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

Study protocol for "Psilocybin in patients with fibromyalgia: brain biomarkers of action"

Background

Chronic pain is a leading cause of disability worldwide. Fibromyalgia is a particularly debilitating form of widespread chronic pain. Fibromyalgia remains poorly understood, and treatment options are limited or moderately effective at best. Here, we present a protocol for a mechanistic study investigating the effects of psychedelic-assisted-therapy in a fibromyalgia population. The principal focus of this trial is the central mechanism(s) of psilocybin-therapy i.e., in the brain and on associated mental schemata, primarily captured by electroencephalography (EEG) recordings of the acute psychedelic state, plus pre and post Magnetic Resonance Imaging (MRI).

Methods

Twenty participants with fibromyalgia will complete 8 study visits over 8 weeks. This will include two dosing sessions where participants will receive psilocybin at least once, with doses varying up to 25mg. Our primary outcomes are 1) Lempel-Ziv complexity (LZc) recorded acutely using EEG, and the 2) the (Brief Experiential Avoidance Questionnaire (BEAQ) measured at baseline and primary endpoint. Secondary outcomes will aim to capture broad aspects of the pain experience and related features through neuroimaging, self-report measures, behavioural paradigms, and qualitative interviews. Pain Symptomatology will be measured using the Brief Pain Inventory Interference Subscale (BPI-IS), physical and mental health-related function will be measured using the 36-Item Short Form Health Survey (SF-36). Further neurobiological investigations will include functional MRI (fMRI) and diffusion tensor imaging (changes from baseline to primary endpoint), and acute changes in pre- vs post-acute spontaneous brain activity - plus event-related potential functional plasticity markers, captured via EEG.

Discussion

The results of this study will provide valuable insight into the brain mechanisms involved in the action of psilocybin-therapy for fibromyalgia with potential implications for the therapeutic action of psychedelic-therapy more broadly. It will also deliver essential data to inform the design of a potential subsequent RCT.

Exercise-induced cortical disinhibition mediates the relationship between fitness and memory in older adults

Regular exercise benefits learning and memory in older adults, but the neural mechanisms mediating these effects remain unclear. Evidence in young adults indicates that acute exercise creates a favourable environment for synaptic plasticity by enhancing cortical disinhibition. As such, we investigated whether plasticity-related disinhibition mediated the relationship between cardiorespiratory fitness and memory function in healthy older adults (n = 16, mean age = 66.06). Participants completed a graded maximal exercise test and assessments of visual and verbal memory, followed by two counterbalanced sessions involving 20 min of either high-intensity interval training exercise or rest. Disinhibition was measured following intermittent theta burst stimulation via paired-pulse transcranial magnetic stimulation. In line with our hypotheses, we observed a positive correlation between cardiorespiratory fitness and verbal memory, which was mediated by plasticity-related cortical disinhibition. Our novel finding implicates cortical disinhibition as a mechanism through which the effects of acute bouts of exercise may translate to improved memory in older adults. This finding extends current understanding of the physiological mechanisms underlying the positive influence of cardiorespiratory fitness for memory function in older adults, and further highlights the importance of promoting exercise engagement to maintain cognitive health in later life. KEY POINTS: There are well established benefits of regular exercise for memory function in older adults, but the mechanisms are unclear. Cortical disinhibition is important for laying down new memories, and is enhanced following acute exercise in young adults, suggesting it is a potential mechanism underlying these benefits in ageing. Older adults completed a fitness test and assessments of memory, followed by two sessions involving either 20 min of exercise or rest. Disinhibition was measured following intermittent theta burst stimulation via paired-pulse transcranial magnetic stimulation. Cardiorespiratory fitness was positively associated with memory performance. Higher fitness was associated with enhanced cortical disinhibition following acute exercise. Cortical disinhibition completely mediated the relationship between fitness and memory. This novel finding provides a mechanistic account for the positive influence of cardiorespiratory fitness on memory in later life, and emphasises the importance of regular exercise for cognitive health in older populations.

