The cannabinoid system and visual processing: a review on experimental findings and clinical presumptions

Compare of optic coherence tomography parameters in recreational synthetic tetrahydrocannabinol use and healthy control

PURPOSE

To evaluate retinal thickness (RT), retinal nerve fiber layer thickness (RNFLT), and choroidal thickness (CT) changes in synthetic cannabinoid (SC) users.

METHODS

This prospective study evaluated the RT, RNFLT, and CT values of 56 SC users and 58 healthy controls. The individuals using SCs were referred to us by our hospital's forensic medicine department. Retinal and choroidal images were obtained using spectral-domain optical coherence tomography (OCT). Measurements (one subfoveal, three temporals, three nasal) were taken at 500 μm intervals up to 1500 μm using the caliper system. Only the right eye was used for subsequent analysis.

RESULTS

Mean ages were 27.7 ± 5.7 years in the SC-user group and 25.4 ± 6.7 in the control group. Subfoveal Global RNFLT was in the SCs group 102.3 ± 10.5 μm and 105.6 ± 20.2 μm in the control group (p = 0.271). Subfoveal CT was in the SC group mean of 316.1 ± 100.2 μm and in the control group mean 346.4 ± 81.8 μm (p = 0.065). RT, T500 (283.3 ± 36.7 μm, 296.6 ± 20.5 μm, p = 0.011) and N1500 (355.1 ± 14.3 μm, 349.3 ± 18.1 μm, p = 0.049) were significantly higher in the SC group than in the control group, respectively.

CONCLUSION

Analysis of OCT findings of individuals who had been using SC for more than one year revealed no statistically significant difference between RNFLT and CT, although N1500 was significantly higher in RT. Further studies in the field of OCT are important to explore the pathology of SC.

Comparison of the influence of alcohol and cannabis on the dynamics of the accommodative response

PURPOSE

To assess and compare the changes produced by the two most commonly used substances, alcohol and cannabis, on accommodation dynamics.

METHODS

A total of 38 young participants (19 females) were enrolled in the study. They were assigned to two groups: a cannabis group (N = 19) and an alcohol group. Participants in the cannabis group underwent two randomized sessions: a baseline session and a session after smoking a cigarette. Participants in the alcohol group underwent three randomized sessions: a baseline session, a session after the intake of 300 ml of red wine (Alcohol 1), and other after the ingestion of 450 ml of wine (Alcohol 2). For the accommodation assessment, the open-field autorefractor WAM-5500 was used.

RESULTS

The decrease of the mean velocity of the accommodative response produced by Alcohol 2 condition was significantly greater than that observed for Alcohol 1 and Cannabis (p = 0.046). The direction of the accommodation (near-distance and distance-near) had no effect on the deterioration of the accommodation dynamics following substance use. The target distance had a significant effect on the decrease of the mean velocity following substance use (p = 0.002). The decrease of the amplitude of the accommodative response was associated with a decrease of the peak velocity (p = 0.004) and the increase of the accommodative lag (p < 0.001).

CONCLUSIONS

A moderate-high dose of alcohol impairs accommodation dynamics to a greater extent that lower dose of alcohol or smoked cannabis. The deterioration of the accommodation mean speed was higher for a shorter target distance.

A Cross Talk between the Endocannabinoid System and Different Systems Involved in the Pathogenesis of Hypertensive Retinopathy

The prognosis of hypertension leads to organ damage by causing nephropathy, stroke, retinopathy, and cardiomegaly. Retinopathy and blood pressure have been extensively discussed in relation to catecholamines of the autonomic nervous system (ANS) and angiotensin II of the renin–angiotensin aldosterone system (RAAS) but very little research has been conducted on the role of the ECS in the regulation of retinopathy and blood pressure. The endocannabinoid system (ECS) is a unique system in the body that can be considered as a master regulator of body functions. It encompasses the endogenous production of its cannabinoids, its degrading enzymes, and functional receptors which innervate and perform various functions in different organs of the body. Hypertensive retinopathy pathologies arise normally due to oxidative stress, ischemia, endothelium dysfunction, inflammation, and an activated renin–angiotensin system (RAS) and catecholamine which are vasoconstrictors in their biological nature. The question arises of which system or agent counterbalances the vasoconstrictors effect of noradrenaline and angiotensin II (Ang II) in normal individuals? In this review article, we discuss the role of the ECS and its contribution to the pathogenesis of hypertensive retinopathy. This review article will also examine the involvement of the RAS and the ANS in the pathogenesis of hypertensive retinopathy and the crosstalk between these three systems in hypertensive retinopathy. This review will also explain that the ECS, which is a vasodilator in its action, either independently counteracts the effect produced with the vasoconstriction of the ANS and Ang II or blocks some of the common pathways shared by the ECS, ANS, and Ang II in the regulation of eye functions and blood pressure. This article concludes that persistent control of blood pressure and normal functions of the eye are maintained either by decreasing systemic catecholamine, ang II, or by upregulation of the ECS which results in the regression of retinopathy induced by hypertension.

Assessing the influence of cannabis and alcohol use on different visual functions: A comparative study

This study aimed to assess and compare the influence of alcohol intake and cannabis smoking on different visual functions. A total of 64 young and healthy volunteers took part in the study. All undertook several randomised experimental sessions in which different visual functions, namely distance stereopsis, retinal straylight, visual discrimination capacity, and contrast sensitivity, were tested. Cannabis smokers (N = 30) took a baseline session and a session after smoking a cannabis cigarette, whereas alcohol users (N = 34) underwent a baseline session, a session after a low alcohol intake (Alcohol 1), and a session after a moderate to high alcohol intake (Alcohol 2). All visual functions were impaired by cannabis and alcohol use, particularly for the Cannabis and Alcohol 2 groups. The deterioration of all visual variables was higher for the Alcohol 2 than for the Alcohol 1 and Cannabis groups, except for retinal straylight, the deterioration of which was equal for the Cannabis group, and distant stereopsis, which was more impaired for the Cannabis group. The Alcohol 2 group experienced the most impairing conditions, although very similar to the cannabis group, and that factors other than the experimental conditions, such as sex and age, also influenced these visual changes. Alcohol and cannabis use clearly impair vision. The deterioration caused by cannabis is similar to, but slightly lower than, that produced by a moderate to high alcohol intake, with the experimental conditions, sex and age all having an impact on the variability of visual deterioration.

