of regular dosing. However modelling the complete timecourse of the ERP with the TCM demonstrated changes in laminar connectivity in primary visual cortex. This primarily included changes to self-gain and inhibitory input parameters acutely. Layer 2/3 to layer 5 excitatory connectivity was also different between LSD and placebo groups. After regular dosing only excitatory input from layer 2/3 into layer 5 and inhibitory input into layer 4 were different between groups.

CONCLUSIONS

Without modulation of the ERPs it is difficult to relate the findings to other studies visually inducing LTP. It also indicates the classic peak analysis may not be sensitive enough to demonstrate evidence for changes in LTP plasticity in humans at such low doses. The TCM provides a more sensitive approach to assessing changes to plasticity as differences in plasticity mediated laminar connectivity were found between the LSD and placebo groups.

TRIAL REGISTRATION

 ANZCTR registration number ACTRN12621000436875; Registered 16/04/2021 https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?id=381476 .

Acute exercise as a modifier of neocortical plasticity and aperiodic activity in the visual cortex

Long-term potentiation (LTP) is a form of neuroplasticity commonly implicated in mechanistic models of learning and memory. Acute exercise can boost LTP in the motor cortex, and is associated with a shift in excitation/inhibition (E:I) balance, but whether this extends to other regions such as the visual cortex is unknown. We investigated the effect of a preceding bout of exercise on LTP induction and the E:I balance in the visual cortex using electroencephalography (EEG). Young adults (N = 20, mean age = 24.20) engaged in 20 min of high-intensity interval training (HIIT) exercise and rest across two counterbalanced sessions. LTP was induced using a high frequency presentation of a visual stimulus; a "visual tetanus". Established EEG markers of visual LTP, the N1b and P2 component of the visual evoked potential, and an EEG-derived measure of the E:I balance, the aperiodic exponent, were measured before and after the visual tetanus. As expected, there was a potentiation of the N1b following the visual tetanus, with specificity to the tetanised stimulus, and a non-specific potentiation of the P2. These effects were not sensitive to a preceding bout of exercise. However, the E:I balance showed a late shift towards inhibition following the visual tetanus. A preceding bout of exercise resulted in specificity of this E:I balance shift to the tetanised stimulus, that was not seen following rest. This novel finding suggests a possible exercise-induced tuning of the visual cortex to stimulus details following LTP induction.

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.

Generalization of sustained neurophysiological effects of short-term auditory 13-Hz stimulation to neighboring frequency representation in humans

A fuller understanding of the effects of auditory tetanization in humans would inform better language and sensory learning paradigms, however, there are still unanswered questions. Here, we probe sustained changes in the event-related potentials (ERPs) to 1020Hz and 980Hz tones following a rapid presentation of 1020Hz tone (every 75 ms, 13.3Hz, tetanization). Consistent with the previous studies (Rygvold, et al., 2021, Mears & Spencer 2012), we revealed the increase in the P2 ERP component after tetanization. Contrary to other studies (Clapp et al., 2005; Lei et al., 2017) we did not observe the expected N1 increase after tetanization even in the experimental sequence identical to Clapp. et al., 2005. We detected a significant N1 decrease after tetanization. Expanding previous research, we showed that P2 increase and N1 decrease is not specific to the stimulus type (tetanized 1020Hz and non-tetanized 980Hz), suggesting the generalizability of tetanization effect to the not-stimulated auditory tones, at least to those of the neighboring frequency. The ERPs tetanization effects were observed for at least 30 min - the most prolonged interval examined, consistent with the duration of long-term potentiation, LTP. In addition, the tetanization effects were detectable in the blocks where the participants watched muted videos, an experimental setting that can be easily used in children and other challenging groups. Thus, auditory 13-Hz stimulation affects brain processing of tones including those of neighboring frequencies.

The association of self-reported physical activity on human sensory long-term potentiation

Exercise has been shown to enhance synaptic plasticity, therefore, potentially affecting memory. While the mechanism(s) responsible for this relationship have been explored in animal models, current research suggests that exercise may possess the ability to induce synaptic long-term potentiation (LTP). Most of the LTP mechanistic work has been conducted in animal models using invasive procedures. For that reason, the purpose of the present experiment was to investigate whether self-reported exercise is related to human sensory LTP-like responses. Nineteen participants (M_AGE = 24 years; 52.6% male) completed the study. Long-term potentiation-like responses were measured by incorporating a non-invasive method that assess the change in potentiation of the N1b component produced from the visual stimulus paradigm presented bilaterally in the visual field. Results demonstrated that those with higher levels of moderate-to-vigorous physical activity (MVPA) had a greater N1b change from baseline to the early time period assessment, r = −0.43, p = 0.06. Our findings provide some suggestive evidence of an association between self-reported MVPA and LTP-like responses. Additional work is needed to support that the potentiation of the human sensory N1b component in the observed study is due to the exercise-induced synaptic changes similar to that detailed in prior animal research.