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 .

Stability of steady-state visual evoked potential contrast response functions

Repetitive sensory stimulation has been shown to induce neuroplasticity in sensory cortical circuits, at least under certain conditions. We measured the plasticity-inducing effect of repetitive contrast-reversal-sweep steady-state visual-evoked potential (ssVEP) stimuli, hoping to employ the ssVEP's high signal-to-noise electrophysiological readout in the study of human visual cortical neuroplasticity. Steady-state VEP contrast-sweep responses were measured daily for 4 days (four 20-trial blocks per day, 20 participants). No significant neuroplastic changes in response amplitude were observed either across blocks or across days. Furthermore, response amplitudes were stable within-participant, with measured across-block and across-day coefficients of variation (CV = SD/mean) of 15-20 ± 2% and 22-25 ± 2%, respectively. Steady-state VEP response phase was also highly stable, suggesting that temporal processing delays in the visual system vary by at most 2-3 ms across blocks and days. While we fail to replicate visual stimulation-dependent cortical plasticity, we show that contrast-sweep steady-state VEPs provide a stable human neurophysiological measure well suited for repeated-measures longitudinal studies.

Quercetin-3-O-β-d-glucuronide in the Nuciferine Leaf Polyphenol Extract Promotes Neurogenesis Involving the Upregulation of the Tropomyosin Receptor Kinase (Trk) Receptor and AKT/Phosphoinositide 3-Kinase Signaling Pathway

Neurogenesis is crucial during the human lifespan for the maintenance of synaptic plasticity and normal function. The impairment of hippocampal neurogenesis in adults may lead to neurodegenerative disease, such as Alzheimer's disease. Miquelianin (quercetin-3-O-β-d-glucuronide, Q3GA) is a constituent of the nuciferine leaf polyphenol extract (NLPE), and it has protective effects against neurodegeneration. In this study, we examined the effect of the NLPE on neurogenesis and the mechanisms underlying Q3GA on neurogenesis. We fed 24-week-old male C57BL/6 mice with 0.1 or 0.25% NLPE for 2 weeks. NLPE treatment increased small spindle-shaped stem cell numbers in the subgranular zone and the number of doublecortin (DCX)- and neuron-specific nuclear protein (NeuN)-expressing neurons. HT22, a hippocampal cell line, treated with Q3GA revealed significant neurite growth and upregulated TrkR and PI3K/Akt levels. The evidence from a model of retinoic acid-induced SH-SY5Y cell differentiation showed that Q3GA or NLPE increases neurite growth significantly. Taken together, the NLPE containing Q3GA to promote neurogenesis involving the upregulation of TrkR and the PI3K/Akt signaling pathway might be potentiated as an alternative strategy for the treatment of neurodegeneration.

An open-label pilot trial assessing tolerability and feasibility of LSD microdosing in patients with major depressive disorder (LSDDEP1)

BACKGROUND

Globally, an estimated 260 million people suffer from depression [1], and there is a clear need for the development of new, alternative antidepressant therapies. In light of problems with the tolerability and efficacy of available treatments [2], a global trend is emerging for patients to self-treat depression with microdoses of psychedelic drugs such as lysergic acid diethylamide (LSD) and psilocybin [3]. Beyond anecdotal reports from those who self-medicate in this way, few clinical trials have evaluated this practice. In our recently published phase 1 study in healthy volunteers [4], we determined that LSD microdosing was relatively safe and well tolerated in that cohort. Furthermore, the data demonstrated that conducting such microdosing trials is broadly feasible, with excellent adherence and compliance to the regimen observed. In this open-label pilot trial of patients with major depressive disorder (LSDDEP1), we will test the tolerability and feasibility of an 8-week regimen of LSD microdosing in this patient group prior to a larger subsequent randomised controlled trial (LSDDEP2).

