Adaptive and maladaptive neural compensatory consequences of sensory deprivation-From a phantom percept perspective

Effective Connectivity Network of Aberrant Prediction Error Processing in Auditory Phantom Perception

Introduction: Prediction error (PE) is key to perception in the predictive coding framework. However, previous studies indicated the varied neural activities evoked by PE in tinnitus patients. Here, we aimed to reconcile the conflict by (1) a more nuanced view of PE, which could be driven by changing stimulus (stimulus-driven PE [sPE]) and violation of current context (context-driven PE [cPE]) and (2) investigating the aberrant connectivity networks that are engaged in the processing of the two types of PEs in tinnitus patients. Methods: Ten tinnitus patients with normal hearing and healthy controls were recruited, and a local-global auditory oddball paradigm was applied to measure the electroencephalographic difference between the two groups during sPE and cPE conditions. Results: Overall, the sPE condition engaged bottom-up and top-down connections, whereas the cPE condition engaged mostly top-down connections. The tinnitus group showed decreased sensitivity to the sPE and increased sensitivity to the cPE condition. Particularly, the auditory cortex and posterior cingulate cortex were the hubs for processing cPE in the control and tinnitus groups, respectively, showing the orientation to an internal state in tinnitus. Furthermore, tinnitus patients showed stronger connectivity to the parahippocampus and pregenual anterior cingulate cortex for the establishment of the prediction during the cPE condition. Conclusion: These results begin to dissect the role of changes in stimulus characteristics versus changes in the context of processing the same stimulus in mechanisms of tinnitus generation. Impact Statement This study delves into the number dynamics of prediction error (PE) in tinnitus, proposing a dual framework distinguishing between stimulus-driven PE (sPE) and context-driven PE (cPE). Electroencephalographic data from tinnitus patients and controls revealed distinct connectivity patterns during sPE and cPE conditions. Tinnitus patients exhibited reduced sensitivity to sPE and increased sensitivity to cPE. The auditory cortex and posterior cingulate cortex emerged as pivotal regions for cPE processing in controls and tinnitus patients, indicative of an internal state orientation in tinnitus. Enhanced connectivity to the parahippocampus and pregenual anterior cingulate cortex underscores the role of context in tinnitus pathophysiology.

A case series of musical hallucinations in psychiatry of old age-in search of the sound of silence

Musical hallucinations (MH) are the subjective experience of hearing music when none is played. They are a rare, understudied area of psychiatry. MH are more common in women and older age and have several underlying aetiologies and predisposing factors such as hearing impairment, mental illness and certain medications. There are no consensus guidelines on treatment; thus, current treatment has two broad approaches: (1) the removal of potential inciting factors (e.g. optimising hearing aids, medications) or (2) pharmacotherapy (antipsychotics, antidepressants, mood stabilisers and cognitive enhancers). This paper presents a case series of patients presenting with MH to a psychiatry of old age service in Dublin City and reviews the current literature of MH. Older age, female gender and hearing impairment are known risk factors for MH. Our findings concurred with the literature-two of three patients were female, and two of three patients suffered from hearing impairment. As this was a psychiatry of old age service, all patients were elderly. One case had a swift resolution of symptoms with a combination of an antipsychotic and antidepressant. The other two cases had limited responses to treatment despite optimising their hearing aids and trials of a number of medications at therapeutic levels. Further research into MH is needed to establish a treatment that is evidence based and symptom focused.

Chronic pain - A maladaptive compensation to unbalanced hierarchical predictive processing

The ability to perceive pain presents an interesting evolutionary advantage to adapt to an ever-changing environment. However, in the case of chronic pain (CP), pain perception hinders the capacity of the system to adapt to changing sensory environments. Similar to other chronic perceptual disorders, CP is also proposed to be a maladaptive compensation to aberrant sensory predictive processing. The local-global oddball paradigm relies on learning hierarchical rules and processing environmental irregularities at a local and global level. Prediction errors (PE) between actual and predicted input typically trigger an update of the forward model to limit the probability of encountering future PEs. It has been hypothesised that CP hinders forward model updating, reflected in increased local deviance and decreased global deviance. In the present study, we used the local-global paradigm to examine how CP influences hierarchical learning relative to healthy controls. As hypothesised, we observed that deviance in the stimulus characteristics evoked heightened local deviance and decreased global deviance of the stimulus-driven PE. This is also accompanied by respective changes in theta phase locking that is correlated with the subjective pain perception. Changes in the global deviant in the stimulus-driven-PE could also be explained by dampened attention-related responses. Changing the context of the auditory stimulus did not however show a difference in the context-driven PE. These findings suggest that CP is accompanied by maladaptive forward model updating where the constant presence of pain perception disrupts local deviance in non-nociceptive domains. Furthermore, we hypothesise that the auditory-processing based biomarker identified here could be a marker of domain-general dysfunction that could be confirmed by future research.

Influence of sensory retraining on cortical reorganization in peripheral neuropathy: A systematic review

OBJECTIVE

This study systematically reviewed the literature about sensory retraining effect in comparison to other rehabilitative techniques on cortical reorganization in patients with peripheral neuropathic pain. TYPE: Systematic review.

LITERATURE SURVEY

After an electronic search of PubMed, Web of Science, and Embase, risk of bias was assessed using the revised Cochrane risk of bias tool for randomized controlled trials and the ROBINS-1 (Risk of bias in non-randomized studies-of interventions) for non-randomized studies of intervention.

METHODOLOGY

The strength of conclusion was determined using the evidence-based guideline development approach.

SYNTHESIS

Limited evidence indicates a higher increase in cortical inhibition and a higher reduction in cortical activation during a motor task of the affected hemisphere after graded motor imagery compared to wait-list. Higher reductions in map volume (total excitability of the cortical representation) of the affected hemisphere after peripheral electrical stimulation (PES) were observed when compared to transcranial direct current stimulation (tDCS) or to sham treatment with limited evidence. No other differences in cortical excitability and representation of the affected and non-affected hemisphere were observed when comparing mirror therapy with sham therapy or tDCS, PES with sham therapy or tDCS, and graded motor imagery with wait-list.

