The role of the retina in visual hallucinations: A review of the literature and implications for psychosis

A three-dimensional model of neural activity and phenomenal-behavioral patterns

How phenomenal experience and behavior are related to neural activity in physiology and psychopathology represents a fundamental question in neuroscience and psychiatry. The phenomenal-behavior patterns may be deconstructed into basic dimensions, i.e., psychomotricity, affectivity, and thought, which might have distinct neural correlates. This work provides a data overview on the relationship of these phenomenal-behavioral dimensions with brain activity across physiological and pathological conditions (including major depressive disorder, bipolar disorder, schizophrenia, attention-deficit/hyperactivity disorder, anxiety disorders, addictive disorders, Parkinson's disease, Tourette syndrome, Alzheimer's disease, and frontotemporal dementia). Accordingly, we propose a three-dimensional model of neural activity and phenomenal-behavioral patterns. In this model, neural activity is organized into distinct units in accordance with connectivity patterns and related input/output processing, manifesting in the different phenomenal-behavioral dimensions. (1) An external neural unit, which involves the sensorimotor circuit/brain's sensorimotor network and is connected with the external environment, processes external inputs/outputs, manifesting in the psychomotor dimension (processing of exteroception/somatomotor activity). External unit hyperactivity manifests in psychomotor excitation (hyperactivity/hyperkinesia/catatonia), while external unit hypoactivity manifests in psychomotor inhibition (retardation/hypokinesia/catatonia). (2) An internal neural unit, which involves the interoceptive-autonomic circuit/brain's salience network and is connected with the internal/body environment, processes internal inputs/outputs, manifesting in the affective dimension (processing of interoception/autonomic activity). Internal unit hyperactivity manifests in affective excitation (anxiety/dysphoria-euphoria/panic), while internal unit hypoactivity manifests in affective inhibition (anhedonia/apathy/depersonalization). (3) An associative neural unit, which involves the brain's associative areas/default-mode network and is connected with the external/internal units (but not with the environment), processes associative inputs/outputs, manifesting in the thought dimension (processing of ideas). Associative unit hyperactivity manifests in thought excitation (mind-wandering/repetitive thinking/psychosis), while associative unit hypoactivity manifests in thought inhibition (inattention/cognitive deficit/consciousness loss). Finally, these neural units interplay and dynamically combine into various neural states, resulting in the complex phenomenal experience and behavior across physiology and neuropsychiatric disorders.

Retina as a potential biomarker in schizophrenia spectrum disorders: a systematic review and meta-analysis of optical coherence tomography and electroretinography

INTRODUCTION

Abnormal findings on optical coherence tomography (OCT) and electroretinography (ERG) have been reported in participants with schizophrenia spectrum disorders (SSDs). This study aims to reveal the pooled standard mean difference (SMD) in retinal parameters on OCT and ERG among participants with SSDs and healthy controls and their association with demographic characteristics, clinical symptoms, smoking, diabetes mellitus, and hypertension.

METHODS

Using PubMed, Scopus, Web of Science, and PSYNDEX, we searched the literature from inception to March 31, 2023, using specific search terms. This study was registered with PROSPERO (CRD4202235795) and conducted according to PRISMA 2020.

RESULTS

We included 65 studies in the systematic review and 44 in the meta-analysis. Participants with SSDs showed thinning of the peripapillary retinal nerve fiber layer (pRNFL), macular ganglion cell layer- inner plexiform cell layer, and retinal thickness in all other segments of the macula. A meta-analysis of studies that excluded SSD participants with diabetes and hypertension showed no change in results, except for pRNFL inferior and nasal thickness. Furthermore, a significant difference was found in the pooled SMD of pRNFL temporal thickness between the left and right eyes. Meta-regression analysis revealed an association between retinal thinning and duration of illness, positive and negative symptoms. In OCT angiography, no differences were found in the foveal avascular zone and superficial layer foveal vessel density between SSD participants and controls. In flash ERG, the meta-analysis showed reduced amplitude of both a- and b-waves under photopic and scotopic conditions in SSD participants. Furthermore, the latency of photopic a-wave was significantly shorter in SSD participants in comparison with HCs.