METHODS

Twenty patients meeting the DSM-5 criteria for major depressive disorder will receive an 8-week LSD microdosing treatment regimen. The treatment protocol will use a sublingual formulation of LSD (MB-22001) delivered twice per week under a titration schedule using a dose of 5-15 µg. Tolerability will be assessed by quantifying the percentage of participants who withdraw from the trial due to adverse events attributable to the treatment regimen, while feasibility will be assessed by quantifying the percentage of attended clinic visits once enrolled. To determine whether there is any antidepressant response to the LSD microdosing regimen, MADRS scores will be assessed at baseline and 2, 4, 6, and 8 weeks after the commencement of the regimen.

DISCUSSION

The results of LSDDEP1 will provide valuable information regarding the tolerability and feasibility of a proposed LSD microdosing regimen in patients with MDD. Such information is critically important to optimise trial design prior to commencing a subsequent and more resource-intensive randomised controlled trial.

TRIAL REGISTRATION

ANZCTR, ACTRN12623000486628. Registered on 12 May 2023.

Baseline long-term potentiation-like cortical plasticity is associated with longitudinal cortical thinning in healthy adults and in adults with bipolar disorder type II

The precise neurobiological processes underlying cerebral cortical thinning in aging and psychiatric illnesses remain undetermined, yet aging- and synaptic dysfunction-related loss of synapses are potentially important mechanisms. We used long-term potentiation-like plasticity of the visual evoked potential as an index of synaptic function in the cortex and hypothesized that plasticity at baseline would be negatively associated with future cortical thinning in healthy adults and in adults with bipolar disorder type II. Thirty-two healthy adults and 15 adults with bipolar disorder type II underwent electroencephalography-based measurement of visual evoked potential plasticity and 3T magnetic resonance imaging of the brain at baseline and a follow-up brain scan on average 2.3 years later. The relationships between visual evoked potential plasticity at baseline and longitudinal cortical thickness changes were examined using Freesurfer and the Permutation Analysis of Linear Models tool. The analyses showed a negative association between the plasticity of the N1 visual evoked potential amplitude at baseline and thinning rate in the medial and lateral parietal and medial occipital cortices in healthy adults and in the right medial occipital cortex in the total sample of healthy adults and adults with bipolar disorder type II, indicating greater thinning over time in subjects with less N1 plasticity (pFWER  < .05). Although preliminary, the results indicate an association between visual evoked potential plasticity and the future rate of cortical thinning in healthy adults and in bipolar disorder type II, supporting the hypothesis that cortical thinning might be related to synaptic dysfunction.

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.

Canalization and plasticity in psychopathology

This theoretical article revives a classical bridging construct, canalization, to describe a new model of a general factor of psychopathology. To achieve this, we have distinguished between two types of plasticity, an early one that we call 'TEMP' for 'Temperature or Entropy Mediated Plasticity', and another, we call 'canalization', which is close to Hebbian plasticity. These two forms of plasticity can be most easily distinguished by their relationship to 'precision' or inverse variance; TEMP relates to increased model variance or decreased precision, whereas the opposite is true for canalization. TEMP also subsumes increased learning rate, (Ising) temperature and entropy. Dictionary definitions of 'plasticity' describe it as the property of being easily shaped or molded; TEMP is the better match for this. Importantly, we propose that 'pathological' phenotypes develop via mechanisms of canalization or increased model precision, as a defensive response to adversity and associated distress or dysphoria. Our model states that canalization entrenches in psychopathology, narrowing the phenotypic state-space as the agent develops expertise in their pathology. We suggest that TEMP - combined with gently guiding psychological support - can counter canalization. We address questions of whether and when canalization is adaptive versus maladaptive, furnish our model with references to basic and human neuroscience, and offer concrete experiments and measures to test its main hypotheses and implications. This article is part of the Special Issue on "National Institutes of Health Psilocybin Research Speaker Series".