CONCLUSIONS

Graded motor imagery and PES result in higher cortical excitability reductions of the affected hemisphere compared to wait-list, tDCS and sham treatment, respectively.

Resting state neurophysiology of agonist-antagonist myoneural interface in persons with transtibial amputation

The agonist-antagonist myoneural interface (AMI) is an amputation surgery that preserves sensorimotor signaling mechanisms of the central-peripheral nervous systems. Our first neuroimaging study investigating AMI subjects conducted by Srinivasan et al. (2020) focused on task-based neural signatures, and showed evidence of proprioceptive feedback to the central nervous system. The study of resting state neural activity helps non-invasively characterize the neural patterns that prime task response. In this study on resting state functional magnetic resonance imaging in AMI subjects, we compared functional connectivity in patients with transtibial AMI (n = 12) and traditional (n = 7) amputations (TA). To test our hypothesis that we would find significant neurophysiological differences between AMI and TA subjects, we performed a whole-brain exploratory analysis to identify a seed region; namely, we conducted ANOVA, followed by t-test statistics to locate a seed in the salience network. Then, we implemented a seed-based connectivity analysis to gather cluster-level inferences contrasting our subject groups. We show evidence supporting our hypothesis that the AMI surgery induces functional network reorganization resulting in a neural configuration that significantly differs from the neural configuration after TA surgery. AMI subjects show significantly less coupling with regions functionally dedicated to selecting where to focus attention when it comes to salient stimuli. Our findings provide researchers and clinicians with a critical mechanistic understanding of the effect of AMI amputation on brain networks at rest, which has promising implications for improved neurorehabilitation and prosthetic control.

Investigating sensitivity to multi-domain prediction errors in chronic auditory phantom perception

The perception of a continuous phantom in a sensory domain in the absence of an external stimulus is explained as a maladaptive compensation of aberrant predictive coding, a proposed unified theory of brain functioning. If this were true, these changes would occur not only in the domain of the phantom percept but in other sensory domains as well. We confirm this hypothesis by using tinnitus (continuous phantom sound) as a model and probe the predictive coding mechanism using the established local-global oddball paradigm in both the auditory and visual domains. We observe that tinnitus patients are sensitive to changes in predictive coding not only in the auditory but also in the visual domain. We report changes in well-established components of event-related EEG such as the mismatch negativity. Furthermore, deviations in stimulus characteristics were correlated with the subjective tinnitus distress. These results provide an empirical confirmation that aberrant perceptions are a symptom of a higher-order systemic disorder transcending the domain of the percept.

Sensory Gating Networks in Normal-Hearing Adults With Minimal Tinnitus

PURPOSE

The goal of this study was to observe sensory gating-related networks underlying cortical auditory evoked potential (CAEP) peak components in individuals with and without minimal tinnitus, as measured using the Tinnitus Handicap Inventory (THI). This analysis was performed on previously published sensory gating responses in normal-hearing adults with and without minimal tinnitus.

METHOD

Independent component analysis was performed for each individual CAEP gating component (Pa, P50, N1, and P2). Significant components were retained for source localization analyses within the following groups: no tinnitus, tinnitus with a THI score ≤ 6, and tinnitus with a THI score > 6. Brain source localization was performed on the gating difference wave for each component using standardized low-resolution brain electromagnetic tomography.

RESULTS

Gating-related networks were identified within each group. Different regional sources were observed between groups, with parietal sources underlying the Pa and P50 components as tinnitus severity increased. A larger prefrontal regional activation was also shown for the N1 gating component as tinnitus severity increased. These results expand upon the functional gating responses via CAEP waveforms in a previously published study.

CONCLUSIONS

The auditory gating response, as measured via CAEPs, has previously been shown to significantly correlate with an increase in tinnitus severity in adults with normal hearing. The corresponding changes in the gating response appear to be supported by different cortical regions in those without tinnitus, those with a THI score ≤ 6, and those with a THI score > 6. Next, functional differences between localized cortical regions should be tested.

Development, Insults and Predisposing Factors of the Brain's Predictive Coding System to Chronic Perceptual Disorders-A Life-Course Examination

The predictive coding theory is currently widely accepted as the theoretical basis of perception and chronic perceptual disorders are explained as the maladaptive compensation of the brain to a prediction error. Although this gives us a general framework to work with, it is still not clear who may be more susceptible and/or vulnerable to aberrations in this system. In this paper, we study changes in predictive coding through the lens of tinnitus and pain. We take a step back to understand how the predictive coding system develops from infancy, what are the different neural and bio markers that characterise this system in the acute, transition and chronic phases and what may be the factors that pose a risk to the aberration of this system. Through this paper, we aim to identify people who may be at a higher risk of developing chronic perceptual disorders as a reflection of aberrant predictive coding, thereby giving future studies more facets to incorporate in their investigation of early markers of tinnitus, pain and other disorders of predictive coding. We therefore view this paper to encourage the thinking behind the development of preclinical biomarkers to maladaptive predictive coding.

Application and underlying mechanism of acupuncture for the nerve repair after peripheral nerve injury: remodeling of nerve system

Peripheral nerve injury (PNI) is a structural event with harmful consequences worldwide. Due to the limited intrinsic regenerative capacity of the peripheral nerve in adults, neural restoration after PNI is difficult. Neurological remodeling has a crucial effect on the repair of the form and function during the regeneration of the peripheral nerve after the peripheral nerve is injured. Several studies have demonstrated that acupuncture is effective for PNI-induced neurologic deficits, and the potential mechanisms responsible for its effects involve the nervous system remodeling in the process of nerve repair. Moreover, acupuncture promotes neural regeneration and axon sprouting by activating related neurotrophins retrograde transport, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), glial cell-derived neurotrophic factor (GDNF), N-cadherin, and MicroRNAs. Peripheral nerve injury enhances the perceptual response of the central nervous system to pain, causing central sensitization and accelerating neuronal cell apoptosis. Together with this, the remodeling of synaptic transmission function would worsen pain discomfort. Neuroimaging studies have shown remodeling changes in both gray and white matter after peripheral nerve injury. Acupuncture not only reverses the poor remodeling of the nervous system but also stimulates the release of neurotrophic substances such as nerve growth factors in the nervous system to ameliorate pain and promote the regeneration and repair of nerve fibers. In conclusion, the neurological remodeling at the peripheral and central levels in the process of acupuncture treatment accelerates nerve regeneration and repair. These findings provide novel insights enabling the clinical application of acupuncture in the treatment of PNI.