DISCUSSION

Considering the prior report of retinal thinning in unaffected first-degree relatives and the results of the meta-analysis, the findings suggest that retinal changes in SSDs have both trait and state aspects. Future longitudinal multimodal retinal imaging studies are needed to clarify the pathophysiological mechanisms of these changes and to clarify their utility in individual patient monitoring efforts.

The association between visual hallucinations and secondary psychosis: a systematic review and meta-analysis

INTRODUCTION

Visual hallucinations are often considered to be suggestive of a secondary cause of psychosis, however, this association has never been assessed meta-analytically. We aimed to compare the presence of visual hallucinations in patients with psychosis due to a primary or secondary cause.

METHOD

We conducted a meta-analysis of case-control studies directly comparing primary and secondary psychosis. A random-effects model, following the DerSimonian and Laird method, was used to pool studies and generate overall odds ratios (OR), 95% confidence intervals (CI) and prediction intervals (PI).

RESULTS

Fourteen studies (904 primary and 804 secondary psychosis patients) were included. Visual hallucinations were significantly associated with secondary psychosis (OR = 3.0, 95% CI = 1.7-5.1, p < 0.001) with moderate between-study heterogeneity (I2 = 70%). Subgroup analysis by type of secondary psychosis (organic, drug-induced, mixed) was non-significant. Analysis of the content of visual hallucinations (51 primary and 142 secondary psychosis patients) found hallucinations of inanimate objects were significantly more likely to be associated with secondary psychosis (OR = 0.1, 95% CI = 0.01-0.8, p = 0.03).

CONCLUSIONS

Visual hallucinations were strongly associated with a secondary cause of psychosis. The presence of visual hallucinations in a patient presenting with psychosis may serve as a potential "red flag" for a secondary cause and warrant further investigation.

Working memory and sensory memory in subclinical high schizotypy: An avenue for understanding schizophrenia?

The search for robust, reliable biomarkers of schizophrenia remains a high priority in psychiatry. Biomarkers are valuable because they can reveal the underlying mechanisms of symptoms and monitor treatment progress and may predict future risk of developing schizophrenia. Despite the existence of various promising biomarkers that relate to symptoms across the schizophrenia spectrum, and despite published recommendations encouraging multivariate metrics, they are rarely investigated simultaneously within the same individuals. In those with schizophrenia, the magnitude of purported biomarkers is complicated by comorbid diagnoses, medications and other treatments. Here, we argue three points. First, we reiterate the importance of assessing multiple biomarkers simultaneously. Second, we argue that investigating biomarkers in those with schizophrenia-related traits (schizotypy) in the general population can accelerate progress in understanding the mechanisms of schizophrenia. We focus on biomarkers of sensory and working memory in schizophrenia and their smaller effects in individuals with nonclinical schizotypy. Third, we note irregularities across research domains leading to the current situation in which there is a preponderance of data on auditory sensory memory and visual working memory, but markedly less in visual (iconic) memory and auditory working memory, particularly when focusing on schizotypy where data are either scarce or inconsistent. Together, this review highlights opportunities for researchers without access to clinical populations to address gaps in knowledge. We conclude by highlighting the theory that early sensory memory deficits contribute negatively to working memory and vice versa. This presents a mechanistic perspective where biomarkers may interact with one another and impact schizophrenia-related symptoms.

Early visual alterations in individuals at-risk of Alzheimer's disease: a multidisciplinary approach

BACKGROUND

The earliest pathological features of Alzheimer's disease (AD) appear decades before the clinical symptoms. The pathology affects the brain and the eye, leading to retinal structural changes and functional visual alterations. Healthy individuals at high risk of developing AD present alterations in these ophthalmological measures, as well as in resting-state electrophysiological activity. However, it is unknown whether the ophthalmological alterations are related to the visual-related electrophysiological activity. Elucidating this relationship is paramount to understand the mechanisms underlying the early deterioration of the system and an important step in assessing the suitability of these measures as early biomarkers of disease.

METHODS

In total, 144 healthy subjects: 105 with family history of AD and 39 without, underwent ophthalmologic analysis, magnetoencephalography recording, and genotyping. A subdivision was made to compare groups with less demographic and more risk differences: 28 high-risk subjects (relatives/APOEɛ4 +) and 16 low-risk (non-relatives/APOEɛ4 -). Differences in visual acuity, contrast sensitivity, and macular thickness were evaluated. Correlations between each variable and visual-related electrophysiological measures (M100 latency and time-frequency power) were calculated for each group.

