The influence of dopamine on spatial vision

Examining the role of the photopigment melanopsin in the striatal dopamine response to light

The mesolimbic dopamine system is a set of subcortical brain circuits that plays a key role in reward processing, reinforcement, associative learning, and behavioral responses to salient environmental events. In our previous studies of the dopaminergic response to salient visual stimuli, we observed that dopamine release in the lateral nucleus accumbens (LNAc) of mice encoded information about the rate and magnitude of rapid environmental luminance changes from darkness. Light-evoked dopamine responses were rate-dependent, robust to the time of testing or stimulus novelty, and required phototransduction by rod and cone opsins. However, it is unknown if these dopaminergic responses also involve non-visual opsins, such as melanopsin, the primary photopigment expressed by intrinsically photosensitive retinal ganglion cells (ipRGCs). In the current study, we evaluated the role of melanopsin in the dopaminergic response to light in the LNAc using the genetically encoded dopamine sensor dLight1 and fiber photometry. By measuring light-evoked dopamine responses across a broad irradiance and wavelength range in constitutive melanopsin (Opn4) knockout mice, we were able to provide new insights into the ability of non-visual opsins to regulate the mesolimbic dopamine response to visual stimuli.

Early-stage visual perception impairment in schizophrenia, bottom-up and back again

Visual perception is one of the basic tools for exploring the world. However, in schizophrenia, this modality is disrupted. So far, there has been no clear answer as to whether the disruption occurs primarily within the brain or in the precortical areas of visual perception (the retina, visual pathways, and lateral geniculate nucleus [LGN]). A web-based comprehensive search of peer-reviewed journals was conducted based on various keyword combinations including schizophrenia, saliency, visual cognition, visual pathways, retina, and LGN. Articles were chosen with respect to topic relevance. Searched databases included Google Scholar, PubMed, and Web of Science. This review describes the precortical circuit and the key changes in biochemistry and pathophysiology that affect the creation and characteristics of the retinal signal as well as its subsequent modulation and processing in other parts of this circuit. Changes in the characteristics of the signal and the misinterpretation of visual stimuli associated with them may, as a result, contribute to the development of schizophrenic disease.

Effect of vitamin D deficiency on spatial contrast sensitivity function

CLINICAL RELEVANCE

Vitamin D has regulatory effects on non-skeletal tissues including neurons. The contrast sensitivity function occurs as a result of interaction between retinal neurons.

BACKGROUND

The association between plasma vitamin D deficiency and contrast sensitivity function was investigated.

METHODS

Forty-one eyes of 41 subjects with vitamin D deficiency with plasma vitamin D level <20 ng/mL (Group 1), and 30 eyes of 30 subjects without vitamin D deficiency with plasma vitamin D level ≥20 ng/mL (Group 2), were included in this prospective study. OPTEC 6500 was used to measure the contrast sensitivity function at all spatial frequencies involving 1.5 cpd, 3 cpd, 6 cpd, 12cpd, and 18 cpd. The average and sectorial retinal nerve fibre layer thickness, the average and minimum ganglion cell-inner plexiform thickness and tear meniscus height were measured by using optical coherence tomography.

RESULTS

A significant difference was present between Group 1 and Group 2 regarding the plasma vitamin D level (12.4 ± 4.7 ng/mL in Group 1 versus 27.1 ± 6.7 ng/mL in Group 2 p < 0.001). All spatial frequencies of contrast sensitivity function were significantly greater in Group 2 than those in Group 1, as follows: 45 ± 22.6 in Group 1 versus 57.5 ± 20.9 in Group 2, p = 0.08 in 1.5cpd; 71.3 ± 31.3 in Group 1 versus 91.8 ± 27.8 in Group 2, p = 0.001 in 3cpd; 77.9 ± 39.9 in Group 1 versus 100.4 ± 38.4 in Group 2, p = 0.013 in 6cpd; 32 ± 17.5 in Group 1 versus 48.8 ± 25.2 in Group 2, p = 0.002 in 12cpd; and 12.1 ± 5 in Group 1 versus 17.5 ± 9.5 in Group 2, p = 0.001 in 18cpd. However, there were no significant difference between two groups in terms of retinal fibre layer thicknesses, ganglion cell-inner plexiform layer thicknesses, and tear meniscus height.