Visual abilities in Severe Alcohol Use Disorder: Preserved spatial but impaired temporal resolution

Visuospatial impairments have long been reported in Severe Alcohol Use Disorder but remain poorly understood, notably regarding the involvement of magnocellular (MC) and parvocellular (PC) pathways. This empirical gap hampers the understanding of the implications of these visual changes, especially since the MC and PC pathways are thought to sustain central bottom-up and top-down processes during cognitive processing. They thus influence our ability to efficiently monitor our environment and make the most effective decisions. To overcome this limitation, we measured PC-inferred spatial and MC-inferred temporal resolution in 35 individuals with SAUD and 30 healthy controls. We used Landolt circles displaying small apertures outside the sensitivity range of MC cells or flickering at a temporal frequency exceeding PC sensitivity. We found evidence of preserved PC spatial resolution combined with impaired MC temporal resolution in SAUD. We also measured how spatial and temporal sensitivity is influenced by the prior presentation of fearful faces - as emotional content could favor MC processing over PC one - but found no evidence of emotional modulation in either group. This spatio-temporal dissociation implies that individuals with SAUD may process visual details efficiently but perceive rapidly updating visual information at a slower pace. This deficit has implications for the tracking of rapidly changing stimuli in experimental tasks, but also for the decoding of crucial everyday visual incentives such as faces, whose micro-expressions vary continuously. Future studies should further specify the visual profile of individuals with SAUD to incorporate disparate findings within a theoretically grounded model of vision.

Impaired P100 among regular cannabis users in response to magnocellular biased visual stimuli

Regular cannabis using causes vision impairment by affecting human retinal neurotransmission. However, studies less considered its impact on the subsequent visual cortical processing, key feature for the integration of the visual signal in brain. We aimed at investigating this purpose in regular cannabis users using spatial frequencies and temporal frequencies filtered visual stimuli. We recruited 45 regular cannabis users and 25 age-matched controls. We recorded visual evoked potentials during the projection of low spatial frequency (0.5 cycles/degree) or high spatial frequency gratings (15 cycles/degree), which were presented statically (0 Hz) or dynamically (8 Hz). We analyzed the amplitude, latency, and area under the curve of both P100 and N170, best EEG markers for early visual processing. Data were compared between groups by repeated measures ANCOVA. Results showed a significant decrease in P100 amplitude among regular cannabis users in low spatial frequency (F(1,67) = 4.43; p = 0.04) and in dynamic condition (F(1,67) = 4.35; p = 0.04). Analysis also reported a decrease in P100 area under the curve in regular cannabis users to low spatial frequency (F(1,67) = 4.31; p = 0.04) and in dynamic condition (F(1,67) = 7.65; p < 0.01). No effect was found on P100 latency, N170 amplitude, latency, or area under the curve. We found alteration of P100 responses to low spatial frequency and dynamic stimuli in regular cannabis users. This result could be interpreted as a preferential magnocellular impairment where such deficit could be linked to glutamatergic dysfunction. As mentioned in the literature, visual and electrophysiological anomalies in schizophrenia are related to a magnocellular dysfunction. Further studies are needed to clarify electrophysiological deficits in both populations. CLINICAL TRIALS REGISTRATION: Electrophysiological Study of the Functioning of Magnocellular Visual Pathway in Regular Cannabis Users (CAUSA MAP). [NCT02864680; ID 2013-A00097-38]. https://clinicaltrials.gov/ct2/show/NCT02864680?cond=Cannabis&cntry=FR&draw=2&rank=1.

Variations of retinal dysfunctions with the level of cannabis use in regular users: Toward a better understanding of cannabis use pathophysiology

The impact of regular cannabis use on retinal function has already been studied using flash (fERG) and pattern (PERG) electroretinogram. Delayed ganglion and bipolar cells responses were observed as showed by increased peak time of PERG N95 and fERG b-wave recorded in photopic condition. Hypoactivity of amacrine cells was also showed by decreased amplitudes of oscillatory potentials (OPs). However, it is unknown how these retinal anomalies evolve according to the level of cannabis use in cannabis users. The aim of this study was to longitudinally assess the retinal function during a treatment aiming to reduce cannabis use. We recorded PERG and fERG in 40 regular cannabis users receiving either an 8 weeks mindfulness-based relapse prevention program or an 8 weeks treatment-as-usual therapy. ERGs were recorded before treatment, at the end of it, and 4 weeks afterward. We found reduced peak times in PERG N95 and fERG b-wave (p = 0.032 and p = 0.024: Dunn's post-hoc test) recorded at week 8 and increased amplitudes in OP2 and OP3 (p = 0.012 and p = 0.030: Dunn's post-hoc test) recorded at week 12 in users with decreased cannabis use. These results support variations of retinal anomalies with the level of cannabis use, implying that reduction of cannabis use could restore retinal function in regular users.

Aberrant inhibitory processing in the somatosensory cortices of cannabis-users

BACKGROUND

Delta-9 tetrahydrocannabinol (THC) is a major exogenous psychoactive agent, which acts as a partial agonist on cannabinoid (CB₁) receptors. THC is known to inhibit presynaptic neurotransmission and has been repeatedly linked to acute decrements in cognitive function across multiple domains. Previous electrophysiological studies of sensory gating have shown specific deficits in inhibitory processing in cannabis-users, but to date these findings have been limited to the auditory cortices, and the degree to which these aberrations extend to other brain regions remains largely unknown.

METHODS

We used magnetoencephalography (MEG) and a paired-pulse somatosensory stimulation paradigm to probe inhibitory processing in 29 cannabis-users (i.e. at least four times per month) and 41 demographically matched non-user controls. MEG responses to each stimulation were imaged in both the oscillatory and time domain, and voxel time-series data were extracted to quantify the dynamics of sensory gating, oscillatory gamma activity, evoked responses, and spontaneous neural activity.

RESULTS

We observed robust somatosensory responses following both stimulations, which were used to compute sensory gating ratios. Cannabis-users exhibited significantly impaired gating relative to non-users in somatosensory cortices, as well as decreased spontaneous neural activity. In contrast, oscillatory gamma activity did not appear to be affected by cannabis use.

CONCLUSIONS

We observed impaired gating of redundant somatosensory information and altered spontaneous activity in the same cortical tissue in cannabis-users compared to non-users. These data suggest that cannabis use is associated with a decline in the brain's ability to properly filter repetitive information and impairments in cortical inhibitory processing.