Evidence for Reduced Long-Term Potentiation-Like Visual Cortical Plasticity in Schizophrenia and Bipolar Disorder

Several lines of research suggest that impairments in long-term potentiation (LTP)-like synaptic plasticity might be a key pathophysiological mechanism in schizophrenia (SZ) and bipolar disorder type I (BDI) and II (BDII). Using modulations of visually evoked potentials (VEP) of the electroencephalogram, impaired LTP-like visual cortical plasticity has been implicated in patients with BDII, while there has been conflicting evidence in SZ, a lack of research in BDI, and mixed results regarding associations with symptom severity, mood states, and medication. We measured the VEP of patients with SZ spectrum disorders (n = 31), BDI (n = 34), BDII (n = 33), and other BD spectrum disorders (n = 2), and age-matched healthy control (HC) participants (n = 200) before and after prolonged visual stimulation. Compared to HCs, modulation of VEP component N1b, but not C1 or P1, was impaired both in patients within the SZ spectrum (χ 2 = 35.1, P = 3.1 × 10-9) and BD spectrum (χ 2 = 7.0, P = 8.2 × 10-3), including BDI (χ 2 = 6.4, P = .012), but not BDII (χ 2 = 2.2, P = .14). N1b modulation was also more severely impaired in SZ spectrum than BD spectrum patients (χ 2 = 14.2, P = 1.7 × 10-4). N1b modulation was not significantly associated with Positive and Negative Syndrome Scale (PANSS) negative or positive symptoms scores, number of psychotic episodes, Montgomery and Åsberg Depression Rating Scale (MADRS) scores, or Young Mania Rating Scale (YMRS) scores after multiple comparison correction, although a nominal association was observed between N1b modulation and PANSS negative symptoms scores among SZ spectrum patients. These results suggest that LTP-like plasticity is impaired in SZ and BD. Adding to previous genetic, pharmacological, and electrophysiological evidence, these results implicate aberrant synaptic plasticity as a mechanism underlying SZ and BD.

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).

The Effects of Human Visual Sensory Stimuli on N1b Amplitude: An EEG Study

Sensory systems are widely known to exhibit adaptive mechanisms. Vision is no exception to input dependent changes in its sensitivity. Recent animal work demonstrates enhanced connectivity between neurons in the visual cortex. The purpose of the present experiment was to evaluate a human model that noninvasively alters the amplitude of the N1b component in the visual cortex of humans by means of rapid visual stimulation. Nineteen participants (M_age = 24 years; 52.6% male) completed a rapid visual stimulation paradigm involving black and white reversal checkerboards presented bilaterally in the visual field. EEG data was collected during the visual stimulation paradigm, which consisted of four main phases, a pre-tetanus block, photic stimulus, early post-tetanus, and late post-tetanus. The amplitude of the N1b component of the pre-tetanus, early post-tetanus and late post-tetanus visual evoked potentials were calculated. Change in N1b amplitude was calculated by subtracting pre-tetanus N1b amplitude from early and late post-tetanus. Results demonstrated a significant difference between pre-tetanus N1b (M = −0.498 µV, SD = 0.858) and early N1b (M = −1.011 µV, SD = 1.088), t (18) = 2.761, p = 0.039, d = 0.633. No difference was observed between pre-tetanus N1b and late N1b (p = 0.36). In conclusion, our findings suggest that it is possible to induce changes in the amplitude of the visually evoked potential N1b waveform in the visual cortex of humans non-invasively. Additional work is needed to corroborate that the potentiation of the N1b component observed in this study is due to similar mechanisms essential to prolonged strengthened neural connections exhibited in cognitive structures of the brain observed in prior animal research. If so, this will allow for the examination of strengthened neural connectivity and its interaction with multiple human sensory stimuli and behaviors.

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.

Review of EEG, ERP, and Brain Connectivity Estimators as Predictive Biomarkers of Social Anxiety Disorder

Social anxiety disorder (SAD) is characterized by a fear of negative evaluation, negative self-belief and extreme avoidance of social situations. These recurrent symptoms are thought to maintain the severity and substantial impairment in social and cognitive thoughts. SAD is associated with a disruption in neuronal networks implicated in emotional regulation, perceptual stimulus functions, and emotion processing, suggesting a network system to delineate the electrocortical endophenotypes of SAD. This paper seeks to provide a comprehensive review of the most frequently studied electroencephalographic (EEG) spectral coupling, event-related potential (ERP), visual-event potential (VEP), and other connectivity estimators in social anxiety during rest, anticipation, stimulus processing, and recovery states. A search on Web of Science provided 97 studies that document electrocortical biomarkers and relevant constructs pertaining to individuals with SAD. This study aims to identify SAD neuronal biomarkers and provide insight into the differences in these biomarkers based on EEG, ERPs, VEP, and brain connectivity networks in SAD patients and healthy controls (HC). Furthermore, we proposed recommendations to improve methods of delineating the electrocortical endophenotypes of SAD, e.g., a fusion of EEG with other modalities such as functional magnetic resonance imaging (fMRI) and magnetoencephalograms (MEG), to realize better effectiveness than EEG alone, in order to ultimately evolve the treatment selection process, and to review the possibility of using electrocortical measures in the early diagnosis and endophenotype examination of SAD.

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.

Ketamine Enhances Visual Sensory Evoked Potential Long-term Potentiation in Patients With Major Depressive Disorder

BACKGROUND

The rapid-acting clinical effects of ketamine as a novel treatment for depression along with its complex pharmacology have made it a growing research area. One of the key mechanistic hypotheses for how ketamine works to alleviate depression is by enhancing long-term potentiation (LTP)-mediated neural plasticity.