Magnetic Resonance T1w/T2w Ratio in the Putamen and Cerebellum as a Marker of Cognitive Impairment in MSA: a Longitudinal Study

The exact pathophysiology of cognitive impairment in multiple system atrophy (MSA) is unclear. In our longitudinal study, we aimed to analyze (I) the relationships between cognitive functions and some subcortical structures, such as putamen and cerebellum assessed by voxel-based morphometry (VBM) and T1-weighted/T2-weighted (T1w/T2w) ratio, and (II) the neuroimaging predictors of the progression of cognitive deficits. Twenty-six patients with MSA underwent a comprehensive neuropsychological battery, motor examination, and brain MRI at baseline (T0) and 1-year follow-up (T1). Patients were then divided according to cognitive status into MSA with normal cognition (MSA-NC) and MSA with mild cognitive impairment (MCI). At T1, we divided the sample according to worsening/non worsening of cognitive status compared to baseline evaluation. Logistic regression analysis showed that age (β =  - 9.45, p = .02) and T1w/T2w value in the left putamen (β = 230.64, p = .01) were significant predictors of global cognitive status at T0, explaining 65% of the variance. Logistic regression analysis showed that ∆-values of WM density in the cerebellum/brainstem (β = 2188.70, p = .02) significantly predicted cognitive worsening at T1, explaining 64% of the variance. Our results suggest a role for the putamen and cerebellum in the cognitive changes of MSA, probably due to their connections with the cortex. The putaminal T1w/T2w ratio may deserve further studies as a marker of cognitive impairment in MSA.

Artificial gravity during a spaceflight analog alters brain sensory connectivity

Spaceflight has numerous untoward effects on human physiology. Various countermeasures are under investigation including artificial gravity (AG). Here, we investigated whether AG alters resting-state brain functional connectivity changes during head-down tilt bed rest (HDBR), a spaceflight analog. Participants underwent 60 days of HDBR. Two groups received daily AG administered either continuously (cAG) or intermittently (iAG). A control group received no AG. We assessed resting-state functional connectivity before, during, and after HDBR. We also measured balance and mobility changes from pre- to post-HDBR. We examined how functional connectivity changes throughout HDBR and whether AG is associated with differential effects. We found differential connectivity changes by group between posterior parietal cortex and multiple somatosensory regions. The control group exhibited increased functional connectivity between these regions throughout HDBR whereas the cAG group showed decreased functional connectivity. This finding suggests that AG alters somatosensory reweighting during HDBR. We also observed brain-behavioral correlations that differed significantly by group. Control group participants who showed increased connectivity between the putamen and somatosensory cortex exhibited greater mobility declines post-HDBR. For the cAG group, increased connectivity between these regions was associated with little to no mobility declines post-HDBR. This suggests that when somatosensory stimulation is provided via AG, functional connectivity increases between the putamen and somatosensory cortex are compensatory in nature, resulting in reduced mobility declines. Given these findings, AG may be an effective countermeasure for the reduced somatosensory stimulation that occurs in both microgravity and HDBR.

Altered Neural Pathways and Related Brain Remodeling: A Rat Study Using Different Nerve Reconstructions

BACKGROUND

Function recovery is related to cortical plasticity. The brain remodeling patterns induced by alterations in peripheral nerve pathways with different nerve reconstructions are unknown.

OBJECTIVE

To explore brain remodeling patterns related to alterations in peripheral neural pathways after different nerve reconstruction surgeries.

METHODS

Twenty-four female Sprague-Dawley rats underwent complete left brachial plexus nerve transection, together with the following interventions: no nerve repair (n = 8), grafted nerve repair (n = 8), and phrenic nerve transfer (n = 8). Resting-state functional MR images of brain were acquired at the end of seventh month postsurgery. Amplitude of low-frequency fluctuation (ALFF), regional homogeneity (ReHo), and functional connectivity (FC) were compared among 3 groups. Behavioral observation and electromyography assessed nerve regeneration.

RESULTS

Compared with brachial plexus injury group, ALFF and ReHo of left entorhinal cortex decreased in nerve repair and nerve transfer groups. The nerve transfer group showed increased ALFF and ReHo than nerve repair group in left caudate putamen, right accumbens nucleus shell (AcbSh), and right somatosensory cortex. The FC between right somatosensory cortex and bilateral piriform cortices and bilateral somatosensory cortices increased in nerve repair group than brachial plexus injury and nerve transfer groups. The nerve transfer group showed increased FC between right somatosensory cortex and areas including left corpus callosum, left retrosplenial cortex, right parietal association cortex, and right dorsolateral thalamus than nerve repair group.

CONCLUSION

Entorhinal cortex is a key brain area in recovery of limb function after nerve reconstruction. Nerve transfer related brain remodeling mainly involved contralateral sensorimotor areas, facilitating directional "shifting" of motor representation.

Acute exercise as active inference in chronic musculoskeletal pain, effects on gait kinematics and muscular activity in patients and healthy participants: a study protocol for a randomised controlled laboratory trial

INTRODUCTION

Chronic musculoskeletal pain is a highly prevalent, complex and distressing condition that may negatively affect all domains of life. In view of an active inference framework, and resting on the concept of allostasis, human movement per se becomes a prerequisite for health and well-being while chronic pain becomes a sign of a system unable to attenuate an allostatic load. Previous studies on different subgroups of chronic pain conditions have demonstrated alterations in gait kinematics and muscle activity, indicating shared disturbances in the motor system from long-term allostatic load. We hypothesise that such alterations exist in heterogenous populations with chronic musculoskeletal pain, and that exposure to acute and controlled exercise may attenuate these alterations. Therefore, the main aim of this study is to investigate the acute effects of exercise on gait kinematics and activity of the back and neck muscles during diverse walking conditions in patients with chronic musculoskeletal pain compared with a reference sample consisting of healthy participants.