The Inhibition of the Degrading Enzyme Fatty Acid Amide Hydrolase Alters the Activity of the Cone System in the Vervet Monkey Retina

Recent studies using full-field electroretinography (ffERG) that triggers a non-specific mass response generated by several retinal sources have attributed an important role for cannabinoid receptors in mediating vision in primates. Specific cone-mediated responses evoked through the photopic flicker ERG appear to be a better way to validate the assumption that endogenous cannabinoids modulate the cone pathway, since FAAH is mainly expressed in the vervet monkey cone photoreceptors. The aim of this study is two-fold: (1) to use the photopic flicker ERG to target the cone pathway specifically, and (2) use URB597 as a selective inhibitor of the endocannabinoid degrading enzyme Fatty Acid Amide Hydrolase (FAAH) to enhance the levels of fatty acid amides, particularly anandamide. We recorded ERGs under four different flicker frequencies (15, 20, 25, and 30 Hz) in light-adapted conditions after intravitreal injections of URB597. Our results show that intravitreal injections of URB597, compared to the vehicle DMSO, increased significantly ffERG amplitudes at 30 Hz, a frequency that solely recruits cone activity. However, at 15 Hz, a frequency that activates both rods and cones, no significant difference was found in the ERG response amplitude. Additionally, we found no differences in implicit times after URB597 injections compared to DMSO vehicle. These results support the role of molecules degraded by FAAH in cone-mediated vision in non-human primates.

Dynamics of the accommodative response after smoking cannabis

PURPOSE

Cannabis is the most widely consumed illicit drug worldwide. It has been suggested that cannabis could generate blurred vision during reading tasks. The goal of this study was to objectively assess the acute effects of smoking cannabis on the dynamics of ocular accommodation. The influence of other factors, including target distance and the direction of accommodation, as well as personal characteristics, were also analysed.

METHODS

Nineteen young people who were occasional cannabis users participated in the study (mean age 22.53 [3.12] years). Their usage profiles were evaluated by means of the Cannabis Use Disorders Identification Test-revised (CUDIT-r). The dynamics of the accommodative response were evaluated using an open-field auto refractor (Grand Seiko WAM-5500). The participants completed two different experimental sessions, one week apart, and in random order (baseline session and after smoking cannabis). During these sessions, the amplitude of the response (D), mean velocity (D/s), peak velocity (D/s), response time (s), accommodative lag (D) and accommodation variability (D) were measured.

RESULTS

The results indicated that cannabis use had a significant main effect on the mean accommodation/disaccommodation velocity (F_1,13  = 7.21; p = 0.02; η p 2  = 0.396). Cannabis consumption also interacted significantly with other factors. Response time showed a significant two-way interaction between condition × target distance (F_1,13  = 11.71; p = 0.005; η p 2  = 0.474) and condition × accommodation direction (F_1,13  = 8.71; p = 0.01; η p 2  = 0.401). For mean velocity, two-way interactions were found between condition × age (F_1,13  = 6.03; p = 0.03; η p 2  = 0.354), condition × CUDIT-r score (F_1,13  = 6.03; p = 0.03; η p 2  = 0.356) and condition × target distance (F_1,13  = 7.20; p = 0.02; η p 2  = 0.396).

CONCLUSIONS

These findings suggest that cannabis use can alter the accommodation process, although further studies should be carried out to explore the role of attention deficits. According to these results, certain daily activities that depend on an accurate accommodative function may be affected by cannabis use.

Impact of Prenatal Exposure to Opioids, Cocaine, and Cannabis on Eye Disorders in Children

OBJECTIVES

Prenatal substance exposure is associated with abnormal visual evoked potentials in offspring, but whether ocular abnormalities are present past infancy is unclear. We determined the association between prenatal substance exposure and hospitalizations for eye disorders in childhood.

METHODS

We conducted a longitudinal cohort study of 794,099 infants born between 2006 and 2016 in all hospital centers in Quebec, Canada. We identified infants prenatally exposed to opioids, cocaine, cannabis, and other illicit substances and followed them over time to assess eye disorders that required in-hospital treatment, including retinal detachment and breaks, strabismus, and other ocular pathologies. We calculated incidence rates and hazard ratios (HR) with 95% confidence intervals (CI) for the association of prenatal substance exposure with risk of eye disorders, adjusted for patient characteristics.

RESULTS

Infants exposed to substances prenatally had a higher incidence of hospitalizations for eye disorders compared with unexposed infants (47.0 vs 32.0 per 10,000 person-years). Prenatal substance exposure was associated with 1.23 times the risk of hospital admission for any eye disorder during childhood compared with no exposure (95% CI 1.04-1.45). Risks were greatest for strabismus (HR 1.55, 95% CI 1.16-2.07) and binocular movement disorders (HR 1.96, 95% CI 1.00-3.83). Opioid use was strongly associated with the risk of ocular muscle disorders (HR 3.15, 95% CI 1.98-5.01).

CONCLUSIONS

Prenatal substance exposure is significantly associated with future hospitalizations for eye disorders in childhood. Efforts to minimize substance use in women of reproductive age are needed in light of the current opioid epidemic.

Oscillatory potentials abnormalities in regular cannabis users: Amacrine cells dysfunction as a marker of central dopaminergic modulation

BACKGROUND

Cannabis is a neuromodulating substance that acts on central synaptic transmission. Regular cannabis use induces a decreased capacity for dopamine synthesis in the brain. The retina is considered an easy means of investigating dysfunctions of synaptic transmission in the brain. We have previously studied the impact of regular cannabis use on retinal function. Using the N95 wave of the pattern electroretinogram, we found a 6 ms-delayed ganglion cells response. Using the b-wave of the photopic flash electroretinogram, we found a 1 ms-delayed bipolar cells response. Here, we investigated amacrine cells function because these cells are located between the bipolar cells and the ganglion cells and contribute to amplifying the signal between these two layers of the retina. We tested the effect of regular cannabis use on these retinal dopaminergic cells. We assessed the role of these cells in amplifying the delay observed previously.

METHODS

We recorded dark-adapted 3.0 flash ERG oscillatory potentials in 56 regular cannabis users and 29 healthy controls. The amplitude and implicit time of OP1, OP2, OP3 and OP4 were evaluated.