METHODS

The objective of this study was to investigate the plasticity hypothesis in 30 patients with depression noninvasively using visual LTP as an index of neural plasticity. In a double-blind, active placebo-controlled crossover trial, electroencephalography-based LTP was recorded approximately 3 to 4 hours following a single 0.44-mg/kg intravenous dose of ketamine or active placebo (1.7 ng/mL remifentanil) in 30 patients. Montgomery-Åsberg Depression Rating Scale scores were used to measure clinical symptoms. Visual LTP was measured as a change in the visually evoked potential following high-frequency visual stimulation. Dynamic causal modeling investigated the underlying neural architecture of visual LTP and the contribution of ketamine.

RESULTS

Montgomery-Åsberg Depression Rating Scale scores revealed that 70% of participants experienced 50% or greater reduction in their depression symptoms within 1 day of receiving ketamine. LTP was demonstrated in the N1 (p = .00002) and P2 (p = 2.31 × 10^-11) visually evoked components. Ketamine specifically enhanced P2 potentiation compared with placebo (p = .017). Dynamic causal modeling replicated the recruitment of forward and intrinsic connections for visual LTP and showed complementary effects of ketamine indicative of downstream and proplasticity modulation.

CONCLUSIONS

This study provides evidence that LTP-based neural plasticity increases within the time frame of the antidepressant effects of ketamine in humans and supports the hypothesis that changes to neural plasticity may be key to the antidepressant properties of ketamine.

Human Sensory LTP Predicts Memory Performance and Is Modulated by the BDNF Val66Met Polymorphism

Background: Long-term potentiation (LTP) is recognised as a core neuronal process underlying long-term memory. However, a direct relationship between LTP and human memory performance is yet to be demonstrated. The first aim of the current study was thus to assess the relationship between LTP and human long-term memory performance. With this also comes an opportunity to explore factors thought to mediate the relationship between LTP and long-term memory. The second aim of the current study was to explore the relationship between LTP and memory in groups differing with respect to brain-derived neurotrophic factor (BDNF) Val⁶⁶Met; a single-nucleotide polymorphism (SNP) implicated in memory function.

Methods: Participants were split into three genotype groups (Val/Val, Val/Met, Met/Met) and were presented with both an EEG paradigm for inducing LTP-like enhancements of the visually-evoked response, and a test of visual memory.

Results: The magnitude of LTP 40 min after induction was predictive of long-term memory performance. Additionally, the BDNF Met allele was associated with both reduced LTP and reduced memory performance.

Conclusions: The current study not only presents the first evidence for a relationship between sensory LTP and human memory performance, but also demonstrates how targeting this relationship can provide insight into factors implicated in variation in human memory performance. It is anticipated that this will be of utility to future clinical studies of disrupted memory function.

The effect of LTP- and LTD-like visual stimulation on modulation of human orientation discrimination

Studies showing that repetitive visual stimulation protocols alter perception and induce cortical reorganization, as well-reported for the tactile domain, have been sparse. In this study, we investigated how “long-term potentiation [LTP]-like” and “long-term depression [LTD]-like” repetitive visual stimulation affects orientation discrimination ability in human observers. LTP-like stimulation with features most closely resembling the stimuli used during behavioral assessment evoked the largest improvement, while the effects were smaller in protocols that differed in shape or orientation features. This gradient suggests lower learning specificity than classical perceptual learning experiments, possibly because of an interplay of task- and feature-based factors. All modulatory effects of repetitive stimulation were superimposed on top of spontaneous task learning. Moreover, blockwise analysis revealed that LTP-like stimulation, in contrast to LTD-like or sham stimulation, prevented a loss of practice-related gain of orientation discrimination thresholds. This observation highlights a critical role of LTP-like stimulation for consolidation, typically observed during sleep.

Neural plasticity is modified over the human menstrual cycle: Combined insight from sensory evoked potential LTP and repetition suppression

In healthy women, fluctuations in hormones including progesterone and oestradiol lead to functional changes in the brain over the course of each menstrual cycle. Though considerable attention has been directed towards understanding changes in human cognition over the menstrual cycle, changes in underlying processes such as neural plasticity have largely only been studied in animals. In this study we explored predictive coding and repetition suppression via the roving mismatch negativity paradigm as a model of short-term plasticity (Garrido, Kilner, Kiebel, et al., 2009), and Hebbian learning via visual sensory long-term potentiation (LTP) as a model of long-term plasticity (Teyler et al., 2005). Electroencephalography (EEG) was recorded in 20 females during their early follicular and mid-luteal phases. Event-related potential (ERP) analyses were complemented with dynamic causal modelling (DCM) to characterise changes in the underlying neural architecture. More sustained variability in the ERP response to a change in tone during the luteal phase are interpreted as a delayed habituation of the P3a component in the luteal relative to the follicular phase. The additional increased forward connection strength over tone repetitions compared to the follicular phase suggests that, in this phase, females may be less efficient when processing deviations from predicted sensory input (error). In contrast, there appears to be no reliable change in sensory LTP. This suggests that predictive coding, but not Hebbian plasticity is modified in the mid-luteal compared to the follicular phase, at least at the days of the menstrual cycle tested. This finding implicates the human menstrual cycle in complex changes in neural plasticity and provides further evidence for the importance of considering the menstrual cycle when including females in electrophysiological research.