CONCLUSION

Vitamin D deficiency can lead to a decrease in contrast sensitivity function that is an indicator of visual quality. This may be an underlying reason for certain visual complaints.

Is contrast sensitivity a physiological marker in attention-deficit hyperactivity disorder?

Attention-deficit hyperactivity disorder (ADHD) is one of the most common childhood-onset psychiatric disorders. Although the etiology is complex and has not yet been clarified, dopamine is thought to play a role in the etiology. Methylphenidate (MPH) is a psychostimulant drug used as first-line treatment for ADHD and it inhibits dopamine and norepinephrine reuptake transporters. Dopamine also has an effect on retina and contrast sensitivity. Despite evidence indicating the effects of dopamine on contrast sensitivity, the results of studies examining contrast sensitivity in ADHD patients are inconsistent. Also, no studies have been encountered examining the possible effect of MPH on contrast sensitivity. The hypotheses of this study are that children with ADHD who have not used MPH will have lower contrast sensitivity levels than the members of the control group, that contrast sensitivity levels increase after the use of MPH, and that contrast sensitivity is a potential physiological marker for ADHD. The study was conducted with 30 children with ADHD and 30 children without ADHD. Psychiatric evaluations of the participants were conducted with the Schedule for Affective Disorders and Schizophrenia for School Age Children-Present and Lifetime-Turkish version, Conner's Parent Rating Scale-Revised Short form and the Turgay DSM-IV-based Child and Adolescent Behavioral Disorders Screening and Rating Scale. Photopic contrast sensitivity was measured using the Functional Acuity Contrast Test (FACT). Results showed that FACT mean values of the control group were significantly higher than those of the ADHD group (pre-treatment) in all spatial frequencies. In four spatial frequencies (CPD 1.5, 3, 12 and 18), the FACT mean values of the control group were significantly higher than the ADHD group (during the OROS-MPH treatment). At all spatial frequencies, the mean values of the ADHD group during the OROS-MPH treatment were significantly higher than before the OROS-MPH treatment. In conclusion, the present study showed that contrast sensitivity is low in children with ADHD and increases significantly after OROS-MPH medication, but still did not reach the levels of the children without ADHD. Our findings suggest that contrast sensitivity may be a potential physiological marker in ADHD.

Colour discrimination among patients with schizophrenia in Lebanon

Background: Abnormalities and contrast sensitivity have already been studied in schizophrenia. However, the relationship between symptom severity in schizophrenia and colour vision sensitivity has not been studied systematically.Aim: Our objective was to evaluate colour discrimination in patients with schizophrenia compared to controls and examine if this colour discrimination is correlated with schizophrenia symptoms' severity.Methods: This case-control study, performed between January and April 2017, included 50 schizophrenic patients and 50 healthy controls matched for age and sex. The Positive and Negative Symptoms Scale (PANSS) was used to determine the schizophrenia symptoms' severity. Colour discrimination was evaluated using the total error score (TES) generated using the Farnsworth D-15 test. The higher the TES, the more severe colourblindness.Results: A significantly higher mean TES was found in schizophrenics (30.32) compared to healthy patients (13.07) (p < 0.001). Colour blindness was correlated to the severity of schizophrenic symptoms only in the subgroup of patients with severe schizophrenia.Conclusion: Colour vision defect is a common feature in schizophrenia, and may be more significant when related to psychotic symptoms.KEY POINTSA significantly higher mean TES was found in schizophrenics compared to healthy patients.Colour blindness was correlated to the severity of schizophrenic symptoms only in the subgroup of patients with severe schizophrenia.Colour vision defect is a common feature in schizophrenia, and may be more significant when related to psychotic symptoms.