RESULTS

Cannabis users showed a significant decrease in OP2 amplitude (p = 0.029, Mann-Whitney test) and OP3 amplitude (p = 0.024, Mann-Whitney test). No significant difference was found between the groups for OP1 and OP4 amplitude or for the implicit time of oscillatory potentials.

CONCLUSIONS

These results reflect the impact of regular cannabis use on amacrine cells function. They highlight abnormalities in dopaminergic transmission and are similar to those found in Parkinson's disease. Oscillatory potentials could be used as markers of central dopaminergic modulation.

Cytotoxic and anti-excitotoxic effects of selected plant and algal extracts using COMET and cell viability assays

Excess glutamate in the central nervous system may be a major cause of neurodegenerative diseases with gradual loss and dysfunction of neurons. Primary or secondary metabolites from medicinal plants and algae show potential for treatment of glutamate-induced excitotoxicity. Three plant extracts were evaluated for impact on glutamate excitotoxicity-induced in primary cultures of retinal ganglion cells (RGC). These cells were treated separately in seven groups: control; Plicosepalus. curviflorus treated; Saussurea lappa treated; Cladophora glomerate treated. Cells were treated independently with 5, 10, 50, or 100 µg/ml of extracts of plant or alga material, respectively, for 2 h. Glutamate-treated cells (48 h with 5, 10, 50, or 100 µM glutamate); and P. curviflorus/glutamate; S. lappa/glutamate; C. glomerata/glutamate [pretreatment with extract for 2 h (50 and 100 µg/ml) before glutamate treatment with 100 µM for 48 h]. Comet and MTT assays were used to assess cell damage and cell viability. The number of viable cells fell significantly after glutamate exposure. Exposure to plant extracts caused no notable effect of viability. All tested plants extracts showed a protective effect against glutamate excitotoxicity-induced RGC death. Use of these extracts for neurological conditions related to excitotoxicity and oxidative stress might prove beneficial.

Aggressive Driving Behaviours in Cannabis Users. The Influence of Consumer Characteristics

This study analysed dangerous driving behaviours in twenty young occasional cannabis users through objective and self-reported data, studying the relationship between the two aspects. Visual function was assessed in a baseline session and after smoking cannabis, as well as speed-related behaviour in a driving simulator. The participants responded to questionnaires on sociodemographic factors, their consumption profile, and the incidence of dangerous behaviours (Dula Dangerous Driving Index; DDDI). After cannabis use, the results revealed a significant deterioration in visual function. In terms of speed management, they showed significantly greater acceleration force in the two different sections of the route, and they drove significantly faster. Our correlations indicate that males and heavier users display more risky speed management. Likewise, the heavier cannabis users admitted to increased dangerous driving behaviour, and an accident in the preceding year was associated with a trend towards aggressive driving behaviour according to the DDDI questionnaire. The findings of this study suggest that cannabis users adopt dangerous behaviours when driving, despite the effect this drug has on certain important functions, such as vision. The results suggest a need for awareness-raising and information campaigns.

Delayed on- and off-retinal responses of cones pathways in regular cannabis users: An On-Off flash electroretinogram case-control study

The retina is considered a useful area for investigating synaptic transmission abnormalities in neuropsychiatric disorders, including as a result of using cannabis, the most widely consumed illicit substance in the developed world. The impact of regular cannabis use on retinal function has already been evaluated, using pattern and flash electroretinogram (ERG) to demonstrate a delay in ganglion and bipolar cell response. Using multifocal ERG, it was showed that the delay to be preferentially located in the central retina. ERG tests do not separately examine the impact of cannabis on the On and Off pathways. The purpose of this study is to assess On and Off pathway function using On-Off ERG. We conducted an On-Off ERG test in 42 regular cannabis users and 26 healthy controls. The protocol was compliant with the International Society for Clinical Electrophysiology of Vision (ISCEV) standards. Amplitude and peak time were measured for the a-, b- and d-waves. Results in the regular cannabis users showed a significant increase in the latencies of both the b- and the d-wave (p = 0.020, p = 0.022, respectively, Mann-Whitney U test), with no change in the wave amplitudes. A-wave peak time and amplitude were unchanged. These findings are reflective of an effect of regular cannabis use on the On and Off pathways and are consistent with previous findings which also identified increases in retinal neuron response times. We confirm here that regular cannabis use impacts the post-receptoral cones pathway at the level of bipolar cells, affecting the On and Off pathways.

Optical Coherence Tomography findings in patients with Multiple Substance Use Disorder

OBJECTIVE

Optical Coherence Tomography (OCT) is a relatively new diagnosis method displaying biological tissue layers by with high-resolution sections. In the present study, the purpose was to examine the OCT findings of patients with Multiple Substance Use Disorder (MSUD) by comparing these findings with healthy controls.

METHODS

The study included 30 MSUD and 30 controls. Detailed biomicroscopic examinations were carried out for all participants, and intraocular pressure, followed by OCT. The central macular thickness (CMT), mean macular thickness (MMT), mean macular volume (MMV), and retinal nerve fibre layer thickness (RNFL) were measured by using OCT.

RESULTS

It was determined that the MMT and CMT were thinned in both eyes compared to the healthy controls. The MMV was decreased in both eyes in patients with substance use disorders compared to healthy controls. The RNFL and total thickness were thickened in temporal and inferior parts in patients with MSUD in both eyes compared to healthy. In the superior quadrant, thickening was detected only in the left eye.

CONCLUSIONS

Based on our results obtained here, it was concluded that vision-related findings should be carefully questioned and evaluated when treatment is planned for patients with substance use.

Effects of cannabis on visual function and self-perceived visual quality

Cannabis is one of the most used drugs of abuse in the world. The objective of this study was to analyze the effects of smoking cannabis on vision and to relate these to those perceived by the user. Thirty-one cannabis users participated in this study. Visual function assessment was carried out in a baseline session as well as after smoking cannabis. We evaluated static visual acuity, contrast sensitivity, stereoacuity, accommodative response, straylight, night-vision disturbances (halos) and pupil size. The participants were also divided into two groups depending on whether they perceived their vision to have worsened after smoking cannabis. A logistic regression analysis was employed to identify which visual test could best predict self-perceived visual effects. The study found that smoking cannabis has significant adverse effects on all the visual parameters analyzed (p < 0.05). Self-perceived visual quality results revealed that about two thirds of the sample think that smoking cannabis impairs their vision. Contrast sensitivity, specifically for the spatial frequency 18 cpd, was identified as the only visual parameter significantly associated with self-perceived visual quality (Odds Ratio: 1.135; p = 0.040). Smoking cannabis is associated with negative effects on visual function. Self-perceived visual quality after smoking cannabis could be related to impaired contrast sensitivity.