METHODS AND ANALYSIS

This two-sample two-armed parallel randomised controlled laboratory trial will include 40 participants with chronic musculoskeletal pain (>3 months) and 40 healthy participants. Participants will be randomly allocated to either 30 min of aerobic exercise or rest. Primary outcomes are gait kinematics (walking speed, step frequency, stride length, lumbar rotation, gait stability) and muscular activity (spatial and temporal) of the back and neck during diverse walking conditions. Secondary outcomes are variability of gait kinematics and muscle activity and subjective pain ratings assessed regularly during the trial.

ETHICS AND DISSEMINATION

The study has been approved by the Regional Ethics Review Board in Uppsala, Sweden (#2018/307). Findings will be disseminated via conference presentations, publications in peer-reviewed journals and engagement with patient support groups and clinicians.

TRIAL REGISTRATION NUMBER

NCT03882333.

Understanding and Measuring the Cognitive Load of Amputees for Rehabilitation and Prosthesis Development

Objective

To derive a definition of cognitive load that is applicable for amputation as well as analyze suitable research models for measuring cognitive load during prosthesis use. Defining cognitive load for amputation will improve rehabilitation methods and enable better prosthesis design.

Data Sources

Elsevier, Springer, PLoS, IEEE Xplore, and PubMed.

Study Selection

Studies on upper limb myoelectric prostheses and neuroprostheses were prioritized. For understanding measurement, lower limb amputations and studies with individuals without lower limb amputations were included.

Data Extraction

Queries including “cognitive load,” “neural fatigue,” “brain plasticity,” “neuroprosthetics,” “upper limb prosthetics,” and “amputation” were used with peer-reviewed journals or articles. Articles published within the last 6 years were prioritized. Articles on foundational principles were included regardless of date. A total of 69 articles were found: 12 on amputation, 15 on cognitive load, 8 on phantom limb, 22 on sensory feedback, and 12 on measurement methods.

Data Synthesis

The emotional, physiological, and neurologic aspects of amputation, prosthesis use, and rehabilitation aspects of cognitive load were analyzed in conjunction with measurement methods, including resolution, invasiveness, and sensitivity to user movement and environmental noise.

Conclusions

Use of “cognitive load” remains consistent with its original definition. For amputation, 2 additional elements are needed: “emotional fatigue,” defined as an amputee's emotional response, including mental concentration and emotions, and “neural fatigue,” defined as the physiological and neurologic effects of amputation on brain plasticity. Cognitive load is estimated via neuroimaging techniques, including electroencephalography, functional magnetic resonance imaging, and functional near-infrared spectroscopy (fNIRS). Because fNIRS measures cognitive load directly, has good temporal and spatial resolution, and is not as restricted by user movement, fNIRS is recommended for most cognitive load studies.

Symptom dimensions to address heterogeneity in tinnitus

Tinnitus, the auditory phantom percept, is a well-known heterogenous disorder with multiple subtypes. Researchers and clinicians have tried to classify these subtypes according to clinical profiles, aetiologies, and response to treatment with little success. The occurrence of overlapping tinnitus subtypes suggests that the disorder exists along a continuum of severity, with no clear distinct boundaries. In this perspective, we propose a neuro-mechanical framework, viewing tinnitus as a dimensional disorder which is a complex interplay of its behavioural, biological and neurophysiological phenotypes. Moreover, we explore the potential of these dimensions as interacting networks without a common existing cause, giving rise to tinnitus. Considering tinnitus as partially overlapping, dynamically changing, interacting networks, each representing a different aspect of the unified tinnitus percept, suggests that the interaction of these networks determines the phenomenology of the tinnitus, ultimately leading to a dimensional spectrum, rather than a categorical subtyping. A combination of a robust theoretical framework and strong empirical evidence can advance our understanding of the functional mechanisms underlying tinnitus and ultimately, improve treatment strategies.

Predisposition to domain-wide maladaptive changes in predictive coding in auditory phantom perception

Tinnitus is hypothesised to be a predictive coding problem. Previous research indicates lower sensitivity to prediction errors (PEs) in tinnitus patients while processing auditory deviants corresponding to tinnitus-specific stimuli. However, based on research with patients with hallucinations and no psychosis we hypothesise tinnitus patients may be more sensitive to PEs produced by auditory stimuli that are not related to tinnitus characteristics. Specifically in patients with minimal to no hearing loss, we hypothesise a more top-down subtype of tinnitus that may be driven by maladaptive changes in an auditory predictive coding network. To test this, we use an auditory oddball paradigm with omission of global and local deviants, a measure that is previously shown to empirically characterise hierarchical prediction errors (PEs). We observe: (1) increased predictions characterised by increased pre-stimulus response and increased alpha connectivity between the parahippocampus, dorsal anterior cingulate cortex and parahippocampus, pregenual anterior cingulate cortex and posterior cingulate cortex; (2) increased PEs characterised by increased P300 amplitude and gamma activity and increased theta connectivity between auditory cortices, parahippocampus and dorsal anterior cingulate cortex in the tinnitus group; (3) increased overall feed-forward connectivity in theta from the auditory cortex and parahippocampus to the dorsal anterior cingulate cortex; (4) correlations of pre-stimulus theta activity to tinnitus loudness and alpha activity to tinnitus distress. These results provide empirical evidence of maladaptive changes in a hierarchical predictive coding network in a subgroup of tinnitus patients with minimal to no hearing loss. The changes in pre-stimulus activity and connectivity to non-tinnitus specific stimuli suggest that tinnitus patients not only produce strong predictions about upcoming stimuli but also may be predisposed to stimulus a-specific PEs in the auditory domain. Correlations with tinnitus-related characteristics may be a biomarker for maladaptive changes in auditory predictive coding.

Auditory speed processing in sighted and blind individuals

Multisensory experience is crucial for developing a coherent perception of the world. In this context, vision and audition are essential tools to scaffold spatial and temporal representations, respectively. Since speed encompasses both space and time, investigating this dimension in blindness allows deepening the relationship between sensory modalities and the two representation domains. In the present study, we hypothesized that visual deprivation influences the use of spatial and temporal cues underlying acoustic speed perception. To this end, ten early blind and ten blindfolded sighted participants performed a speed discrimination task in which spatial, temporal, or both cues were available to infer moving sounds' velocity. The results indicated that both sighted and early blind participants preferentially relied on temporal cues to determine stimuli speed, by following an assumption that identified as faster those sounds with a shorter duration. However, in some cases, this temporal assumption produces a misperception of the stimulus speed that negatively affected participants' performance. Interestingly, early blind participants were more influenced by this misleading temporal assumption than sighted controls, resulting in a stronger impairment in the speed discrimination performance. These findings demonstrate that the absence of visual experience in early life increases the auditory system's preference for the time domain and, consequentially, affects the perception of speed through audition.