RESULTS

High-risk groups showed increased visual acuity. Visual acuity was also related to a lower M100 latency and a greater power time-frequency cluster in the high-risk group. Low-risk groups did not show this relationship. High-risk groups presented trends towards a greater contrast sensitivity that did not remain significant after correction for multiple comparisons. The highest-risk group showed trends towards the thinning of the inner plexiform and inner nuclear layers that did not remain significant after correction. The correlation between contrast sensitivity and macular thickness, and the electrophysiological measures were not significant after correction. The difference between the high- and low- risk groups correlations was no significant.

CONCLUSIONS

To our knowledge, this paper is the first of its kind, assessing the relationship between ophthalmological and electrophysiological measures in healthy subjects at distinct levels of risk of AD. The results are novel and unexpected, showing an increase in visual acuity among high-risk subjects, who also exhibit a relationship between this measure and visual-related electrophysiological activity. These results have not been previously explored and could constitute a useful object of research as biomarkers for early detection and the evaluation of potential interventions' effectiveness.

Abnormal sensory perception masks behavioral performance of Grin1 knockdown mice

The development and function of sensory systems require intact glutamatergic neurotransmission. Changes in touch sensation and vision are common symptoms in autism spectrum disorders, where altered glutamatergic neurotransmission is strongly implicated. Further, cortical visual impairment is a frequent symptom of GRIN disorder, a rare genetic neurodevelopmental disorder caused by pathogenic variants of GRIN genes that encode NMDA receptors. We asked if Grin1 knockdown mice (Grin1KD), as a model of GRIN disorder, had visual impairments resulting from NMDA receptor deficiency. We discovered that Grin1KD mice had deficient visual depth perception in the visual cliff test. Since Grin1KD mice are known to display robust changes in measures of learning, memory, and emotionality, we asked whether deficits in these higher-level processes could be partly explained by their visual impairment. By changing the experimental conditions to improve visual signals, we observed significant improvements in the performance of Grin1KD mice in tests that measure spatial memory, executive function, and anxiety. We went further and found destabilization of the outer segment of retina together with the deficient number and size of Meissner corpuscles (mechanical sensor) in the hind paw of Grin1KD mice. Overall, our findings suggest that abnormal sensory perception can mask the expression of emotional, motivational and cognitive behavior of Grin1KD mice. This study demonstrates new methods to adapt routine behavioral paradigms to reveal the contribution of vision and other sensory modalities in cognitive performance.

Retinal electroretinogram features can detect depression state and treatment response in adults: A machine learning approach

BACKGROUND

Major depressive disorder (MDD) is a major public health problem. The retina is a relevant site to indirectly study brain functioning. Alterations in retinal processing were demonstrated in MDD with the pattern electroretinogram (PERG). Here, the relevance of signal processing and machine learning tools applied on PERG was studied.

METHODS

PERG - whose stimulation is reversible checkerboards - was performed according to the International Society for Clinical Electrophysiology of Vision (ISCEV) standards in 24 MDD patients and 29 controls at the inclusion. PERG was recorded every 4 weeks for 3 months in patients. Amplitude and implicit time of P50 and N95 were evaluated. Then, time/frequency features were extracted from the PERG time series based on wavelet analysis. A statistical model has been learned in this feature space and a metric aiming at quantifying the state of the MDD patient has been derived, based on minimum covariance determinant (MCD) mahalanobis distance.

RESULTS

MDD patients showed significant increase in P50 and N95 implicit time (p = 0,006 and p = 0,0004, respectively, Mann-Whitney U test) at the inclusion. The proposed metric extracted from the raw PERG provided discrimination between patients and controls at the inclusion (p = 0,0001). At the end of the follow-up at week 12, the difference between the metrics extracted on controls and patients was not significant (p = 0,07), reflecting the efficacy of the treatment.

CONCLUSIONS

Signal processing and machine learning tools applied on PERG could help clinical decision in the diagnosis and the follow-up of MDD in measuring treatment response.