Cannabinoids in Glaucoma Patients: The Never-Ending Story

Glaucoma is one of the principal causes of irreversible blindness worldwide. Yet, intraocular pressure (IOP) is the main modifiable risk factor for disease progression. In the never-ending challenge to develop new and effective drugs, several molecules have been tested as anti-glaucoma agents thanks to their pressure-lowering capabilities. Among these molecules, the cannabinoids have been investigated as possible anti-glaucoma drugs since the early 1970s. Cannabinoids are a large class of chemical compounds that exploit their effects by interaction with cannabinoid receptors 1 and 2. These receptors are widely expressed in the human retina where they may influence important functions such as photo-transduction, amacrine cell network maintenance, and IOP regulation. Therefore, in past years several studies have been conducted in order to assess the IOP lowering effects of cannabinoids. PRISMA guidelines have been used to perform a literature search on Pubmed and Scopus aiming to investigate the mechanism of IOP lowering effects and the potential benefits of orally administered, inhaled, topical, and intravenous cannabinoids in the treatment of glaucoma patients.

Effects of Smoking Cannabis on Visual Function and Driving Performance. A Driving-Simulator Based Study

Cannabis is the most widely used illegal drug in the world. Limited information about the effects of cannabis on visual function is available, and more detail about the possible impact of visual effects on car driving is required. This study investigated the effects of smoking cannabis on vision and driving performance, and whether these effects are correlated. Twenty drivers and occasional users were included (mean (SE) age, 23.3 (1.0) years; five women). Vision and simulated driving performance were evaluated in a baseline session and after smoking cannabis. Under the influence of cannabis, certain visual functions such as visual acuity (p < 0.001), contrast sensitivity (p = 0.004) and stereoacuity (far, p < 0.001; near, p = 0.013) worsened. In addition, there was an overall deterioration of driving performance, with the task of keeping the vehicle in the lane proving more difficult (p < 0.05). A correlation analysis showed significant associations between driving performance and visual function. Thus, the strongest correlations were found between the distance driven onto the shoulder and stereoacuity, for near (ρ = 0.504; p = 0.001) and far distances (ρ = 0.408; p = 0.011). This study provides the first evidence to show that the visual effects of cannabis could impact driving performance, compromising driving safety. The results indicate that information and awareness campaigns are essential for reducing the incidence of driving under the influence of cannabis.

[Retinal investigations in patients with major depressive disorder, bipolar disorder or schizophrenia: A review of the literature]

Major depressive disorder, bipolar disorder and schizophrenia are currently among the most common psychiatric disorders, known to constitute a serious public health issue in terms of morbidity, mortality and functional handicap. Their pathophysiology is still unclear, but there is now increasing evidence supporting the existence of abnormalities of neurotransmission. As the retina is an extension of the central nervous system, it may be an interesting site of study which might provide a better understanding of the pathophysiology of psychiatric disorders. Several studies have demonstrated retinal abnormalities, with abnormal cone and rod responses on electroretinography (ERG), suggesting a process of functional neuronal loss, structurally supported by a decrease in the retinal nerve fiber layer thickness (RNFL) on optical coherence tomography (OCT), which suggests involvement of the molecular signal pathways of neurotransmission. These tests could be useful tools for diagnosing and monitoring psychiatric disorders. This article is an overview of the literature on retinal abnormalities observed in patients with major depressive disorder, bipolar disorder or schizophrenia, and discusses how they could be pathophysiologic markers.

Study of retinal structure and function in patients with major depressive disorder, bipolar disorder or schizophrenia: A review of the literature

Major depressive disorder, bipolar disorder and schizophrenia are currently among the most common psychiatric disorders, known to constitute a serious public health issue in terms of morbidity, mortality and functional handicap. Their pathophysiology is still unclear, but there is now increasing evidence supporting the existence of abnormalities of neurotransmission. As the retina is an extension of the central nervous system, it may be an interesting site of study which might provide a better understanding of the pathophysiology of psychiatric disorders. Several studies have demonstrated retinal abnormalities, with abnormal cone and rod responses on electroretinography (ERG), suggesting a process of functional neuronal loss, structurally supported by a decrease in the retinal nerve fiber layer thickness (RNFL) on optical coherence tomography (OCT), which suggests involvement of the molecular signal pathways of neurotransmission. These tests could be useful tools for diagnosing and monitoring psychiatric disorders. This article is an overview of the literature on retinal abnormalities observed in patients with major depressive disorder, bipolar disorder or schizophrenia, and discusses how they could be pathophysiologic markers.

Spatial localization of retinal anomalies in regular cannabis users: The relevance of the multifocal electroretinogram

Widely used in industrialized countries, cannabis is a neuromodulator substance. The cannabinoid system is present at critical stages of retinal processing. We have recently shown a delay in bipolar and ganglion cell responses in regular cannabis users, as observed using flash (fERG) and pattern (PERG) electroretinogram. Although the results obtained during these tests provide information about global retinal responses, they do not give any indication about the spatial localization of the anomalies that were detected. The latter may be analyzed, however, by means of multifocal electroretinogram (mfERG). We recorded the mfERG responses in 49 regular cannabis users and 21 healthy subjects. The amplitudes and implicit times of the mfERG N1, P1 and N2 waves were recorded. The results showed that in regular cannabis users: in the <2° region, a significant increase in the N2 implicit time (p = 0.037); in the 2-5° region, a significant increase in the N2 (p = 0.018) and P1 (p = 0.046) implicit times; in the 5-10° region, a significant increase in the P1 (p = 0.006) and N1 (p = 0.034) implicit times; and in the 10-15° region, a significant increase in the P1 implicit time (p = 0.014). An isolated decrease in the N1 amplitude in the 2-5° region (p = 0.044) was also found. This indicates that there is a delay in the transmission of visual information from the central retina to the near periphery in cannabis users suggesting potential alterations in precise and color vision.