A long noncoding RNA cluster-based genomic locus maintains proper development and visual function

Long noncoding RNAs (lncRNAs) represent a group of regulatory RNAs that play critical roles in numerous cellular events, but their functional importance in development remains largely unexplored. Here, we discovered a series of previously unidentified gene clusters harboring conserved lncRNAs at the nonimprinting regions in brain (CNIBs). Among the seven identified CNIBs, human CNIB1 locus is located at Chr 9q33.3 and conserved from Danio rerio to Homo sapiens. Chr 9q33.3-9q34.11 microdeletion has previously been linked to human nail-patella syndrome (NPS) which is frequently accompanied by developmental and visual deficiencies. By generating CNIB1 deletion alleles in zebrafish, we demonstrated the requirement of CNIB1 for proper growth and development, and visual activities. Furthermore, we found that the role of CNIB1 on visual activity is mediated through a regulator of ocular development-lmx1bb. Collectively, our study shows that CNIB1 lncRNAs are important for zebrafish development and provides an lncRNA cluster-mediated pathophysiological mechanism for human Chr 9q33.3-9q34.11 microdeletion syndrome.

Psychophysical assessment of koniocellular pathway in patients with schizophrenia versus healthy controls

This study was designed to perform psychophysical assessment of koniocellular pathway in patients with schizophrenia versus healthy controls. A total of 26 patients diagnosed with schizophrenia and 15 healthy controls were included. Snellen Visual Acuity Chart scores and Short Wavelength Automated Perimetry (SWAP) visual field testing including global visual field indices [mean deviation (MD), pattern standard deviation (PSD), test time (min)], reliability parameters [false negative responses (%), false positive responses (%) and fixed losses (%)] and average threshold sensitivity [central (parafovea), peripheral area, and four quadrants] were recorded in both groups. Significantly lower MD scores, higher PSD scores and lower average threshold sensitivity at each location across the visual field were noted in schizophrenia relative to control group. In conclusion, our findings revealed a deficit in koniocellular pathway with impaired SWAP global indices and lower threshold sensitivity at each location across the visual field among chronic schizophrenic patients as compared with control subjects. Our findings emphasize potential application of SWAP outside its original intended purpose as a glaucoma test, to provide deeper understanding of the specific contribution of lateral geniculate nucleus to the visual and cognitive disturbances of schizophrenia.

Veering in hemi-Parkinson's disease: Primacy of visual over motor contributions

Veering while walking is often reported in individuals with Parkinson's disease (PD), with potential mechanisms being vision-based (asymmetrical perception of the visual environment) or motoric (asymmetry in stride length between relatively affected and non-affected body side). We examined these competing hypotheses by assessing veering in 13 normal control participants (NC) and 20 non-demented individuals with PD: 9 with left-side onset of motor symptoms (LPD) and 11 with right-side onset (RPD). Participants walked in a corridor under three conditions: eyes-open, egocentric reference point (ECRP; walk toward a subjectively perceived center of a target at the end of the corridor), and vision-occluded. The visual hypothesis predicted that LPD participants would veer rightward, in line with their rightward visual-field bias, whereas those with RPD would veer leftward. The motor hypothesis predicted the opposite pattern of results, with veering toward the side with shorter stride length. Results supported the visual hypothesis. Under visual guidance, RPD participants significantly differed from NC, veering leftward despite a shorter right- than left-stride length, whereas LPD veered rightward (not significantly different from NC), despite shorter left- than right-stride length. LPD participants showed significantly reduced rightward veering and stride asymmetry when they walked in the presence of a visual landmark (ECRP) than in the eyes-open condition without a target. There were no group differences in veering in the vision-occluded condition. The findings suggest that interventions to correct walking abnormalities such as veering in PD should incorporate vision-based strategies rather than solely addressing motor asymmetries, and should be tailored to the distinctive navigational profiles of LPD and RPD.