Psychedelics therapeutics: What we know, what we think, and what we need to research

Psychedelic therapy is perhaps the most exciting new development in psychiatry. Not only does it offer a radical new approach to treatment where mainstream approaches have proven ineffective, but the growing evidence for transdiagnostic efficacy is eliciting a re-think of current diagnostic and symptom-specific approaches to psychiatry. This excitement has led to a massive investment in this field with many tens of new pharmaceutical companies being set up to research the effects of known psychedelics and develop new patentable molecules. Whilst this enthusiasm is to be welcomed, it is important that new research is properly grounded in established facts and reflects current knowledge. In this commentary we lay out the knowledge framework that should be taken into account by all innovative researchers in this field.

Sustained effects of single doses of classical psychedelics in humans

The serotonergic classical psychedelics include compounds that primarily activate the brain's serotonin 2 A receptor (5-HT2AR), such as LSD, psilocybin, and DMT (ayahuasca). The acute effects of these compounds are well-known as are their ability to increase the emotional state both in healthy people and in those with neuropsychiatric disorders. In particular psilocybin, the psychoactive constituent in "magic mushrooms", has shown great potential for treatment of anxiety and depression. A unique and compelling feature of psychedelics is that intake of just a single psychedelic dose is associated with long-lasting effects. This includes effects on personality, e.g., higher openness, and amelioration of depressive symptoms. This review focuses on these stunning effects and summarizes our current knowledge on which behavioral, biochemical, neuroimaging, and electrophysiological data support that the intriguing effects of psychedelics on the human brain and mind are based on neural plasticity. The review also points to so far understudied areas and suggests research questions to be addressed in future studies which potentially can help to understand the intriguing long-term effects after intake of a single (or a few) psychedelic doses.

Long term potentiation-like neural plasticity and performance-based memory function

OBJECTIVE

Experience-dependent modulation of the visual evoked potential (VEP) has emerged as a promising non-invasive proxy for assaying long term potentiation (LTP)-like plasticity in the cerebral cortex. LTP is considered the principal candidate mechanism underlying learning and memory. There is, however, a paucity of evidence exploring associations between LTP-like plasticity and performance-based learning and memory. The present study aimed to explore the relationship between VEP-plasticity and higher-order learning and memory in healthy adults.

METHOD

Visual and verbal learning and memory was assessed using the Aggie Figures Learning Test (AFLT) and the Rey Auditory Verbal Learning Test (RAVLT). The study included 111 healthy adults (61.1% females; mean age 37.6 years, range 17-71) who underwent a VEP paradigm employing visual high-frequency stimulation to induce a change in visual evoked responses recorded by scalp EEG. In addition, a more comprehensive neuropsychological assessment was administered.

RESULTS

Several significant moderate age-corrected positive correlations were found between modulation of the later VEP components (N1 and P1-N1 peak-to-peak) and both visual and verbal learning and memory performance. Further, there were significant differences in learning and memory performance between participants showing a higher degree of modulation (>1 SD above mean) compared to participants showing a lower degree of modulation. No significant associations were found between VEP-plasticity and other neurocognitive domains.

CONCLUSIONS

The current results suggest that LTP-like plasticity indexed by VEP modulation reflect processes specific to learning and memory. Future research is needed to further delineate the complex relationship between neural plasticity and learning and memory, specifically concerning possible clinical implications in populations with deficits in learning and memory function.

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.

Epigenetic conditioning induces intergenerational resilience to dementia in a mouse model of vascular cognitive impairment

INTRODUCTION

Epigenetic stimuli induce beneficial or detrimental changes in gene expression, and consequently, phenotype. Some of these phenotypes can manifest across the lifespan-and even in subsequent generations. Here, we used a mouse model of vascular cognitive impairment and dementia (VCID) to determine whether epigenetically induced resilience to specific dementia-related phenotypes is heritable by first-generation progeny.

METHODS

Our systemic epigenetic therapy consisted of 2 months of repetitive hypoxic "conditioning" (RHC) prior to chronic cerebral hypoperfusion in adult C57BL/6J mice. Resultant changes in object recognition memory and hippocampal long-term potentiation (LTP) were assessed 3 and 4 months later, respectively.