An Integrated Neuroimaging Approach to Inform Transcranial Electrical Stimulation Targeting in Visual Hallucinations

For decades, noninvasive brain stimulation (NIBS), such as transcranial electrical stimulation (tES), has been used to directly modulate human brain mechanisms of visual perception, setting the groundwork for the development of novel circuit-based therapies. While the field of NIBS has grown considerably over recent years, few studies have used these technologies to treat visual hallucinations (VH). Here, we review the NIBS-VH literature and find mixed results due to shortcomings that may potentially be addressed with a unique multimodal neuroimaging-NIBS approach. We highlight methodological advances in NIBS research that have provided researchers with more precise anatomical measurements that may improve our ability to influence brain activity. Specifically, we propose a methodology that combines neuroimaging advances, clinical neuroscience developments such as the identification of brain regions causally involved in VH, and personalized NIBS approaches that improve anatomical targeting. This methodology may enable us to reconcile existing discrepancies in tES-VH research and pave the way for more effective, VH-specific protocols for treating a number of neuropsychiatric disorders with VH as a core symptom.

Altered central vision and amacrine cells dysfunction as marker of hypodopaminergic activity in treated patients with schizophrenia

BACKGROUND

Retinal dysfunction is widely documented in schizophrenia using flash (fERG) and pattern electroretinograms (PERG), but the role of dopamine transmission has seldom been explored.

METHODS

We explored the role of dopamine transmission by evaluating the spatial location of retinal anomalies using multifocal ERG (mfERG) in photopic condition and the oscillatory potentials (OPs) extracted from fERG measured in scotopic condition in 29 patients with schizophrenia and 29 healthy controls.

RESULTS

With the mfERG, our main results revealed reduced amplitudes in the center of the retina: P1 (p < .005) and N2 amplitudes (p < .01) in the <2° region, N1 (p < .0005) and P1 amplitudes (p < .001) in the 2-5° region and P1 amplitude (p < .05) in the 5-10° region. For OPs, our results showed a decrease in the O1 (p < .005), O2 (p < .005), O3 (p < .05) and overall O1, O2, O3 index amplitudes (p < .005) in patients with schizophrenia.

CONCLUSIONS

Both the central location of retinal dysfunctions of the mfERG and OPs results could reflect a hypodopaminergic effect in patients with schizophrenia. In future studies, OPs should be considered as a measure to evaluate the hypodopaminergy in patients.

Retinal ganglion cell dysfunction is correlated with disturbed visual cognition in schizophrenia patients with visual hallucinations

Patients with schizophrenia have altered visual cognition and retinal functions. No studies have explored if retinal anomalies are related to visual cognition and the presence of visual hallucinations (VH). We explored functional responses of the retinal ganglion cells in schizophrenia patients with or without VH and conducted a neuropsychological evaluation to explore the links between cognition and retinal function. The VH+ group showed poorer visual cognition and we found correlations between the amplitudes of the P50 and the N95 waves and visual cognition. Our results provide arguments for a link between retinal dysfunction, impaired visual processing and VH in schizophrenia.

Retinal dysfunctions in regular tobacco users: The retina as a window to the reward circuit in addictive disorders

The nicotine contained in tobacco is a neuromodulator which affects neurotransmission within the brain. The retina is an easy way to study central synaptic transmission dysfunctions in neuropsychiatric disorders. The purpose of this study is to assess the impact of regular tobacco use on retinal function using pattern (PERG), flash (fERG) and multifocal (mfERG) electroretinogram (ERG). We recorded PERG, fERG and mfERG for 24 regular tobacco users and 30 healthy non-smoking subjects. The protocol was compliant with International Society for Clinical Electrophysiology of Vision standards. The amplitudes and peak times (PT) of P50, N95 waves (PERG), a-, b- and oscillatory potentials (fERG), and N1, P1, N2 (mfERG) were evaluated. Compared to non-smokers, the results (Mann-Whitney U test, Bonferroni correction) for tobacco users suggested a significant increase of ~ 1 ms in the PT of light-adapted 3.0 fERG b-wave (p = 0.002). Using mfERG, we observed the following increases in tobacco users: in ring 3 for P1 PT of ~1,5 ms and in ring 5 for P1 PT of ~ 1 ms and for N2 PT of ~ 1 ms (p = 0.002, p = 0.002 and p = 0.006). It is our hypothesis that these results reflect the consequences of regular tobacco use on retinal synaptic transmission, and more specifically on dopaminergic and cholinergic transmission. We deduce that the retina may provide a crucial site of investigation for neurotransmission modulation of the reward circuit in regular tobacco users.