A review of plasticity induced by auditory and visual tetanic stimulation in humans

Long-term potentiation is a form of synaptic plasticity thought to play an important role in learning and memory. Recently noninvasive methods have been developed to induce and measure activity similar to long-term potentiation in humans. Sensory tetani (trains of quickly repeating auditory or visual stimuli) alter the electroencephalogram in a manner similar to electrical stimulation that results in long-term potentiation. This review briefly covers the development of long-term potentiation research before focusing on in vivo human studies that produce long-term potentiation-like effects using auditory and visual stimulation. Similarities and differences between traditional (animal and brain tissue) long-term potentiation studies and human sensory tetanization studies will be discussed, as well as implications for perceptual learning. Although evidence for functional consequences of sensory tetanization remains scarce, studies involving clinical populations indicate that sensory induced plasticity paradigms may be developed into diagnostic and research tools in clinical settings. Individual differences in the effects of sensory tetanization are not well-understood and provide an interesting avenue for future research. Differences in effects found between research groups that have emerged as the field has progressed are also yet to be resolved.

Longitudinal and cross-sectional investigations of long-term potentiation-like cortical plasticity in bipolar disorder type II and healthy individuals

Visual evoked potential (VEP) plasticity is a promising assay for noninvasive examination of long-term potentiation (LTP)-like synaptic processes in the cerebral cortex. We conducted longitudinal and cross-sectional investigations of VEP plasticity in controls and individuals with bipolar disorder (BD) type II. VEP plasticity was assessed at baseline, as described previously (Elvsåshagen et al. Biol Psychiatry 2012), and 2.2 years later, at follow-up. The longitudinal sample with VEP data from both time points comprised 29 controls and 16 patients. VEP data were available from 13 additional patients at follow-up (total n = 58). VEPs were evoked by checkerboard reversals in two premodulation blocks before and six blocks after a plasticity-inducing block of prolonged (10 min) visual stimulation. VEP plasticity was computed by subtracting premodulation VEP amplitudes from postmodulation amplitudes. Saliva samples for cortisol analysis were collected immediately after awakening in the morning, 30 min later, and at 12:30 PM, at follow-up. We found reduced VEP plasticity in BD type II, that impaired plasticity was present in the euthymic phases of the illness, and that VEP plasticity correlated negatively with depression severity. There was a positive association between VEP plasticity and saliva cortisol in controls, possibly reflecting an inverted U-shaped relationship between cortisol and synaptic plasticity. VEP plasticity exhibited moderate temporal stability over a period of 2.2 years. The present study provides additional evidence for impaired LTP-like cortical plasticity in BD type II. VEP plasticity is an accessible method, which may help elucidate the pathophysiological and clinical significance of synaptic dysfunction in psychiatric disorders.

Indexing sensory plasticity: Evidence for distinct Predictive Coding and Hebbian learning mechanisms in the cerebral cortex

The Roving Mismatch Negativity (MMN), and Visual LTP paradigms are widely used as independent measures of sensory plasticity. However, the paradigms are built upon fundamentally different (and seemingly opposing) models of perceptual learning; namely, Predictive Coding (MMN) and Hebbian plasticity (LTP). The aim of the current study was to compare the generative mechanisms of the MMN and visual LTP, therefore assessing whether Predictive Coding and Hebbian mechanisms co-occur in the brain. Forty participants were presented with both paradigms during EEG recording. Consistent with Predictive Coding and Hebbian predictions, Dynamic Causal Modelling revealed that the generation of the MMN modulates forward and backward connections in the underlying network, while visual LTP only modulates forward connections. These results suggest that both Predictive Coding and Hebbian mechanisms are utilized by the brain under different task demands. This therefore indicates that both tasks provide unique insight into plasticity mechanisms, which has important implications for future studies of aberrant plasticity in clinical populations.

Cognitive correlates of visual neural plasticity in schizophrenia

Neuroplasticity may be an important treatment target to improve the cognitive deficits in schizophrenia (SZ). Yet, it is poorly understood and difficult to assess. Recently, a visual high-frequency stimulation (HFS) paradigm that potentiates electroencephalography (EEG)-based visual evoked potentials (VEP) has been developed to assess neural plasticity in the visual cortex. Using this paradigm, we examined visual plasticity in SZ patients (N=64) and its correlations with clinical symptoms, neurocognition, functional capacity, and community functioning. VEPs were assessed prior to (baseline), and 2-, 4-, and 20-min after (Post-1, Post-2, and Post-3, respectively) 2min of visual HFS. Cluster-based permutation tests were conducted to identify time points and electrodes at which VEP amplitudes were significantly different after HFS. Compared to baseline, there was increased negativity between 140 and 227ms for the early post-HFS block (average of Post-1 and Post-2), and increased positivity between 180 and 281ms for the late post-HFS block (Post-3), at parieto-occipital and occipital electrodes. The increased negativity in the early post-HFS block did not correlate with any of the measures, whereas increased positivity in the late post-HFS block correlated with better neurocognitive performance. Results suggest that SZ patients exhibit both short- and long-term plasticity. The long-term plasticity effect, which was present 22min after HFS, was evident relatively late in the VEP, suggesting that neuroplastic changes in higher-order visual association areas, rather than earlier short-term changes in primary and secondary visual cortex, may be particularly important for the maintenance of neurocognitive function in SZ.