Robustness of neuronal tuning to binaural sound localization cues against age-related loss of inhibitory synaptic inputs

Sound localization relies on minute differences in the timing and intensity of sound arriving at both ears. Neurons of the lateral superior olive (LSO) in the brainstem process these interaural disparities by precisely detecting excitatory and inhibitory synaptic inputs. Aging generally induces selective loss of inhibitory synaptic transmission along the entire auditory pathways, including the reduction of inhibitory afferents to LSO. Electrophysiological recordings in animals, however, reported only minor functional changes in aged LSO. The perplexing discrepancy between anatomical and physiological observations suggests a role for activity-dependent plasticity that would help neurons retain their binaural tuning function despite loss of inhibitory inputs. To explore this hypothesis, we use a computational model of LSO to investigate mechanisms underlying the observed functional robustness against age-related loss of inhibitory inputs. The LSO model is an integrate-and-fire type enhanced with a small amount of low-voltage activated potassium conductance and driven with (in)homogeneous Poissonian inputs. Without synaptic input loss, model spike rates varied smoothly with interaural time and level differences, replicating empirical tuning properties of LSO. By reducing the number of inhibitory afferents to mimic age-related loss of inhibition, overall spike rates increased, which negatively impacted binaural tuning performance, measured as modulation depth and neuronal discriminability. To simulate a recovery process compensating for the loss of inhibitory fibers, the strength of remaining inhibitory inputs was increased. By this modification, effects of inhibition loss on binaural tuning were considerably weakened, leading to an improvement of functional performance. These neuron-level observations were further confirmed by population modeling, in which binaural tuning properties of multiple LSO neurons were varied according to empirical measurements. These results demonstrate the plausibility that homeostatic plasticity could effectively counteract known age-dependent loss of inhibitory fibers in LSO and suggest that behavioral degradation of sound localization might originate from changes occurring more centrally.

The cerebellum could serve as a potential imaging biomarker of dementia conversion in patients with amyloid-negative amnestic mild cognitive impairment

BACKGROUND AND PURPOSE

As part of network-specific neurodegeneration, changes in cerebellar gray matter (GM) volume and impaired cerebello-cerebral functional networks have been reported in Alzheimer disease (AD). Compared with healthy controls, a volume loss in the cerebellum has been observed in patients with continuum of AD. However, little is known about the anatomical or functional changes in patients with clinical AD but no brain amyloidosis. We aimed to identify the relationship between cerebellar volume and dementia conversion of amyloid-negative mild cognitive impairment (MCI).

METHODS

This study was a retrospective cohort study of patients over the age 50 years with amyloid-negative amnestic MCI who visited the memory clinic of Asan Medical Center with no less than a 36-month follow-up period. All subjects underwent detailed neuropsychological tests, 3 T brain magnetic resonance imaging scans including three-dimensional T1 imaging, and fluorine-18[F¹⁸ ]-florbetaben amyloid positron emission tomography scans. A spatially unbiased atlas template of the cerebellum and brainstem was used for analyzing cerebellar GM volume.

RESULTS

During the 36 months of follow-up, 39 of 107 (36.4%) patients converted to dementia from amnestic MCI. The converter group had more severe impairments in all visual memory tasks. In terms of volumetric analysis, reduced crus I/II volume adjusted with total intracranial volume, and age was observed in the converter group.

CONCLUSIONS

Significant cerebellar GM atrophy involving the bilateral crus I/II may be a novel imaging biomarker for predicting dementia progression in amyloid-negative amnestic MCI patients.

Spontaneous brain activity underlying auditory hallucinations in the hearing-impaired

Auditory hallucinations, the perception of a sound without a corresponding source, are common in people with hearing impairment. Two forms can be distinguished: simple (i.e., tinnitus) and complex hallucinations (speech and music). Little is known about the precise mechanisms underlying these types of hallucinations. Here we tested the assumption that spontaneous activity in the auditory pathways, following deafferentation, underlies these hallucinations and is related to their phenomenology. By extracting (fractional) Amplitude of Low Frequency Fluctuation [(f)ALFF] scores from resting state fMRI of 18 hearing impaired patients with complex hallucinations (voices or music), 18 hearing impaired patients with simple hallucinations (tinnitus or murmuring), and 20 controls with normal hearing, we investigated differences in spontaneous brain activity between these groups. Spontaneous activity in the anterior and posterior cingulate cortex of hearing-impaired groups was significantly higher than in the controls. The group with complex hallucinations showed elevated activity in the bilateral temporal cortex including Wernicke's area, while spontaneous activity of the group with simple hallucinations was mainly located in the cerebellum. These results suggest a decrease in error monitoring in both hearing-impaired groups. Spontaneous activity of language-related areas only in complex hallucinations suggests that the manifestation of the spontaneous activity represents the phenomenology of the hallucination. The link between cerebellar activity and simple hallucinations, such as tinnitus, is new and may have consequences for treatment.

Frightening Complex Visual Hallucinations in an Elderly Patient with Ophthalmological Pathology and Vascular Dementia

A lady in her 90s was referred to the Later Life Team (LLT) in a rural area of the United Kingdom with complex visual hallucinations (VH). She had significant ophthalmological pathology, including cataracts, a branch retinal vein occlusion, and vitreous haemorrhage. The hallucinations included seeing monkeys ripping the heads off of her cats and lions prowling the garden. The patient was distressed by the hallucinations and believed them to be real events. Her management involved low dose olanzapine and requesting that her ophthalmological surgery be expedited. The surgery resulted in a significant reduction in VH. A diagnosis of vascular dementia went on to be made following cognitive testing and imaging. The cognitive impairment may have contributed to the patient's inability to identify her experiences as hallucinations and thus render her without insight. A review of the computed tomography (CT) scans performed prior to the patient's presentation to our service confirmed significant vascular pathology including small vessel disease and lacunar infarcts. Cognitive testing confirmed a cognitive impairment which had gone unnoticed by her family. This case leads to an interesting discussion regarding diagnosis in complex VH in cases of significant ophthalmological pathology but a lack of insight. Various authors have proposed theories to explain VH; cortical release and the Perception and Attention Deficit (PAD) model are explored as possible explanations for the experiences of this patient.