The Phenomenology and Neurobiology of Visual Distortions and Hallucinations in Schizophrenia: An Update

Schizophrenia is characterized by visual distortions in ~60% of cases, and visual hallucinations (VH) in ~25-50% of cases, depending on the sample. These symptoms have received relatively little attention in the literature, perhaps due to the higher rate of auditory vs. visual hallucinations in psychotic disorders, which is the reverse of what is found in other neuropsychiatric conditions. Given the clinical significance of these perceptual disturbances, our aim is to help address this gap by updating and expanding upon prior reviews. Specifically, we: (1) present findings on the nature and frequency of VH and distortions in schizophrenia; (2) review proposed syndromes of VH in neuro-ophthalmology and neuropsychiatry, and discuss the extent to which these characterize VH in schizophrenia; (3) review potential cortical mechanisms of VH in schizophrenia; (4) review retinal changes that could contribute to VH in schizophrenia; (5) discuss relationships between findings from laboratory measures of visual processing and VH in schizophrenia; and (6) integrate findings across biological and psychological levels to propose an updated model of VH mechanisms, including how their content is determined, and how they may reflect vulnerabilities in the maintenance of a sense of self. In particular, we emphasize the potential role of alterations at multiple points in the visual pathway, including the retina, the roles of multiple neurotransmitters, and the role of a combination of disinhibited default mode network activity and enhanced state-related apical/contextual drive in determining the onset and content of VH. In short, our goal is to cast a fresh light on the under-studied symptoms of VH and visual distortions in schizophrenia for the purposes of informing future work on mechanisms and the development of targeted therapeutic interventions.

Schizophrenia and the retina: Towards a 2020 perspective

BACKGROUND

Differences between people with schizophrenia and psychiatrically healthy controls have been consistently demonstrated on measures of retinal function such as electroretinography (ERG), and measures of retinal structure such as optical coherence tomography (OCT). Since our 2015 review of this literature, multiple new studies have been published using these techniques. At the same time, the accumulation of data has highlighted the "fault lines" in these fields, suggesting methodological considerations that need greater attention in future studies.

METHODS

We reviewed studies of ERG and OCT in schizophrenia, as well as data from studies whose findings are relevant to interpreting these papers, such as those on effects of the following on ERG and OCT data: comorbid medical conditions that are over-represented in schizophrenia, smoking, antipsychotic medication, substance abuse, sex and gender, obesity, attention, motivation, and influences of brain activity on retinal function.

RESULTS

Recent ERG and OCT studies continue to support the hypothesis of retinal structural and functional abnormalities in schizophrenia, and suggest that these are relevant to understanding broader aspects of pathophysiology, neurodevelopment, and neurodegeneration in this disorder. However, there are differences in findings which suggest that the effects of multiple variables on ERG and OCT data need further clarification.

CONCLUSIONS

The retina, as the only component of the CNS that can be imaged directly in live humans, has potential to clarify important aspects of schizophrenia. With greater attention to specific methodological issues, the true potential of ERG and OCT as biomarkers for important clinical phenomena in schizophrenia should become apparent.

Retinal ganglion cells dysfunctions in schizophrenia patients with or without visual hallucinations

The electroretinogram has revealed photoreceptor, bipolar cell, and, in one prior study, retinal ganglion cell (RGC) dysfunction in schizophrenia. The structural abnormalities of the RGC are well documented in schizophrenia and such abnormalities have been associated with visual hallucinations (VH) in neurological disorders. The goals of this study were: 1) to examine the functional responses of photoreceptors and RGC in schizophrenia patients in comparison with healthy controls; and 2) to compare the extent of retinal dysfunction in schizophrenia patients with or without VH. We recorded the flash electroretinogram in scotopic and photopic conditions, and the pattern electroretinogram, in schizophrenia patients (n = 29) and healthy controls (n = 29). Schizophrenia patients were divided in two groups: schizophrenia patients with VH (VH group, n = 12) and schizophrenia patients with auditory hallucinations or no hallucinations (AHNH group, n = 17). Our results replicate previous findings regarding photoreceptor dysfunction in schizophrenia. PERG results showed a significant increase of the P50 implicit time in schizophrenia patients compared with controls (t(55) = 2.1, p < .05, d = 0.55) and a significant increase of the N95 implicit time in schizophrenia patients compared with controls (t(55) = 4.2; p < .001, d = 0.66). We found an increased rod b-wave implicit time (dark-adapted 0.01 ERG) in the VH group compared to the AHNH group and to the control group, which was associated with lifetime VH score. Our results demonstrate a slowing of RGC signaling in schizophrenia patients, which could affect the quality of visual information reaching the visual cortex. The implications of the data for understanding VH in schizophrenia are discussed.