Glutamatergic metabolites are associated with visual plasticity in humans

Long-term potentiation (LTP) is a basic cellular mechanism underlying learning and memory. LTP-like plasticity in the visual cortex can be induced by high frequency visual stimulation in rodents and humans. Since glutamate plays a fundamental role in LTP, this study investigated if visual cortical glutamate and glutamine levels, measured by proton magnetic resonance spectroscopy (MRS), relate to visual plasticity in humans. Since plasticity requires a delicate excitation and inhibition balance, GABA was also explored. Eighteen healthy participants completed MRS and a visual fMRI paradigm. Results revealed enhanced fMRI activations after high frequency visual stimulation, suggesting visual plasticity occurred. Higher activations were associated with higher resting glutamine levels after family wise error-correction. Exploratory analyses revealed that higher resting glutamate and GABA levels were associated with visual plasticity, suggesting there may be a critical excitation-inhibition balance necessary for experience dependent plasticity. This is the first empirical evidence that resting glutamine levels and potentially glutamate and GABA levels are associated with visual plasticity in humans.

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.

Influence of Physical Activity on Human Sensory Long-Term Potentiation

A growing body of literature has explored the influence of physical activity on brain structure and function. While the mechanisms of this relationship remain largely speculative, recent research suggests that one of the effects of physical exercise is an increase in synaptic long-term potentiation (LTP). This has not yet been explored directly in humans due to the difficulty of measuring LTP non-invasively. However, we have previously established that LTP-like changes in visual-evoked potentials (VEPs) can be measured in humans. Here, we investigated whether physical fitness status affects the degree of visual sensory LTP. Using a self-report measure of physical activity, participants were split into two groups: a high-activity group, and a low-activity group. LTP was measured and compared between the two groups using the previously established electroencephalography-LTP paradigm, which assesses the degree to which the N1b component of the VEP elicited by a sine grating is potentiated (enhanced) following a rapid "tetanic" presentation of that grating. Both groups demonstrated increased negativity in the amplitude of the N1b component of the VEP immediately after presentation of the visual "tetanus," indicating potentiation. However, after a 30-min rest period, the N1b for the high-activity group remained potentiated while the N1b for the low-activity group returned to baseline. This study presents the first evidence for the impact of self-reported levels of physical activity on LTP in humans, and sheds light on potential neurological mechanisms underlying the relationship between physical fitness and cognition.

LTP-like plasticity in the visual system and in the motor system appear related in young and healthy subjects

LTP-like plasticity measured by visual evoked potentials (VEP) can be induced in the intact human brain by presenting checkerboard reversals. Also associated with LTP-like plasticity, around two third of participants respond to transcranial magnetic stimulation (TMS) with a paired-associate stimulation (PAS) protocol with a potentiation of their motor evoked potentials. LTP-like processes are also required for verbal and motor learning tasks. We compared effect sizes, responder rates and intercorrelations as well as the potential influence of attention between these four assessments in a group of 37 young and healthy volunteers. We observed a potentiation effect of the N75 and P100 VEP component which positively correlated with plasticity induced by PAS. Subjects with a better subjective alertness were more likely to show PAS and VEP potentiation. No correlation was found between the other assessments. Effect sizes and responder rates of VEP potentiation were higher compared to PAS. Our results indicate a high variability of LTP-like effects and no evidence for a system-specific nature. As a consequence, studies wishing to assess individual levels of LTP-like plasticity should employ a combination of multiple assessments.

In vivo evidence for neuroplasticity in older adults

Neuroplasticity can be conceptualized as an intrinsic property of the brain that enables modification of function and structure in response to environmental demands. Neuroplastic strengthening of synapses is believed to serve as a critical mechanism underlying learning, memory, and other cognitive functions. Ex vivo work investigating neuroplasticity has been done on hippocampal slices using high frequency stimulation. However, in vivo neuroplasticity in humans has been difficult to demonstrate. Recently, a long-term potentiation-like phenomenon, a form of neuroplastic change, was identified in young adults by differences in visual evoked potentials (VEPs) that were measured before and after tetanic visual stimulation (TVS). The current study investigated whether neuroplastic changes in the visual pathway can persist in older adults. Seventeen healthy subjects, 65 years and older, were recruited from the community. Subjects had a mean age of 77.4 years, mean education of 17 years, mean MMSE of 29.1, and demonstrated normal performance on neuropsychological tests. 1Hz checkerboard stimulation, presented randomly to the right or left visual hemi-field, was followed by 2min of 9Hz stimulation (TVS) to one hemi-field. After 2min of rest, 1Hz stimulation was repeated. Temporospatial principal component analysis was used to identify the N1b component of the VEPs, at lateral occipital locations, in response to 1Hz stimulation pre- and post-TVS. Results showed that the amplitude of factors representing the early and late N1b component was substantially larger after tetanic stimulation. These findings indicate that high frequency visual stimulation can enhance the N1b in cognitively high functioning old adults, suggesting that neuroplastic changes in visual pathways can continue into late life. Future studies are needed to determine the extent to which this marker of neuroplasticity is sustained over a longer period of time, and is influenced by age, cognitive status, and neurodegenerative disease.