Cerebellar Volume Is Associated with Cognitive Decline in Mild Cognitive Impairment: Results from ADNI

Alzheimer's disease (AD) is a disease with dysfunctional brain network. Previous studies found the cerebellar volume changes over the course of AD disease progression; however, whether cerebellar volume change contributes to the cognitive decline in AD, or its earlier disease stage (i.e., mild cognitive impairment [MCI]) remains unclear. In ADNI, cognitive function was assessed using Alzheimer's Disease Assessment Scale-Cognitive Behavior section (ADAS-Cog). We used linear regression and linear mixed effects models to examine whether cerebellar volume is associated with either baseline cognition or with cognitive changes over time in MCI or in AD. We used logistic regression to assess the relationship between cerebellar volume and disease progression to MCI and AD. We found that cerebellar volume is associated with cognition in patients with MCI, after adjusting for age, gender, education, hippocampal volume, and APOE4 status. Consistently, cerebellar volume is associated with increased odds of the disease stages of MCI and AD when compared to controls. However, cerebellar volume is not associated with cognitive changes over time in either MCI or AD. In summary, cerebellar volume may contribute to cognition level in MCI, but not in AD, indicating that the cerebellar network might modulate the cognitive function in the early stage of the disease. The cerebellum may be a potential target for neuromodulation in treating MCI.

[Psychopathological consequences of confinement]

The psychological effects of isolation have already been described in the literature (polar expeditions, submarines, prison). Nevertheless, the scale of confinement implemented during the COVID-19 pandemic is unprecedented. In addition to reviewing the published studies, we need to anticipate the psychological problems that could arise during or at a distance from confinement. We have gone beyond the COVID-19 literature in order to examine the implications of the known consequences of confinement, like boredom, social isolation, stress, or sleep deprivation. Anxiety, post-traumatic stress disorder, depression, suicidal or addictive behaviours, domestic violence are described effects of confinement, but the mechanisms of emergence of these disorders and their interrelationships remain to be studied. For example, what are the mechanisms of emergence of post-traumatic stress disorders in the context of confinement? We also remind the reader of points of vigilance to be kept in mind with regard to eating disorders and hallucinations. Hallucinations are curiously ignored in the literature on confinement, whereas a vast literature links social isolation and hallucinations. Due to the broad psychopathological consequences, we have to look for these various symptoms to manage them. We quickly summarize the diagnostic and therapeutic approaches already in place, such as telemedicine, which is undergoing rapid development during the COVID-19 crisis.

Deafferentation as a cause of hallucinations

PURPOSE OF REVIEW

The association between hallucinations and sensory loss, especially vision- and hearing-impairment, has been firmly established over the past years. The deafferentation theory, a decrease of the threshold for activation in the brain and the consequential imbalance between excitatory and inhibitory brain networks, is hypothesized to underly this relationship. Here we review the studies investigating this theory with a focus on the most recent literature to better understand the contribution of sensory loss to hallucinations.

RECENT FINDINGS

A large cross-sectional study has recently confirmed the relationship between auditory impairment and deafferentation. However, the underlying mechanisms of deafferentation are still under debate, with hyperexcitability and deviations in bottom-up and top-down processes being the most likely explanations. Social isolation following sensory impairment increases the risk for hallucinations. Better knowledge and awareness about the contribution of deafferentation and loneliness would benefit diagnosis and treatment of hallucinations.

SUMMARY

Studies imply activity in higher order areas, corresponding to the functional mapping of sensory system, and a general state of higher excitability as neurobiological explanation. Auditory deafferentation, tinnitus and other auditory hallucinations, likely lie on a continuum. Social isolation mediates psychotic symptoms in sensory-impaired individuals. Currently, there is no standard treatment for deafferentation hallucinations.

Investigating functional changes in the brain to intermittently induced auditory illusions and its relevance to chronic tinnitus

Several studies have demonstrated the neural correlates of chronic tinnitus. However, we still do not understand what happens in the acute phase. Past studies have established Zwicker tone (ZT) illusions as a good human model for acute tinnitus. ZT illusions are perceived following the presentation of a notched noise stimulus, that is, broadband noise with a narrow band-stop filter (notch). In the current study, we compared the neural correlates of the reliable perception of a ZT illusion to that which is not. We observed changes in evoked and total theta power in wide-spread regions of the brain particularly in the temporal-parietal junction, pregenual anterior cingulate cortex/ventromedial prefrontal cortex (pgACC/vmPFC), parahippocampus during perception of the ZT illusion. Furthermore, we observe that increased theta power significantly predicts a gradual positive change in the intensity of the ZT illusion. Such changes may suggest a malfunction of the sensory gating system that enables habituation to redundant stimuli and suppresses hyperactivity. It could also suggest a successful retrieval of the memory of the missing frequencies, resulting in their conscious perception indicating the role of higher-order processing in the mechanism of action of ZT illusions. To establish a more concrete relationship between ZT illusion and chronic tinnitus, future longitudinal studies following up a much larger sample of participants who reliably perceive a ZT illusion to see if they develop tinnitus at a later stage is essential. This could inform us if the ZT illusion may be a precursor to chronic tinnitus.

Fractal properties in sensorimotor variability unveil internal adaptations of the organism before symptomatic functional decline

If health can be defined as adaptability, then measures of adaptability are crucial. Convergent findings across clinical areas established the notion that fractal properties in bio-behavioural variability characterize the healthy condition of the organism, and its adaptive capacities in general. However, ambiguities remain as to the significance of fractal properties: the literature mainly discriminated between healthy vs. pathological states, thereby loosing perspective on the progression in between, and overlooking the distinction between adaptability and effective adaptations of the organism. Here, we design an experimental tapping paradigm involving gradual feedback deprivation in groups of healthy subjects and one deafferented man as a pathological-limit case. We show that distinct types of fractal properties in sensorimotor behaviour characterize, on the one hand impaired functional ability, and on the other hand internal adaptations for maintaining performance despite the imposed constraints. Findings may prove promising for early detection of internal adaptations preceding symptomatic functional decline.