Genetic polymorphisms of transient receptor potential melastatin 1 correlate with voriconazole-related visual adverse events

BACKGROUND

Causes of voriconazole-related visual adverse events (VVAE) remained controversial.

OBJECTIVES

We aimed to explore the relationship between voriconazole serum concentrations and VVAE as well as the potential influence of transient receptor potential melastatin 1 (TRPM1) on VVAE.

PATIENTS/METHODS

This prospective observational cohort study was done in two stages. Patients who received voriconazole for invasive fungal diseases were consecutively enrolled. Correlations between voriconazole trough levels and VVAE were explored in 76 patients. Genotyping was further conducted for 17 tag SNPs of TRPM1 in a larger population of 137 patients. Genotype distributions were compared between patients with and without VVAE.

RESULT

Of the 76 patients, a total of 229 steady-state voriconazole trough levels were evaluated, 69.9% of which were within the target range (1-5.5 mg/L). No correlations were found between voriconazole trough levels and VVAE. Of the total 137 patients, VVAE occurred in 37 (27.0%) patients, including visual hallucination (13.9%, 19/137) and visual disturbances (19.0%, 26/137). Significant difference in TRPM1 genotype distribution was only observed in patients with visual hallucination but not with visual disturbances. We found that rs890160 G/T genotype was under-presented (OR, 0.11; 95% CI, 0.01-0.84; P = .011) and rs1378847 C/C genotype was more frequently detected (OR, 8.89; 95% CI, 1.14-69.02; P = .013) in patients with visual hallucination when compared with those without.

CONCLUSION

Transient receptor potential melastatin 1 was genetically associated with voriconazole-related visual hallucination. The correlation was failed to found between voriconazole trough levels and VVAE.

Retinal Changes in Schizophrenia: A Systematic Review and Meta-analysis Based on Individual Participant Data

BACKGROUND

Retinal assessment has indicated the presence of neuronal loss in neurodegenerative disorders, but its role in schizophrenia remains unclear. We sought to synthesize the available evidence considering 3 noninvasive modalities: optical coherence tomography, electroretinography, and fundus photography, and examine their diagnostic accuracy based on unpublished individual participant data, when provided by the primary study authors.

METHODS

We searched MEDLINE, SCOPUS, clinicaltrials.gov, PSYNDEX, Cochrane Controlled Register of Trials (CENTRAL), WHO International Clinical Trials Registry Platform, and Google Scholar, up to October 30, 2018. Authors were contacted and invited to share anonymized participant-level data. Aggregate data were pooled using random effects models. Diagnostic accuracy meta-analysis was based on multiple cutoffs logistic generalized linear mixed modeling. This study was registered with PROSPERO, number CRD42018109344.

RESULTS

Pooled mean differences of peripapillary retinal nerve fiber layer thickness in micrometer between 694 eyes of 432 schizophrenia patients and 609 eyes of 358 controls, from 11 case-control studies, with corresponding 95% confidence intervals (CIs) by quadrant were the following: -4.55, 95% CI: -8.28, -0.82 (superior); -6.25, 95% CI: -9.46, -3.04 (inferior); -3.18, 95% CI: -5.04, -1.31 (nasal); and -2.7, 95% CI: -4.35, -1.04 (temporal). Diagnostic accuracy, based on 4 studies, was fair to poor, unaffected by age and sex; macular area measurements performed slightly better.

CONCLUSION

The notion of structural and functional changes in retinal integrity of patients with schizophrenia is supported with current evidence, but diagnostic accuracy is limited. The potential prognostic, theranostic, and preventive role of retinal evaluation remains to be examined.