Brain-derived neurotrophic factor Val66Met polymorphism, human memory, and synaptic neuroplasticity

Some people have much better memory than others, and there is compelling evidence that a considerable proportion of this variation in memory ability is genetically inherited. A form of synaptic plasticity known as long-term potentiation (LTP) is the principal candidate mechanism underlying memory formation in neural circuits, and it might be expected, therefore, that a genetic influence on the degree of LTP might in turn influence memory abilities. Of the genetic variations thought to significantly influence mnemonic ability in humans, the most likely to have its effect via LTP is a single nucleotide polymorphism affecting brain-derived neurotrophic factor [BDNF (Val66Met)]. However, although it is likely that BDNF influences memory via a modulation of acute plasticity (i.e., LTP), BDNF also has considerable influence on structural development of neural systems. Thus, the influence of BDNF (Val66Met) on mnemonic performance via influences of brain structure as well as function must also be considered. In this brief review, we will describe the phenomenon of LTP and its study in non-human animals. We will discuss the relatively recent attempts to translate this work to studies in humans. We will describe how this has enabled investigation of the effect of the BDNF polymorphism on LTP, on brain structure, and on memory performance.

Impaired visual cortical plasticity in schizophrenia

BACKGROUND

Impaired cortical plasticity may be part of the core pathophysiology of schizophrenia (SZ). Long-term potentiation is a form of neuroplasticity that has been recently demonstrated in humans by showing that repetitive visual stimulation produces lasting enhancement of visual evoked potentials (VEP). Using this paradigm, we examined whether visual cortical plasticity is impaired in SZ.

METHODS

Electroencephalographic data were recorded from 19 SZ and 22 healthy control (HC) subjects during a visual long-term potentiation paradigm. Visual evoked potentials were elicited by standard visual stimuli (∼.83 Hz, 2-minute blocks) at baseline and at 2, 4, and 20 minutes following exposure to visual high-frequency stimulation (HFS) (∼8.8 Hz, 2 minutes) designed to induce VEP potentiation. To ensure attentiveness during VEP assessments, subjects responded with a button press to infrequent (10%) target stimuli. Visual evoked potentials were subjected to principal components analysis. Two negative-voltage components prominent over occipital-parietal electrode sites were evident at 92 msec (C1) and at 146 msec (N1b). Changes in C1 and N1b component scores from baseline to the post-HFS assessments were compared between groups.

RESULTS

High-frequency stimulation produced sustained potentiation of visual C1 and N1b in HCs but not in SZs. The HCs and SZs had comparable HFS-driven electroencephalographic visual steady state responses. However, greater visual steady state responses to the HFS predicted greater N1b potentiation in HCs but not in SZs. Schizophrenia patients with greater N1b potentiation decreased their reaction times to target stimuli.

CONCLUSIONS

Visual cortical plasticity is impaired in schizophrenia, consistent with hypothesized deficits in N-methyl-D-aspartate receptor function.

Electrophysiological assessment of auditory stimulus-specific plasticity in schizophrenia

BACKGROUND

Disrupted neuroplasticity may be an important aspect of the neural basis of schizophrenia. We used event-related brain potentials (ERPs) to assay neuroplasticity after auditory conditioning in chronic schizophrenia patients (SZ) and matched healthy control subjects (HC).

METHODS

Subjects (15 HC, 14 SZ) performed an auditory oddball task during electroencephalogram recording before and after auditory tetanic stimulation (Pre/Post Blocks). Each oddball block consisted of 1000-Hz and 1500-Hz standards and 400-Hz targets. During tetanic conditioning, 1000-Hz tones were presented at 11 Hz for 2.4 min. We analyzed the standard trials, comparing the ERPs evoked by the tetanized stimuli (1000 Hz tones: TS+) and untetanized stimuli (1500 Hz tones: TS-) in the Post Blocks with ERPs from the Pre Blocks (averaged into Baseline ERPs).

RESULTS

In Post Block 1 in HC, TS+ tones evoked a negative shift (60-350 msec) at right temporal electrodes relative to Baseline. No pre-/post-tetanus effects were found in SZ. In Post Block 2 in HC, TS+ tones evoked a positive shift (200-300 msec) at bilateral frontal electrodes. In SZ, TS+ tones evoked a positive shift (100-400 msec) at right frontotemporal electrodes. No pre-/post-tetanus effects were found in either subject group for the TS- tones. The right temporal Post Block 1 and 2 effects were correlated in SZ, suggesting a trade-off in the expression of these effects.

CONCLUSIONS

These results suggest that stimulus-specific auditory neuroplasticity is abnormal in schizophrenia. The electrophysiologic assessment of stimulus-specific plasticity may yield novel targets for drug treatment in schizophrenia.