RESULTS

Hypoperfusion-induced memory/plasticity deficits were abrogated by RHC. Moreover, similarly robust dementia resilience was documented in untreated cerebral hypoperfused animals derived from RHC-treated parents.

CONCLUSIONS

Our results in experimental VCID underscore the efficacy of epigenetics-based treatments to prevent memory loss, and demonstrate for the first time the heritability of an induced resilience to dementia.

Enhancement of visual perception by combining transcranial electrical stimulation and visual perceptual training

The visual system remains highly malleable even after its maturity or impairment. Our visual function can be enhanced through many ways, such as transcranial electrical stimulation (tES) and visual perceptual learning (VPL). TES can change visual function rapidly, but its modulation effect is short-lived and unstable. By contrast, VPL can lead to a substantial and long-lasting improvement in visual function, but extensive training is typically required. Theoretically, visual function could be further improved in a shorter time frame by combining tES and VPL than by solely using tES or VPL. Vision enhancement by combining these two methods concurrently is both theoretically and practically significant. In this review, we firstly introduced the basic concept and possible mechanisms of VPL and tES; then we reviewed the current research progress of visual enhancement using the combination of two methods in both general and clinical population; finally, we discussed the limitations and future directions in this field. Our review provides a guide for future research and application of vision enhancement and restoration by combining VPL and tES.

Construction of complex memories via parallel distributed cortical–subcortical iterative integration

The construction of complex engrams requires hippocampal-cortical interactions. These include both direct interactions and ones via often-overlooked subcortical loops. Here, we review the anatomical organization of a hierarchy of parallel ‘Papez’ loops through the hypothalamus that are homologous in mammals from rats to humans. These hypothalamic loops supplement direct hippocampal-cortical connections with iterative re-processing paced by theta rhythmicity. We couple existing anatomy and lesion data with theory to propose that recirculation in these loops progressively enhances desired connections, while reducing interference from competing external goals and internal associations. This increases the signal-to-noise ratio in the distributed engrams (neocortical and cerebellar) necessary for complex learning and memory. The hypothalamic nodes provide key motivational input for engram enhancement during consolidation.

Study Protocol for "Psilocybin as a Treatment for Anorexia Nervosa: A Pilot Study"

Background: Anorexia nervosa (AN) is a serious and life-threatening psychiatric condition. With a paucity of approved treatments, there is a desperate need for novel treatment avenues to be explored. Here, we present (1) an overview of the ways through which Public Patient Involvement (PPI) has informed a trial of psilocybin-assisted therapy for AN and (2) a protocol for a pilot study of psilocybin-assisted therapy in AN currently underway at Imperial College London. The study aims to assess the feasibility, brain mechanisms and preliminary outcomes of treating anorexia nervosa with psilocybin. Methods: (1) PPI: Across two online focus groups, eleven individuals with lived experience of AN were presented with an overview of the protocol. Their feedback not only identified solutions to possible barriers for future participants, but also helped the research team to better understand the concept of "recovery" from the perspective of those with lived experience. (2) Protocol: Twenty female participants [21-65 years old, body mass index (BMI) 15 kg/m2 or above] will receive three oral doses of psilocybin (up to 25 mg) over a 6-week period delivered in a therapeutic environment and enveloped by psychological preparation and integration. We will work with participant support networks (care teams and an identified support person) throughout and there will be an extended remote follow-up period of 12 months. Our two-fold primary outcomes are (1) psychopathology (Eating Disorder Examination) across the 6-month follow-up and (2) readiness and motivation to engage in recovery (Readiness and Motivation Questionnaire) across the 6-week trial period. Neurophysiological outcome measures will be: (1) functional magnetic resonance imaging (fMRI) brain changes from baseline to 6-week endpoint and (2) post-acute changes in electroencephalography (EEG) activity, including an electrophysiological marker of neuronal plasticity. Discussion: The results of this pilot study will not only shed light on the acceptability, brain mechanisms, and impression of the potential efficacy of psilocybin as an adjunct treatment for AN but will be essential in shaping a subsequent Randomised Control Trial (RCT) that would test this treatment against a suitable control condition. Clinical Trial Registration: identifier: NCT04505189.