Changes in visual function and retinal structure in the progression of Alzheimer's disease

BACKGROUND

Alzheimer's Disease (AD) can cause degeneration in the retina and optic nerve either directly, as a result of amyloid beta deposits, or secondarily, as a result of the degradation of the visual cortex. These effects raise the possibility that tracking ophthalmologic changes in the retina can be used to assess neurodegeneration in AD. This study aimed to detect retinal changes and associated functional changes in three groups of patients consisting of AD patients with mild disease, AD patients with moderate disease and healthy controls by using non-invasive psychophysical ophthalmological tests and optical coherence tomography (OCT).

METHODS

We included 39 patients with mild AD, 21 patients with moderate AD and 40 age-matched healthy controls. Both patients and controls were ophthalmologically healthy. Visual acuity, contrast sensitivity, colour perception, visual integration, and choroidal thicknesses were measured. In addition, OCT and OCT angiography (OCTA) were applied.

FINDINGS

Visual acuity, contrast sensitivity, colour perception, and visual integration were significantly lower in AD patients than in healthy controls. Compared to healthy controls, macular thinning in the central region was significant in the mild AD patients, while macular thickening in the central region was found in the moderate AD group. The analysis of macular layers revealed significant thinning of the retinal nerve fibre layer, the ganglion cell layer and the outer plexiform layer in AD patients relative to controls. Conversely, significant thickening was observed in the outer nuclear layer of the patients. However, mild AD was associated with significant thinning of the subfovea and the nasal and inferior sectors of the choroid. Significant superonasal and inferotemporal peripapillary thinning was observed in patients with moderate disease.

CONCLUSIONS

The first changes in the mild AD patients appear in the psychophysical tests and in the central macula with a decrease in the central retinal thickness. When there was a disease progression to moderate AD, psychophysical tests remained stable with respect to the decrease in mild AD, but significant thinning in the peripapillary retina and thickening in the central retina appeared. The presence of AD is best indicated based on contrast sensitivity.

Altered dynamics of visual contextual interactions in Parkinson's disease

Over the last decades, psychophysical and electrophysiological studies in patients and animal models of Parkinson's disease (PD), have consistently revealed a number of visual abnormalities. In particular, specific alterations of contrast sensitivity curves, electroretinogram (ERG), and visual-evoked potentials (VEP), have been attributed to dopaminergic retinal depletion. However, fundamental mechanisms of cortical visual processing, such as normalization or "gain control" computations, have not yet been examined in PD patients. Here, we measured electrophysiological indices of gain control in both space (surround suppression) and time (sensory adaptation) in PD patients based on steady-state VEP (ssVEP). Compared with controls, patients exhibited a significantly higher initial ssVEP amplitude that quickly decayed over time, and greater relative suppression of ssVEP amplitude as a function of surrounding stimulus contrast. Meanwhile, EEG frequency spectra were broadly elevated in patients relative to controls. Thus, contrary to what might be expected given the reduced contrast sensitivity often reported in PD, visual neural responses are not weaker; rather, they are initially larger but undergo an exaggerated degree of spatial and temporal gain control and are embedded within a greater background noise level. These differences may reflect cortical mechanisms that compensate for dysfunctional center-surround interactions at the retinal level.

Retinal layers in Parkinson's disease: A meta-analysis of spectral-domain optical coherence tomography studies

BACKGROUND

Patients with Parkinson's disease experience visual symptoms, partially originating from retinal changes. Since 2011, multiple case-control studies using spectral-domain OCT, which allows for studying individual retinal layers, have been published. The aim of this study was to substantiate the occurrence, extent, and location of retinal degeneration in Parkinson's by meta-analysis.

METHODS

Spectral-domain OCT case-control data were collected by performing a search in PubMed and Embase with terms: "optical coherence tomography" and "parkinson", up to November 5th, 2018. Studies with fewer than 10 patients or controls were excluded. We performed a random effects meta-analysis. Heterogeneity was evaluated with I² statistics; publication bias with Egger's and Begg's tests.

RESULTS

Out of 77 identified studies, 36 were included, totaling 1916 patients and 2006 controls. A significant thinning of the peripapillary retinal nerve fiber layer (d = -0.42; 95% confidence interval -0.54 to -0.29) and the combined ganglion cell and inner plexiform layers (d = -0.40; -0.72, to -0.07) was found. The inner nuclear layer and outer plexiform layer did not show significant changes. Heterogeneity ranged from 3 to 92%; no publication bias was found.