Stimulus-selective response plasticity in the visual cortex: an assay for the assessment of pathophysiology and treatment of cognitive impairment associated with psychiatric disorders

Long-term potentiation (LTP) is a form of experimentally induced enhancement of chemical synaptic transmission that has long been proposed as a model of the endogenous processes of synaptic plasticity that mediate memory. There is a large body of evidence that the molecular mechanisms underlying experimentally induced LTP also subserve various forms of naturally occurring, experience-dependent synaptic plasticity in animals and humans. Here we describe a phenomenon called stimulus-specific response potentiation (SRP), which occurs in the primary visual cortex of mice as a result of repeated exposure to visual stimuli and is believed to reveal the mechanisms that underlie perceptual learning. We first describe evidence that SRP represents naturally occurring LTP of thalamo-cortical synaptic transmission. We then discuss the potential value of SRP as a preclinical assay for the assessment of putative drug treatments on synaptic plasticity. Stimulus-specific response potentiation is not only easy to assay and robust but captures features of feed-forward glutamatergic function and visual learning that are deficient in human psychiatric disorders, notably including schizophrenia. We suggest that phenomena analogous to SRP in humans are likely to be useful biomarkers of altered cortical LTP and of treatment response in diseases associated with impaired cognition.

Translating long-term potentiation from animals to humans: a novel method for noninvasive assessment of cortical plasticity

Long-term potentiation (LTP) is a synaptic mechanism underlying learning and memory that has been studied extensively in laboratory animals. The study of LTP recently has been extended into humans with repetitive sensory stimulation to induce cortical LTP. In this review article, we will discuss past results from our group demonstrating that repetitive sensory stimulation (visual or auditory) induces LTP within the sensory cortex (visual/auditory, respectively) and can be measured noninvasively with electroencephalography or functional magnetic resonance imaging. We will discuss a number of studies that indicate that this form of LTP shares several characteristics with the synaptic LTP described in animals: it is frequency dependent, long-lasting (> 1 hour), input-specific, depotentiates with low-frequency stimulation, and is blocked by N-methyl-D-aspartate receptor blockers in rats. In this review, we also present new data with regard to the behavioral significance of human sensory LTP. These advances will permit enquiry into the functional significance of LTP that has been hindered by the absence of a human model. The ability to elicit LTP with a natural sensory stimulus noninvasively will provide a model system allowing the detailed examination of synaptic plasticity in normal subjects and might have future clinical applications in the diagnosis and assessment of neuropsychiatric and neurocognitive disorders.

About similar characteristics of visual perceptual learning and LTP

Perceptual learning is an implicit form of learning which induces long-lasting perceptual enhancements. Perceptual learning shows intriguing characteristics. For example, a minimal number of trials per session is needed for learning and the interleaved presentation of more than one stimulus type can hinder learning. Here, we show that these and other characteristics of perceptual learning are very similar to characteristics of long-term potentiation (LTP), the basic mechanism of memory formation. We outline these characteristics and discuss results of electrophysiological experiments which indirectly link LTP and perceptual learning.

Evidence for impaired neocortical synaptic plasticity in bipolar II disorder

BACKGROUND

Synaptic plasticity might play an important role in the pathophysiology and treatment of bipolar disorders. There is, however, a paucity of human evidence supporting this hypothesis, mainly due to a lack of methods for noninvasive assessment of synaptic plasticity. It has recently been demonstrated that plasticity of the visual evoked potential (VEP) induced by repeated visual stimulation might reflect synaptic plasticity. In this study, we examined VEP plasticity in healthy control subjects and patients with bipolar II disorder (BD-II).

METHODS

Forty healthy control subjects and 26 individuals with a DSM-IV diagnosis of BD-II matched for age and gender participated. The VEPs were evoked by checkerboard reversal stimulation before and after a modulation block of prolonged (10 min) visual stimulation.

RESULTS

The modulation block resulted in significant VEP plasticity in healthy control subjects. The VEP plasticity was significantly impaired in patients with BD-II. Explorative analyses indicated a trend toward a less severe impairment in medicated than in unmedicated patients.

CONCLUSIONS

Visual evoked potential plasticity might represent a reliable and robust assay for studies of synaptic plasticity in vivo in humans. In addition, our findings support the hypothesis of impaired synaptic plasticity in BD-II. Longitudinal studies are needed to fully clarify the effects of medication and mood state on VEP plasticity.

Visual experience induces long-term potentiation in the primary visual cortex

Stimulus-specific response potentiation (SRP) is a robust form of experience-dependent plasticity that occurs in primary visual cortex. In awake mice, visual evoked potentials (VEPs) recorded in layer 4 of binocular visual cortex undergo increases in amplitude with repeated presentation of a sinusoidal grating stimulus over days. This effect is highly specific to the experienced stimulus. Here, we test whether the mechanisms of thalamocortical long-term potentiation (LTP), induced with a theta burst electrical stimulation (TBS) of the dorsal lateral geniculate nucleus, are sufficient to account for SRP. First, we demonstrate that LTP similarly enhances the amplitude of VEPs, but in a way that generalizes across multiple stimuli, spatial frequencies, and contrasts. Second, we show that LTP occludes the subsequent expression of SRP. Third, we reveal that previous SRP occludes TBS-induced LTP of the VEP evoked by the experienced stimulus, but not by unfamiliar stimuli. Finally, we show that SRP is rapidly and selectively reversed by local cortical infusion of a peptide that inhibits PKMζ, a constitutively active kinase known to maintain NMDA receptor-dependent LTP and memory. Thus, SRP is expressed by the same core mechanisms as LTP. SRP therefore provides a simple assay to assess the integrity of LTP in the intact nervous system. Moreover, the results suggest that LTP of visual cortex, like SRP, can potentially be exploited to improve vision.