Hallucinogen persisting perception disorder: A literature review and three case reports

The paper describes diagnostic criteria, clinical presentation and types of hallucinogen persisting perception disorder (HPPD), as well as current approaches to the treatment of this phenomenon using available scientific sources. Three case reports are also presented to demonstrate different types of this disorder. The first case report describes a 23-year old patient with a previous history of cannabis consumption who reported HPDD type I after the use of psilocybin mushrooms with small amounts of alcohol and hash. A month later, after cannabis use, the same visual and auditory distortions appeared again. During the following year, hallucinations recurred with the consumption of natural cannabinoids but not with alcohol intake. The symptoms have reduced within a year. Surprisingly, both other cases belonging to HPDD type II appeared in patients who consumed ecstasy, although MDMA is generally not considered a hallucinogen and hallucinations are not frequently reported after MDMA consumption. In both cases of HPPD type II after the use of ecstasy, the condition was very stressful and frightening. Both patients sought medical help and received tofisopam, lamotrigine and sertraline. After that, in both cases visual impairments have smoothed, but have not passed completely. Scientific sources suggest that HPPD may affect more than 50% of hallucinogen users and this disorder is often underdiagnosed. Therefore, patients suffering from HPPD can present in various clinical settings, and clinicians should be aware of this condition.

Association between increased retinal background noise and co-occurrent regular cannabis and alcohol use

BACKGROUND

Cannabis consumption is widespread across the world, and the co-occurrence of cannabis use and alcohol consumption is common. The study of background noise - resting-state neural activity, in the absence of stimulation - is an approach that could enable the neurotoxicity of these substances to be explored. Preliminary results have shown that delta-9-tetrahydrocannabinol (Δ9-THC) causes an increase in neural noise in the brain. Neurons in the brain and the retina share a neurotransmission system and have similar anatomical and functional properties. Retinal function, evaluated using an electroretinogram (ERG), may therefore reflect central neurochemistry. This study analyses retinal background noise in a population of regular co-occurrent cannabis and alcohol consumers.

METHODS

We recorded the flash ERGs of 26 healthy controls and 45 regular cannabis consumers, separated into two groups based on their alcohol consumption: less than or equal to 4 glasses per week (CU ≤ 4) or strictly >4 glasses per week (CU >4). In order to extract the background noise, the Fourier transform of the pseudo-periodic and sinusoidal signals of the 3.0 flicker-response sequence was calculated. This sequence represents the vertical transmission of the signal from cones to bipolar cells. The magnitude of the background noise is defined as the average of the magnitudes of the two neighbouring harmonics: harmonic -1 (low frequency noise) and harmonic +1 (high frequency noise).

RESULTS

The magnitude of harmonic -1 was significantly increased between the groups CU > 4 (6.78 (±1.24)) and CU ≤ 4 (5.69 (±1.80)) among regular users of cannabis and alcohol. A significant increase in the average magnitude of the two harmonics was found between the groups CU > 4 (5.12 (±0.92)) and CU ≤ 4 (4.36 (±1.14)). No significant difference was observed with regard to the magnitude of the harmonic +1.

CONCLUSIONS

The increase in background noise may reflect the neurotoxicity of cannabis, potentiated by alcohol consumption, on retinal neurons dynamic. This neural disruption of the response generated by retinal stimulation may be attributable to altered neurotransmitter release.

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.

Contrast sensitivity and motion discrimination in cannabis users

RATIONALE

Cannabis use impairs visual attention; however, it is unclear whether cannabis use also impairs low level visual processing or whether low level visual deficits can be related to lower dopaminergic functioning found in cannabis users.

OBJECTIVES

To investigate whether spatiotemporal contrast sensitivity and motion discrimination under normal and low luminance conditions differ in cannabis users and non-users.

METHODS

Control (n = 20) and cannabis (n = 21) participants completed a visual acuity test, a saliva test and self-report measures. Spatial and temporal contrast thresholds, motion coherence thresholds for translational and radial motion and the spontaneous eye blink rate were then collected.

RESULTS

Cannabis users showed decreased spatial contrast sensitivity under low luminance conditions and increased motion coherence thresholds under all luminance levels tested compared to non-users. No differences in temporal contrast sensitivity were found between the groups. Frequency of cannabis use correlated significantly and negatively with contrast sensitivity, both spatial and temporal, in the cannabis group and higher motion coherence thresholds for radial motion were also associated with more frequent cannabis use in this group. The eye blink rate was significantly lower in cannabis users compared to non-users.

CONCLUSIONS

The present study shows that cannabis use is associated with deficits in low level visual processing. Such deficits are suggested to relate to lower dopamine, in a similar manner as in clinical populations. The implications for driving safety under reduced visibility (e.g. night) in abstaining cannabis users are discussed.

Delayed bipolar and ganglion cells neuroretinal processing in regular cannabis users: The retina as a relevant site to investigate brain synaptic transmission dysfunctions

Cannabis use is widespread worldwide, but the impact of smoking cannabis regularly on brain synaptic transmission has only been partially elucidated. The retina is considered as an easy means of determining dysfunction in brain synaptic transmission. The endocannabinoid system is involved in regulating retinal synaptic transmission, which might also be affected by tobacco. Previous preliminary results have shown impairments in retinal ganglion cell response in cannabis users. Here, we test the extent to which earlier retinal levels-bipolar cells and photoreceptors-are affected in cannabis users, i.e. by the association of tobacco and cannabis. We recorded pattern (PERG) and flash (fERG) ERG in 53 regular cannabis users and 29 healthy controls. Amplitude and peak time of P50 and N95 (PERG) and of a- and b-waves (fERG) were evaluated. Cannabis users showed a significant increase in PERG N95 peak time and in fERG light-adapted 3.0 b-wave peak time, compared with controls (p = 0.0001 and p = 0.002, respectively; Mann-Whitney U test). No significant difference was found between the groups in terms of wave amplitude (p = 0.525 and p = 0.767 for the N95 and light-adapted 3.0 b-wave amplitude respectively; Mann-Whitney U test). The results demonstrated delayed ganglion and bipolar cell responses in cannabis users. These results reflect a delay in the transmission of visual information from the retina to the brain. This retinal dysfunction may be explained by an effect of cannabis use on retinal synaptic transmission. Main limitations of these results concern tobacco and alcohol use that differed between groups. The consequences of these anomalies on visual perception along with the molecular mechanisms underlying this retinal dysfunction should be explored in future human and animal studies.