Schizophrenia and the eye

Although visual processing impairments are common in schizophrenia, it is not clear to what extent these originate in the eye vs. the brain. This review highlights potential contributions, from the retina and other structures of the eye, to visual processing impairments in schizophrenia and high-risk states. A second goal is to evaluate the status of retinal abnormalities as biomarkers for schizophrenia. The review was motivated by known retinal changes in other disorders (e.g., Parkinson’s disease, multiple sclerosis), and their relationships to perceptual and cognitive impairments, and disease progression therein. The evidence reviewed suggests two major conclusions. One is that there are multiple structural and functional disturbances of the eye in schizophrenia, all of which could be factors in the visual disturbances of patients. These include retinal venule widening, retinal nerve fiber layer thinning, dopaminergic abnormalities, abnormal ouput of retinal cells as measured by electroretinography (ERG), maculopathies and retinopathies, cataracts, poor acuity, and strabismus. Some of these are likely to be illness-related, whereas others may be due to medication or comorbid conditions. The second conclusion is that certain retinal findings can serve as biomarkers of neural pathology, and disease progression, in schizophrenia. The strongest evidence for this to date involves findings of widened retinal venules, thinning of the retinal nerve fiber layer, and abnormal ERG amplitudes. These data suggest that a greater understanding of the contribution of retinal and other ocular pathology to the visual and cognitive disturbances of schizophrenia is warranted, and that retinal changes have untapped clinical utility.

Ophthalmology issues in schizophrenia

Schizophrenia is a complex mental disorder associated with not only cognitive dysfunctions, such as memory and attention deficits, but also changes in basic sensory processing. Although most studies on schizophrenia have focused on disturbances in higher-order brain functions associated with the prefrontal cortex or frontal cortex, recent investigations have also reported abnormalities in low-level sensory processes, such as the visual system. At very early stages of the disease, schizophrenia patients frequently describe in detail symptoms of a disturbance in various aspects of visual perception that may lead to worse clinical symptoms and decrease in quality of life. Therefore, the aim of this review is to describe the various studies that have explored the visual issues in schizophrenia.

Processing of spatial-frequency altered faces in schizophrenia: effects of illness phase and duration

Low spatial frequency (SF) processing has been shown to be impaired in people with schizophrenia, but it is not clear how this varies with clinical state or illness chronicity. We compared schizophrenia patients (SCZ, n = 34), first episode psychosis patients (FEP, n = 22), and healthy controls (CON, n = 35) on a gender/facial discrimination task. Images were either unaltered (broadband spatial frequency, BSF), or had high or low SF information removed (LSF and HSF conditions, respectively). The task was performed at hospital admission and discharge for patients, and at corresponding time points for controls. Groups were matched on visual acuity. At admission, compared to their BSF performance, each group was significantly worse with low SF stimuli, and most impaired with high SF stimuli. The level of impairment at each SF did not depend on group. At discharge, the SCZ group performed more poorly in the LSF condition than the other groups, and showed the greatest degree of performance decline collapsed over HSF and LSF conditions, although the latter finding was not significant when controlling for visual acuity. Performance did not change significantly over time for any group. HSF processing was strongly related to visual acuity at both time points for all groups. We conclude the following: 1) SF processing abilities in schizophrenia are relatively stable across clinical state; 2) face processing abnormalities in SCZ are not secondary to problems processing specific SFs, but are due to other known difficulties constructing visual representations from degraded information; and 3) the relationship between HSF processing and visual acuity, along with known SCZ- and medication-related acuity reductions, and the elimination of a SCZ-related impairment after controlling for visual acuity in this study, all raise the possibility that some prior findings of impaired perception in SCZ may be secondary to acuity reductions.