Long-term potentiation and long-term depression: a clinical perspective

Long-term potentiation and long-term depression are enduring changes in synaptic strength, induced by specific patterns of synaptic activity, that have received much attention as cellular models of information storage in the central nervous system. Work in a number of brain regions, from the spinal cord to the cerebral cortex, and in many animal species, ranging from invertebrates to humans, has demonstrated a reliable capacity for chemical synapses to undergo lasting changes in efficacy in response to a variety of induction protocols. In addition to their physiological relevance, long-term potentiation and depression may have important clinical applications. A growing insight into the molecular mechanisms underlying these processes, and technological advances in non-invasive manipulation of brain activity, now puts us at the threshold of harnessing long-term potentiation and depression and other forms of synaptic, cellular and circuit plasticity to manipulate synaptic strength in the human nervous system. Drugs may be used to erase or treat pathological synaptic states and non-invasive stimulation devices may be used to artificially induce synaptic plasticity to ameliorate conditions arising from disrupted synaptic drive. These approaches hold promise for the treatment of a variety of neurological conditions, including neuropathic pain, epilepsy, depression, amblyopia, tinnitus and stroke.

Long-term potentiation (LTP) of human sensory-evoked potentials

Long-term potentiation (LTP) is the principal candidate synaptic mechanism underlying learning and memory, and has been studied extensively at the cellular and molecular level in laboratory animals. Inquiry into the functional significance of LTP has been hindered by the absence of a human model as, until recently, LTP has only been directly demonstrated in humans in isolated cortical tissue obtained from patients undergoing surgery, where it displays properties identical to those seen in non-human preparations. In this brief review, we describe the results of paradigms recently developed in our laboratory for inducing LTP-like changes in visual-, and auditory-evoked potentials. We describe how rapid, repetitive presentation of sensory stimuli leads to a persistent enhancement of components of sensory-evoked potential in normal humans. Experiments to date, investigating the locus, stimulus specificity, and NMDA receptor dependence of these LTP-like changes suggest that they have the essential characteristics of LTP seen in experimental animals. The ability to elicit LTP from non-surgical patients will provide a human model system allowing the detailed examination of synaptic plasticity in normal subjects and may have future clinical applications in the assessment of cognitive disorders. Copyright © 2010 John Wiley & Sons, Ltd. For further resources related to this article, please visit the WIREs website.

Induction of orientation-specific LTP-like changes in human visual evoked potentials by rapid sensory stimulation

Recent research suggests that rapid visual stimulation can induce long-term potentiation-like effects non-invasively in humans. However, to date, this research has provided only limited evidence for input-specificity, a fundamental property of cellular long-term potentiation. In the present study we extend the evidence for input-specificity by investigating the effect of stimulus orientation. We use sine wave gratings of two different orientations to show that rapid visual stimulation can induce orientation-specific potentiation, as indexed by changes in the amplitude of a late phase of the N1 complex of the visual-evoked potential. This result suggests that discrete populations of orientation-tuned neurons can be selectively potentiated by rapid visual stimulation. Furthermore, our results support earlier studies that have suggested that the locus of potentiation induced by rapid visual stimulation is visual cortex.

Long-term plasticity of visually evoked potentials in humans is altered in major depression

BACKGROUND

Long-term synaptic plasticity is a ubiquitous form of neuronal plasticity that regulates the strength of synaptic transmission in many brain areas. However, most data on long-term potentiation and long-term depression rely on research in animal brain slices. The role of synaptic plasticity in physiology and pathology of the functioning human brain remains obscure. Considering recent studies that provided evidence for a dysfunction of brain plasticity as the neurobiological basis of affective disorders, we assessed neural transmission and its plastic modulation in the visual pathway in healthy control subjects and patients with major depression.

METHODS

Recordings of visually evoked potentials (VEPs) in humans.

RESULTS

Prolonged visual presentation of checkerboard reversals resulted in a sustained amplitude modulation of early components of subsequent VEPs. After a 10-min block of visual stimulation (two checkerboard reversals per second), the C1 component was reduced, whereas P1 and N1 were both significantly increased for >30 min. Chronic application of the selective serotonin reuptake inhibitor sertraline in healthy control subjects augmented these effects, whereas the polarity of the modulation was inverted in patients with severe major depression. Moreover, early VEP amplitudes were decreased in depressed patients when compared with matched control subjects and increased in normal control subjects after chronic intake of an antidepressant.

CONCLUSIONS

Our results demonstrate that stimulus-induced response plasticity of VEPs can be induced in the human brain and is sharing properties with hebbian forms of synaptic plasticity. Major depression and antidepressant treatment of healthy control subjects differentially modulate amplitude and plasticity of evoked potentials. This study provides direct evidence in humans for a central role of synaptic plasticity in the pathophysiology of depression.