CONCLUSIONS

Parkinson's patients show significant thinning of the inner retinal layers, resembling changes found in glaucoma and other neurodegenerative diseases like Alzheimer's. Study of different cell layers in-vivo is possible by moving from time-to spectral domain OCT. Retinal degeneration may be affiliated with neurodegenerative pathology overall, and could serve as a biomarker in neurodegenerative disorders. Longitudinal research including clinical correlations is needed to determine usefulness in Parkinson's disease.

Cannabis use and human retina: The path for the study of brain synaptic transmission dysfunctions in cannabis users

Owing to the difficulty of obtaining direct access to the functioning brain, new approaches are needed for the indirect exploration of brain disorders in neuroscience research. Due to its embryonic origin, the retina is part of the central nervous system and is well suited to the investigation of neurological functions in psychiatric and addictive disorders. In this review, we focus on cannabis use, which is a crucial public health challenge, since cannabis is one of the most widely used addictive drugs in industrialized countries. We first explain why studying retinal function is relevant when exploring the effects of cannabis use on brain function. Next, we describe both the retinal electrophysiological measurements and retinal dysfunctions observed after acute and regular cannabis use. We then discuss how these retinal dysfunctions may inform brain synaptic transmission abnormalities. Finally, we present various directions for future research on the neurotoxic effects of cannabis use.

Retinal explant culture: A platform to investigate human neuro-retina

The retina is the tissue responsible for light detection, in which retinal neurons convert light energy into electrical signals to be transported towards the visual cortex. Damage of retinal neurons leads to neuronal cell death and retinal pathologies, compromising visual acuity and eventually leading to irreversible blindness. Models of retinal neurodegeneration include 2D systems like cell lines, disassociated cultures and co-cultures, and 3D models like organoids, organotypic retinal cultures and animal models. Of these, ex vivo human retinal cultures are arguably the most suitable models for translational research as they retain complex inter-cellular interactions of the retina and precisely mimic in-situ responses. In this review, we summarize the distinguishing features of the human retina which are important to preserve in experimental culture, the historical development of human retinal culture systems, the factors affecting ex vivo human retinal culture and the applications and challenges associated with current methods of human retinal explant culture.

The utility and application of electrophysiological methods in the study of visual hallucinations

Visual hallucinations (VH) are a common symptom in multiple clinical and non-clinical populations. Although structural and functional neuroimaging has informed the understanding of VH, temporal resolution is limited. Electrophysiological techniques provide a complementary perspective on dynamic and temporal aspects of neural functioning, offering greater insight into the mechanisms underlying their formation. In this review we examine and critically evaluate the emerging evidence base utilising electrophysiological approaches in the study of VH. Overall, increased visual system excitability, dysfunctional visual processing and network connectivity, and cholinergic dysfunction have been consistently observed in VH-prone pathologies. However, a major limitation is in the lack of robust experimental studies and the reliance on single case reports. We conclude that electrophysiology provides tentative evidence for the contribution of bottom-up, top-down, and network dysfunction in the aetiology of VH, supporting several existing VH models. Furthermore, we discuss how electrophysiology has been directly utilised in specific clinical interventions for VH. Further exploration utilising electrophysiology in combination with, for example, neuroimaging will help better understand VH aetiology while aiding in the development of novel therapeutic interventions for this difficult to treat symptom.

Distressing Visual Hallucinations after Treatment with Trazodone

Trazodone, a second-generation atypical antidepressant, is increasingly being used off-label, in the treatment of insomnia. Although generally well tolerated, trazodone treatment can be associated with some complications. We describe a case of a 60-year-old man who received trazodone for primary insomnia. He returned, to the emergency department, two days later with distressing visual hallucinations, which prompted inpatient treatment. Trazodone was discontinued, leading to a complete resolution of his visual hallucinations, and he was treated with mirtazapine for 6 months. There has been no relapse in a follow-up period of two years. Patients presenting with visual hallucinations without significant psychiatry history can be a challenging situation. We highlight the importance of careful anamnesis with an accurate medication history. Given the widespread use of trazodone, clinicians should be aware of this possible side effect.