Prediction and perception: Insights for (and from) tinnitus

More than 150 years have passed since Helmholtz first described perception as a process of unconscious inference about the causes of sensations. His ideas have since inspired a wealth of literature investigating the mechanisms underlying these inferences. In recent years, much of this work has converged on the notion that the brain is a hierarchical generative model of its environment that predicts sensations and updates itself based on prediction errors. Here, we build a case for modeling tinnitus from this perspective, i.e. predictive coding. We emphasize two key claims: (1) acute tinnitus reflects an increase in sensory precision in related frequency channels and (2) chronic tinnitus reflects a change in the brain's default prediction. We further discuss specific neural biomarkers that would constitute evidence for or against these claims. Finally, we explore the implications of our model for clinical intervention strategies. We conclude that predictive coding offers the basis for a unifying theory of cognitive neuroscience, which we demonstrate with several examples linking tinnitus to other lines of brain research.

Frontostriatal network dysfunction as a domain-general mechanism underlying phantom perception

In the present study, we use resting state fMRI to investigate whether nucleus accumbens (NAc) and extended frontostriatal networks are involved in the pathology of auditory phantom perception, i.e., tinnitus, through a study of functional connectivity. We hypothesize that resting state functional connectivity involving NAc will be increased relative to what is observed in healthy subjects and that this connectivity will correlate with clinical measures of tinnitus such as percept loudness, duration of symptoms, etc. We show that a large sample of patients with chronic tinnitus (n = 90) features extensive functional connectivity involving NAc that is largely absent in healthy subjects (n = 94). We further show that connectivity involving NAc correlates significantly with tinnitus percept loudness and the duration of tinnitus symptoms, even after controlling for the effects of age and hearing loss. The loudness correlation, which involves NAc and parahippocampal cortex, is consistent with existing literature identifying the parahippocampus as a tinnitus generator. Our results further suggest that frontostriatal connectivity may predict the transition from acute to chronic tinnitus, analogous to what is seen in the pain literature. We discuss these ideas and suggest fruitful avenues for future research.

Top-down and Bottom-up Regulated Auditory Phantom Perception

Auditory phantom percepts such as tinnitus are associated with auditory deafferentation. The idea is that auditory deafferentation limits the amount of information the brain can acquire to make sense of the world. Because of this, auditory deafferentation increases the uncertainty of the auditory environment. To minimize uncertainty, the deafferented brain will attempt to obtain or fill in the missing information. A proposed multiphase compensation model suggests two distinct types of bottom-up related tinnitus: an auditory cortex related tinnitus and a parahippocampal cortex related tinnitus. The weakness of this model is that it cannot explain why some people without hearing loss develop tinnitus, whereas conversely others with hearing loss do not develop tinnitus. In this human study, we provide evidence for a top-down type of tinnitus associated with a deficient noise-cancelling mechanism. A total of 72 participants (age: 40.96 ± 7.67 years; males: 48; females: 24) were recruited for this study. We demonstrate that top-down related tinnitus is related to a change in the pregenual anterior cingulate cortex that corresponds to increased activity in the auditory cortex. This is in accordance with the idea that tinnitus can have different generators as proposed in a recent model that suggests that different compensation mechanisms at a cortical level can be linked to phantom percepts.SIGNIFICANCE STATEMENT Chronic tinnitus affects 15% of the population worldwide. The term "tinnitus" however represents a highly heterogeneous condition, as evidenced by the fact that there are no effective treatments or even an adequate understanding of the underlying neural mechanisms. Consistent with this idea, our research shows that tinnitus indeed has different subtypes related to hearing loss. In a human study tightly controlled for hearing loss, we establish a tinnitus subtype associated with a deficient top-down noise-cancelling mechanism, which distinguishes it from bottom-up subtypes. We demonstrate that top-down related tinnitus relates to a change in the pregenual anterior cingulate cortex that corresponds to increased activity in the auditory cortex, whereas bottom-up tinnitus instead relates to changes in the parahippocampal cortex.

Neural basis of induced phantom limb pain relief

OBJECTIVE

Phantom limb pain (PLP) is notoriously difficult to treat, partly due to an incomplete understanding of PLP-related disease mechanisms. Noninvasive brain stimulation (NIBS) is used to modulate plasticity in various neuropathological diseases, including chronic pain. Although NIBS can alleviate neuropathic pain (including PLP), both disease and treatment mechanisms remain tenuous. Insight into the mechanisms underlying both PLP and NIBS-induced PLP relief is needed for future implementation of such treatment and generalization to related conditions.

METHODS

We used a within-participants, double-blind, and sham-controlled design to alleviate PLP via task-concurrent NIBS over the primary sensorimotor missing hand cortex (S1/M1). To specifically influence missing hand signal processing, amputees performed phantom hand movements during anodal transcranial direct current stimulation. Brain activity was monitored using neuroimaging during and after NIBS. PLP ratings were obtained throughout the week after stimulation.

RESULTS

A single session of intervention NIBS significantly relieved PLP, with effects lasting at least 1 week. PLP relief associated with reduced activity in the S1/M1 missing hand cortex after stimulation. Critically, PLP relief and reduced S1/M1 activity correlated with preceding activity changes during stimulation in the mid- and posterior insula and secondary somatosensory cortex (S2).

INTERPRETATION

The observed correlation between PLP relief and decreased S1/M1 activity confirms our previous findings linking PLP with increased S1/M1 activity. Our results further highlight the driving role of the mid- and posterior insula, as well as S2, in modulating PLP. Lastly, our novel PLP intervention using task-concurrent NIBS opens new avenues for developing treatment for PLP and related pain conditions. ANN NEUROL 2019;85:59-73.