Perceptual and cognitive effects of antipsychotics in first-episode schizophrenia: the potential impact of GABA concentration in the visual cortex

Schizophrenia is characterized by anomalous perceptual experiences (e.g., sensory irritation, inundation, and flooding) and specific alterations in visual perception. We aimed to investigate the effects of short-term antipsychotic medication on these perceptual alterations. We assessed 28 drug-naïve first episode patients with schizophrenia and 20 matched healthy controls at baseline and follow-up 8 weeks later. Contrast sensitivity was measured with steady- and pulsed-pedestal tests. Participants also received a motion coherence task, the Structured Interview for Assessing Perceptual Anomalies (SIAPA), and the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). Proton magnetic resonance spectroscopy was used to measure gamma-aminobutyric acid (GABA) levels in the occipital cortex (GABA/total creatine [Cr] ratio). Results revealed that, comparing baseline and follow-up values, patients with schizophrenia exhibited a marked sensitivity reduction on the steady-pedestal test at low spatial frequency. Anomalous perceptual experiences were also significantly ameliorated. Antipsychotic medications had no effect on motion perception. RBANS scores showed mild improvements. At baseline, but not at follow-up, patients with schizophrenia outperformed controls on the steady-pedestal test at low spatial frequency. The dysfunction of motion perception (higher coherence threshold in patients relative to controls) was similar at both assessments. There were reduced GABA levels in schizophrenia at both assessments, which were not related to perceptual functions. These results suggest that antipsychotics dominantly affect visual contrast sensitivity and anomalous perceptual experiences. The prominent dampening effect on low spatial frequency in the steady-pedestal test might indicate the normalization of putatively overactive magnocellular retino-geniculo-cortical pathways.

Schizophrenia spectrum participants have reduced visual contrast sensitivity to chromatic (red/green) and luminance (light/dark) stimuli: new insights into information processing, visual channel function, and antipsychotic effects

Background: Individuals with schizophrenia spectrum diagnoses have deficient visual information processing as assessed by a variety of paradigms including visual backward masking, motion perception and visual contrast sensitivity (VCS). In the present study, the VCS paradigm was used to investigate potential differences in magnocellular (M) vs. parvocellular (P) channel function that might account for the observed information processing deficits of schizophrenia spectrum patients. Specifically, VCS for near threshold luminance (black/white) stimuli is known to be governed primarily by the M channel, while VCS for near threshold chromatic (red/green) stimuli is governed by the P channel.

Methods: VCS for luminance and chromatic stimuli (counterphase-reversing sinusoidal gratings, 1.22 c/degree, 8.3 Hz) was assessed in 53 patients with schizophrenia (including 5 off antipsychotic medication), 22 individuals diagnosed with schizotypal personality disorder and 53 healthy comparison subjects.

Results: Schizophrenia spectrum groups demonstrated reduced VCS in both conditions relative to normals, and there was no significant group by condition interaction effect. Post-hoc analyses suggest that it was the patients with schizophrenia on antipsychotic medication as well as SPD participants who accounted for the deficits in the luminance condition.

Conclusions: These results demonstrate visual information processing deficits in schizophrenia spectrum populations but do not support the notion of selective abnormalities in the function of subcortical channels as suggested by previous studies. Further work is needed in a longitudinal design to further assess VCS as a vulnerability marker for psychosis as well as the effect of antipsychotic agents on performance in schizophrenia spectrum populations.

Low Spatial Frequency Bias in Schizophrenia is Not Face Specific: When the Integration of Coarse and Fine Information Fails

Studies have shown that patients with schizophrenia exhibit visual processing impairments, particularly regarding the processing of spatial frequencies. In a previous work, we found that, compared to healthy volunteers, patients were biased toward low spatial frequencies (LSF) to identify facial expression at a glance. Given the ubiquity of faces in visual perception, it remains an open question whether the LSF bias is face specific or also occurs with other visual objects. Here, 15 patients with schizophrenia and 11 healthy control adults performed a categorization task with hybrid stimuli. These stimuli were single images consisting of two different objects, a fruit and an animal, each in a specific spatial frequency range, either low (LSF) or high (HSF). Observers were asked to report if they saw an animal or a fruit. The reported category demonstrated which spatial scale was preferentially perceived in each trial. In a control experiment, participants performed the same task but with images of only a single object, either a LSF or HSF filtered animal or fruit, to verify that participants could perceive both HSF or LSF when presented in isolation. The results on the categorization task showed that patients chose more frequently LSF with hybrid stimuli compared to healthy controls. However, both populations performed equally well with HSF and LSF filtered pictures in the control experiment, demonstrating that the LSF preference found with hybrid stimuli in patients was not due to an inability to perceive HSF. The LSF preference found in schizophrenia confirms our previous study conducted with faces, and shows that this LSF bias generalizes to other categories of objects. When a broad range of spatial frequencies are present in the image, as in normal conditions of viewing, patients preferentially rely on coarse visual information contained in LSF. This result may be interpreted as a dysfunction of the guidance of HSF processing by LSF processing.