The adverse health effects and harms related to marijuana use: an overview review

BACKGROUND

With impending marijuana legislation in Canada, a broad understanding of the harms associated with marijuana use is needed to inform the clinical community and public, and to support evidence-informed public policy development. The purpose of the review was to synthesize the evidence on adverse health effects and harms of marijuana use.

METHODS

We searched MEDLINE, The Cochrane Database of Systematic Reviews, Embase, PsycINFO, the Cumulative Index to Nursing and Allied Health Literature, and the Health Technology Assessment Database from the inception of each database to May 2018. Given that systematic reviews evaluating one or other specific harm have been published, this is an overview review with the primary objective of assessing a health effect or harm. Data on author, country and year of publication, search strategy and results, and outcomes were extracted. Quality was assessed using the AMSTAR (A Measurement Tool to Assess Systematic Reviews) checklist.

RESULTS

The final analysis included 68 reviews. Evidence of harm was reported in 62 reviews for several mental health disorders, brain changes, cognitive outcomes, pregnancy outcomes and testicular cancer. Inconclusive evidence was found for 20 outcomes (some mental health outcomes, other types of cancers and all-cause mortality). No evidence of harm was reported for 6 outcomes.

INTERPRETATION

Harm was associated with most outcomes assessed. These results should be viewed with concern by physicians and policy-makers given the prevalence of use, the persistent reporting of a lack of recognition of marijuana as a possibly harmful substance and the emerging context of legalization for recreational use.

Hallucinogen Persisting Perception Disorder: Etiology, Clinical Features, and Therapeutic Perspectives

Hallucinogen Persisting Perception Disorder (HPPD) is a rare, and therefore, poorly understood condition linked to hallucinogenic drugs consumption. The prevalence of this disorder is low; the condition is more often diagnosed in individuals with a history of previous psychological issues or substance misuse, but it can arise in anyone, even after a single exposure to triggering drugs. The aims of the present study are to review all the original studies about HPPD in order to evaluate the following: (1) the possible suggested etiologies; (2) the possible hallucinogens involved in HPPD induction; (3) the clinical features of both HPPD I and II; (4) the possible psychiatric comorbidities; and (5) the available and potential therapeutic strategies. We searched PubMed to identify original studies about psychedelics and Hallucinogen Persisting Perception Disorder (HPPD). Our research yielded a total of 45 papers, which have been analyzed and tabled to provide readers with the most updated and comprehensive literature review about the clinical features and treatment options for HPPD.

Impaired contrast sensitivity at low spatial frequency in cannabis users with early onset

The regular use of cannabis generates pronounced cognitive disorders, especially in users who begin before the age of 15-16. However, less is known about the impact of regular cannabis on visual function, especially in the case of early onset. Cannabinoid receptors (CB1) are expressed in areas of the visual system, like the thalamus and primary cortex, which might originate sensory disorders. Hence, we measured contrast sensitivity (CS) in three groups, i.e. cannabis users with late onset of cannabis use (after 16 years old), cannabis users with early onset". We used a constant method which allowed us to control for biased responses. Stimuli were presented at high and low spatial frequencies and in both static and dynamic conditions (8Hz). As contrast sensitivity is measured behaviorally based on an explicit response and could thus be impacted by attentional or vigilance disorders, participants' attention and vigilance were carefully monitored by means of the D2 test, CPT-AX for attention and pupillography for vigilance. Cannabis users with early onset were significantly impaired only at low spatial frequency. This effect was independent of response bias, vigilance and attention. These results show for the first time that early cannabis use impacts contrast sensitivity at low spatial frequency.

Association of anandamide with altered binocular depth inversion illusion in schizophrenia

OBJECTIVES

Binocular depth inversion illusion (BDII) represents an illusion of visual perception that involves higher-order visual and cognitive processes. Its impairment has been linked to psychotic conditions and identified as a marker for at-risk mental states. The endogenous cannabinoid system (ECS) is involved in various neurophysiological processes. One of its key components, anandamide, is involved in the pathophysiology of schizophrenia. Little is known about its impact on BDII alterations. Therefore, we explored associations between BDII and anandamide levels.

METHODS

BDII was conducted and blood and CSF were taken in 28 first-episode antipsychotic-naïve schizophrenia (SZ) patients and 81 healthy controls (HC). Serum and CSF anandamide levels were determined by high-performance liquid chromatography/mass spectrometry.

RESULTS

BDII scores were significantly elevated in SZ versus HC, indicating a disruption of illusionary revision of percepts in SZ. Anandamide levels were significantly higher in CSF of SZ compared to HC, while serum anandamide was not. However, we found specific association differences of anandamide levels and BDII scores between schizophrenia patients and controls in serum.

CONCLUSIONS

These findings support the hypothesis of an involvement of anandamide in cognitive processes impaired in schizophrenia and are consistent with a protective effect of elevated anandamide levels herein.

Looking into the brain through the retinal ganglion cells in psychiatric disorders: A review of evidences

Psychiatry and neuroscience research need novel approaches to indirectly investigate brain function. As the retina is an anatomical and developmental extension of the central nervous system (CNS), changes in retinal function may reflect neurological dysfunctions in psychiatric disorders. The last and most integrated retinal relay before visual information transfer to the brain is the ganglion cell layer. Here, based on collected arguments, we argue that these cells offer a crucial site for indirectly investigating brain function. We describe the anatomical and physiological properties of these cells together with measurements of their functional properties named pattern electroretinogram (PERG). Based on ganglion cell dysfunctions measured with PERG in neurological disorders, we argue for the relevance of studying ganglion cell function in psychiatric research. We review studies that have evaluated ganglion cell function in psychiatric and addictive disorders and discuss how changes in PERG measurements could be functional markers of pathophysiological mechanisms of psychiatric disorders.