Functional brain changes in auditory phantom perception evoked by different stimulus frequencies

Bayesian models of brain function such as active inference and predictive coding offer a general theoretical framework with which to explain several aspects of normal and disordered brain function. Of particular interest to the present study is the potential for such models to explain the pathology of auditory phantom perception, i.e. tinnitus. To test this framework empirically, we perform an fMRI experiment on a large clinical sample (n = 75) of the human chronic tinnitus population. The experiment features a within-subject design based on two experimental conditions: subjects were presented with sound stimuli matched to their tinnitus frequency (TF) as well as similar stimuli presented at a control frequency (CF). The responses elicited by these stimuli, as measured using both activity and functional connectivity, were then analyzed both within and between conditions. Given the Bayesian-brain framework, we hypothesize that TF stimuli will elicit greater activity and/or functional connectivity in areas related to the cognitive and emotional aspects of tinnitus, i.e. tinnitus-related distress. We conversely hypothesize that CF stimuli will elicit greater activity/connectivity in areas related to auditory perception and attention. We discuss our results in the context of this framework and suggest future directions for empirical testing.

Effect of distress on transient network dynamics and topological equilibrium in phantom sound perception

Distress is a domain-general behavioral symptom whose neural correlates have been under investigation for a long time now. Although some studies suggest that distress is encoded by changes in alpha activity and functional connectivity between specific brain regions, no study that we know has delved into the whole brain temporal dynamics of the distress component. In the current study, we compare the changes in the mean and variance of functional connectivity and small-worldness parameter over 3 min of resting state EEG to analyze the fluctuation in transient stable states, and network structure. On comparing these measures between healthy controls and patients experiencing low and high levels of distress due to a continuous ringing in the ear (tinnitus), we observe an increase in fluctuation between transient stable states characterized by an increase in both variance of functional connectivity and the small-worldness parameter. This results in a possible increase in degrees of freedom leading to a paradoxical equilibrium of the network structure in highly distressed patients. This may also be interpreted as a maladaptive compensation to look for information in order to reduce the hyper-salience in highly distressed individuals. In addition, this is correlated with the amount of distress only in the high distress tinnitus group, suggesting a catastrophic breakdown of the brain's resilience. Distress not only accompanies tinnitus, but other disorders such as somatic disorders, fibromyalgia, post-traumatic stress disorder, etc. Since the current study focuses on a disorder-general distress symptom, the methods and results of the current study have a wide application in different neuropathologies.

Cortical remodeling after electroacupuncture therapy in peripheral nerve repairing model

Electroacupuncture (EA) is an alternative therapy for peripheral nerve injury (PNI). The treatment relies on post-therapeutic effect rather than real-time effect. We utilized fMRI to clarify the resting-state alteration caused by sustained effect of EA on peripheral nerve repairing model. Twenty-four rats were divided equally into three groups: normal group, model group and intervention group. Rats of the model and intervention group underwent sciatic nerve transection and direct anastomosis. EA intervention at ST-36 and GB-30 was conducted continuously for 4 months on the intervention group. Behavioral assessments and fMRI were performed 1 month and 4 months after surgery. Intervention group showed significant improvement on the gait parameters max contact mean intensity (MCMI) and thermal withdrawal latency (TWL) than model group. EA-related sustained effects of amplitude of low frequency fluctuations (ALFF) could be described as a remolding pattern of somatosensory area and sensorimotor integration regions which presented higher ALFF in the contralateral hemisphere and lower in the ipsilateral hemisphere than model group. Interhemispheric functional connectivity (FC) analysis showed a significantly lower FC after EA therapy between the largest significantly different clusters in bilateral somatosensory cortices than the model group 4 months after surgery(p < 0.05). And the model group presented significantly higher FC than the normal group at both two time-points (p < 0.01). The sustained effect of EA on peripheral nerve repairing rats appeared to induce both regional and extensive neuroplasticity in bilateral hemispheres. We proposed that such EA-related effect was a reverse of maladaptive plasticity caused by PNI.

The neural correlates of the unified percept of alcohol-related craving: a fMRI and EEG study

Alcohol addiction is accompanied by aberrant neural activity. Previously, task-based fMRI and resting-state EEG studies have revealed that craving, a critical component of addiction, is linked to abnormal activity in cortical regions including the dorsal anterior cingulate cortex (dACC), nucleus accumbens (NAcc), posterior cingulate cortex (PCC) and pregenual anterior cingulate cortex (pgACC), etc. In this study, we combine these two imaging techniques to investigate a group of alcohol-addicted patients and provide convergent evidence for the neural correlates of craving not only in alcohol but substance abuse in general. We observe abnormal BOLD signal levels in the dACC, NAcc, pgACC, PCC, amygdala, and parahippocampus (PHC) in a cue-reactivity fMRI experiment. These findings are consistent with increased beta-band activity in the dACC and pgACC in resting-state EEG. We further observe desynchronization characterized by decreased functional connectivity in cue-based fMRI and hypersynchronization characterized by increased functional connectivity between these regions in the theta frequency band. The results of our study show a consistent pattern of alcohol craving elicited by external cues and internal desires. Given the advantage of superior spatial and temporal resolution, we hypothesize a "central craving network" that integrates the different aspects of alcohol addiction into a unified percept.

Motor correlates of phantom limb pain

Following amputation, individuals ubiquitously report experiencing lingering sensations of their missing limb. While phantom sensations can be innocuous, they are often manifested as painful. Phantom limb pain (PLP) is notorious for being difficult to monitor and treat. A major challenge in PLP management is the difficulty in assessing PLP symptoms, given the physical absence of the affected body part. Here, we offer a means of quantifying chronic PLP by harnessing the known ability of amputees to voluntarily move their phantom limbs. Upper-limb amputees suffering from chronic PLP performed a simple finger-tapping task with their phantom hand. We confirm that amputees suffering from worse chronic PLP had worse motor control over their phantom hand. We further demonstrate that task performance was consistent over weeks and did not relate to transient PLP or non-painful phantom sensations. Finally, we explore the neural basis of these behavioural correlates of PLP. Using neuroimaging, we reveal that slower phantom hand movements were coupled with stronger activity in the primary sensorimotor phantom hand cortex, previously shown to associate with chronic PLP. By demonstrating a specific link between phantom hand motor control and chronic PLP, our findings open up new avenues for PLP management and improvement of existing PLP treatments.