Deficient visual sensitivity in schizotypal personality disorder

Schizotypal personality disorder is a personality disorder in the schizophrenia spectrum, sharing genetic and neurobiologic characteristics with schizophrenia. Visual contrast detection, found to be abnormal in chronic schizophrenia, was investigated in schizotypal personality disorder (SPD). Since dopamine in the retina enhances visual contrast detection and SPD patients have relatively reduced dopaminergic activity in the brain compared to schizophrenia patients, it was hypothesized that SPD patients would have decreased to normal contrast sensitivity. Twenty-one subjects with DSM-IV diagnosed SPD, 18 healthy controls, and 12 subjects with a personality disorder unrelated to schizophrenia (OPD) were evaluated for contrast detection using a sinusoidal grating presented at varying temporal frequencies. Subjects also were evaluated neuropsychologically using several standardized neurocognitive tests. A significant effect of subject group was found on the contrast detection threshold (p<0.01) with a significant difference between the SPD group and the healthy control group but not between the OPD group and the healthy control group. The SPD group had higher contrast detection thresholds at all temporal frequencies tested. Correlations were found between contrast detection and performance on the Trail-Making, N-Back, and CPT tasks in SPD patients. These results, based upon a paradigm reflecting dopamine activity in the early visual system, highlight the differences as well as similarities between SPD and schizophrenia with regard to the dopamine system in schizophrenia spectrum (Siever and Davis, 2004).

Altered spatial frequency content in paintings by artists with schizophrenia

While it is difficult to imagine the way someone with mental illness perceives the world, paintings produced by mental illness sufferers with artistic talents offer a hint of this experience. Here we analyze these images in terms of statistics related to low-level visual processing. It is known that art in general possesses regular spatial frequency amplitude spectra, probably due to factors including luminance compression, approximation of natural scene spatial statistics, media, and aesthetics. Whatever the contributions of those factors may be, would the same ones apply for artists with schizophrenia? We find that spatial frequency content in paintings by five artists with schizophrenia or schizoaffective disorder is significantly different from that found in a large sample of art by painters without schizophrenia, while other basic spatial and intensity statistics are not different for the two groups. In particular, amplitude spectrum slopes are significantly steeper for paintings by artists with schizophrenia. A separate study of the works of one artist diagnosed with schizoaffective disorder confirmed these findings and showed no effect of medication type on amplitude spectrum slope. We suggest that these results support the notion that people with schizophrenia show decreased contrast sensitivity at low spatial frequencies. If people with schizophrenia cannot perceive low frequencies at the same level of contrast as that at which healthy individuals can, it follows that on average they will portray such components with higher contrast, resulting in steeper spectra.

The possible role of retinal dopaminergic system in visual performance

It is a well-known fact that the retina is one of the tissues in the body, which is richest in dopamine (DA), yet the role of this system in various visual functions remains unclear. We have identified 13 types of DA retinal pathologies, and 15 visual functions. The pathologies were arranged in this review on a net grid, where one axis was "age" (i.e., from infancy to old age) and the other axis the level of retinal DA (i.e., from DA deficiency to DA excess, from Parkinson disorder to Schizophrenia). The available data on visual dysfunction(s) is critically presented for each of the DA pathologies. Special effort was made to evaluate whether the site of DA malfunction in the different DA pathologies and visual function is at retinal level or in higher brain centers. The mapping of DA and visual pathologies demonstrate the pivot role of retinal DA in mediating visual functions and also indicate the "missing links" in our understanding of the mechanisms underlying these relationships.