High-Frequency Visual Stimulation Primes Gamma Oscillations for Visually Evoked Phase Reset and Enhances Spatial Acuity

The temporal frequency of sensory stimulation is a decisive factor in the plasticity of perceptual detection thresholds. However, surprisingly little is known about how distinct temporal parameters of sensory input differentially recruit activity of neuronal circuits in sensory cortices. Here we demonstrate that brief repetitive visual stimulation induces long-term plasticity of visual responses revealed 24 h after stimulation and that the location and generalization of visual response plasticity is determined by the temporal frequency of the visual stimulation. Brief repetitive low-frequency stimulation (2 Hz) is sufficient to induce a visual response potentiation that is expressed exclusively in visual cortex layer 4 and in response to a familiar stimulus. In contrast, brief, repetitive high-frequency stimulation (HFS, 20 Hz) is sufficient to induce a visual response potentiation that is expressed in all cortical layers and transfers to novel stimuli. HFS induces a long-term suppression of the activity of fast-spiking interneurons and primes ongoing gamma oscillatory rhythms for phase reset by subsequent visual stimulation. This novel form of generalized visual response enhancement induced by HFS is paralleled by an increase in visual acuity, measured as improved performance in a visual detection task.

MDLSD: study protocol for a randomised, double-masked, placebo-controlled trial of repeated microdoses of LSD in healthy volunteers

BACKGROUND

Regular ingestion of sub-hallucinogenic doses of psychedelics, referred to as "microdosing", has gained increasing popularity and attention in the press and in online forums, with reported benefits across multiple cognitive and emotional domains. Rigorously controlled studies to date, however, have been limited in scope and have failed to produce results comparable to those reported in the grey literature.

METHODS

Eighty healthy male participants will receive 14 doses of placebo or 10 μg lysergic acid diethylamide orally every 3rd day over a 6-week treatment protocol. A battery of personality, creativity, mood, cognition, and EEG plasticity measures, as well as resting-state fMRI imaging, will be administered at baseline and at the end of the protocol. Creativity, mood, and plasticity measures will additionally be assessed in the acute phase of the first dose. Daily functioning will be monitored with questionnaires and a wearable sleep and activity tracker.

DISCUSSION

This study will rigorously examine the claims presented in the microdosing grey literature by pairing a comparable dosing protocol with objective measures. Potential therapeutic implications include future clinical trials to investigate microdosed psychedelics as a standalone treatment or as an augmentation of psychotherapy in the treatment of depression, addiction, eating disorders, obsessive-compulsive disorders, and palliative care.

TRIAL REGISTRATION

ACTRN12621000436875 . Registered on 19 February 2021.

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.

The aging mouse brain: cognition, connectivity and calcium

Aging is a complex process that differentially impacts multiple cognitive, sensory, neuronal and molecular processes. Technological innovations now allow for parallel investigation of neuronal circuit function, structure and molecular composition in the brain of awake behaving adult mice. Thus, mice have become a critical tool to better understand how aging impacts the brain. However, a more granular systems-based approach, which considers the impact of age on key features relating to neural processing, is required. Here, we review evidence probing the impact of age on the mouse brain. We focus on a range of processes relating to neuronal function, including cognitive abilities, sensory systems, synaptic plasticity and calcium regulation. Across many systems, we find evidence for prominent age-related dysregulation even before 12 months of age, suggesting that emerging age-related alterations can manifest by late adulthood. However, we also find reports suggesting that some processes are remarkably resilient to aging. The evidence suggests that aging does not drive a parallel, linear dysregulation of all systems, but instead impacts some processes earlier, and more severely, than others. We propose that capturing the more fine-scale emerging features of age-related vulnerability and resilience may provide better opportunities for the rejuvenation of the aged brain.

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).