Emotion regulation deficits in regular marijuana users

Effective regulation of negative affective states has been associated with mental health. Impaired regulation of negative affect represents a risk factor for dysfunctional coping mechanisms such as drug use and thus could contribute to the initiation and development of problematic substance use. This study investigated behavioral and neural indices of emotion regulation in regular marijuana users (n = 23) and demographically matched nonusing controls (n = 20) by means of an fMRI cognitive emotion regulation (reappraisal) paradigm. Relative to nonusing controls, marijuana users demonstrated increased neural activity in a bilateral frontal network comprising precentral, middle cingulate, and supplementary motor regions during reappraisal of negative affect (P < 0.05, FWE) and impaired emotion regulation success on the behavioral level (P < 0.05). Amygdala-focused analyses further revealed impaired amygdala downregulation in the context of decreased amygdala-dorsolateral prefrontal cortex functional connectivity (P < 0.05, FWE) during reappraisal in marijuana users relative to controls. Together, the present findings could reflect an unsuccessful attempt of compensatory recruitment of additional neural resources in the context of disrupted amygdala-prefrontal interaction during volitional emotion regulation in marijuana users. As such, impaired volitional regulation of negative affect might represent a consequence of, or risk factor for, regular marijuana use. Hum Brain Mapp 38:4270-4279, 2017. © 2017 Wiley Periodicals, Inc.

The arguments for and against cannabinoids application in glaucomatous retinopathy

Glaucoma represents several optic neuropathies leading to irreversible blindness through progressive retinal ganglion cell (RGC) loss. Reduction of intraocular pressure (IOP) is known as the only modifiable factor in the treatment of this disorder. Application of exogenous cannabinoids to lower IOP has attracted attention of scientists as potential agents for the treatment of glaucoma. Accordingly, neuroprotective effect of these agents has been recently described through modulation of endocannabinoid system in the eye. In the present work, pertinent information regarding ocular endocannabinoid system, mechanism of exogenous cannabinoids interaction with the ocular endocannabinoid system to reduce IOP, and neuroprotection property of cannabinoids will be discussed according to current scientific literature. In addition to experimental studies, bioavailability of cannabinoids, clinical surveys, and adverse effects of application of cannabinoids in glaucoma will be reviewed.

Cannabinoid Receptors CB1 and CB2 Modulate the Electroretinographic Waves in Vervet Monkeys

The expression patterns of the cannabinoid receptor type 1 (CB1R) and the cannabinoid receptor type 2 (CB2R) are well documented in rodents and primates. In vervet monkeys, CB1R is present in the retinal neurons (photoreceptors, horizontal cells, bipolar cells, amacrine cells, and ganglion cells) and CB2R is exclusively found in the retinal glia (Müller cells). However, the role of these cannabinoid receptors in normal primate retinal function remains elusive. Using full-field electroretinography in adult vervet monkeys, we recorded changes in neural activity following the blockade of CB1R and CB2R by the intravitreal administration of their antagonists (AM251 and AM630, resp.) in photopic and scotopic conditions. Our results show that AM251 increases the photopic a-wave amplitude at high flash intensities, whereas AM630 increases the amplitude of both the photopic a- and b-waves. In scotopic conditions, both blockers increased the b-wave amplitude but did not change the a-wave amplitude. These findings suggest an important role of CB1R and CB2R in primate retinal function.

The Endocannabinoid System in the Retina: From Physiology to Practical and Therapeutic Applications

Cannabis is one of the most prevalent drugs used in industrialized countries. The main effects of Cannabis are mediated by two major exogenous cannabinoids: ∆9-tetrahydroxycannabinol and cannabidiol. They act on specific endocannabinoid receptors, especially types 1 and 2. Mammals are endowed with a functional cannabinoid system including cannabinoid receptors, ligands, and enzymes. This endocannabinoid signaling pathway is involved in both physiological and pathophysiological conditions with a main role in the biology of the central nervous system. As the retina is a part of the central nervous system due to its embryonic origin, we aim at providing the relevance of studying the endocannabinoid system in the retina. Here, we review the distribution of the cannabinoid receptors, ligands, and enzymes in the retina and focus on the role of the cannabinoid system in retinal neurobiology. This review describes the presence of the cannabinoid system in critical stages of retinal processing and its broad involvement in retinal neurotransmission, neuroplasticity, and neuroprotection. Accordingly, we support the use of synthetic cannabinoids as new neuroprotective drugs to prevent and treat retinal diseases. Finally, we argue for the relevance of functional retinal measures in cannabis users to evaluate the impact of cannabis use on human retinal processing.

Endogenous and Synthetic Cannabinoids as Therapeutics in Retinal Disease

The functional significance of cannabinoids in ocular physiology and disease has been reported some decades ago. In the early 1970s, subjects who smoked Cannabis sativa developed lower intraocular pressure (IOP). This led to the isolation of phytocannabinoids from this plant and the study of their therapeutic effects in glaucoma. The main treatment of this disease to date involves the administration of drugs mediating either the decrease of aqueous humour synthesis or the increase of its outflow and thus reduces IOP. However, the reduction of IOP is not sufficient to prevent visual field loss. Retinal diseases, such as glaucoma and diabetic retinopathy, have been defined as neurodegenerative diseases and characterized by ischemia-induced excitotoxicity and loss of retinal neurons. Therefore, new therapeutic strategies must be applied in order to target retinal cell death, reduction of visual acuity, and blindness. The aim of the present review is to address the neuroprotective and therapeutic potential of cannabinoids in retinal disease.

The emerging field of retinal electrophysiological measurements in psychiatric research: A review of the findings and the perspectives in major depressive disorder

Major depressive disorder (MDD) is a severe mental illness leading to long-term disabilities. One of the current challenges in psychiatric research is to develop new approaches to investigate the pathophysiology of MDD and monitor drug response in order to provide better therapeutic strategies to the patients. Since the retina is considered as part of the central nervous system, it was suggested that it constitutes an appropriate site to investigate mental illnesses. In the past years, several teams assessed the retinal function of patients with mood disorders and many relevant abnormalities have been reported. Investigation of the retinal electrophysiological abnormalities in MDD remains a young emerging field, but we believe that the current findings are very promising and we argue that objective retinal electrophysiological measurements may eventually become relevant tools to investigate the pathophysiology of MDD. Here, we review the retinal abnormalities detected with objective electrophysiological measurements such as the flash electroretinogram (fERG), the pattern electroretinogram (PERG) and the electrooculogram (EOG) in patients with MDD. We discuss how these changes might reflect the pathophysiology of MDD in both clinical and scientific points of view, according especially to the monoamine neurotransmission deficiency hypothesis. We also discuss the technical details that must be taken into consideration for a potential use of the objective retinal electrophysiological measurements as tools to investigate the pathophysiology of MDD.