Spatio-temporal luminance contrast sensitivity and visual backward masking in schizophrenia

The aim of two experiments was to investigate the relationship between spatio-temporal contrast sensitivity and visual backward masking in normal observers and in subgroups with positive or negative symptoms in schizophrenia. Experiment 1 measured contrast sensitivity for stationary and counterphase-modulated sinusoidal gratings at four spatial (0.5, 2.0, 4.0, 8.0 cycles/degree) and four temporal frequencies (0, 4.0, 8.0, 16.0 Hz). The results showed that there were no differences in spatio-temporal contrast sensitivity between the control and positive-symptom group, and in comparison with these groups, contrast sensitivity was significantly lower at all spatial and temporal frequencies in the negative-symptom group. Experiment 2 measured the visibility of a Landolt C target with a constant target stimulus duration of 4.0 ms followed by a 150-ms backward mask, which was presented at 12 stimulus onset asynchronies from 0 to 110 ms in the same groups of observers. Consistent with the findings of the previous experiment, there were no significant differences in backward masking between the control and positive-symptom group, and in comparison with these groups, visual backward masking was significantly higher at all stimulus onset asynchronies from 40 to 110 ms in the negative-symptom group. The present findings show that there were no significant differences in contrast sensitivity and in backward masking between normal observers and a group with positive symptoms in schizophrenia. It was concluded that the reduction in contrast sensitivity for low spatial frequency counterphase flicker in the negative-symptom group is consistent with a reduction in the 'contrast gain control' mechanism of magnocellular channels, and that the reduction in contrast sensitivity for medium and high stationary gratings is consistent with a disorder in parvocellular channels. It was proposed that a disorder in magnocellular channels in the negative-symptom group may enforce a reliance on parvocellular channels that results in longer temporal summation and visible persistence, slower visual processing of single target stimuli at threshold and higher levels of sensory integration, and backward masking when multiple stimuli are presented rapidly in time.

Vernier Threshold in Patients With Schizophrenia and in Their Unaffected Siblings

The aim of this study was to investigate magnocellular (M) and parvocellular (P) visual functions in nonmedicated patients with schizophrenia and in their unaffected siblings. Possible abnormalities in cortical integration of retinal receptive fields also were addressed. Twenty-two nonmedicated patients with schizophrenia, their unaffected siblings, and 20 age- and IQ-matched healthy control subjects received 4 vernier acuity tasks (blue-on-yellow, frequency-doubling, achromatic low and high contrast conditions) in which they were asked to detect the spatial alignment of dots and gratings. Results revealed that the patients with schizophrenia and their unaffected siblings showed selective dysfunctions in the frequency-doubling and achromatic low contrast conditions, which were devoted to investigate M pathways. In the isoluminant blue-on-yellow and high contrast achromatic conditions, there were no significant differences between the experimental groups. These results suggest that the deficit of M pathway is an endophenotype of schizophrenia.

The effect of peripheral visual motion on focal contrast sensitivity in positive- and negative-symptom schizophrenia

The aim of the present research was to investigate the effect of peripheral (ambient) stimulation on focal visual processing using the far-out jerk effect in normal observers and subgroups with positive- and negative-symptoms in schizophrenia. The far-out jerk effect refers to a reduction in sensitivity of a briefly presented stimulus in central vision in the presence of a sudden movement or oscillation of a stimulus in peripheral vision. In order to measure the far-out jerk effect the focal contrast sensitivity of 5.0Hz modulated sinusoidal target gratings (0.5, 1.0, 2.0, 4.0, and 8.0 number of cycles per degree (c/degrees )) was measured in the presence of three kinds of peripheral surround: a blank field, a stationary 0.75 c/degrees grating, and a 5.0Hz drifting 0.75 c/degrees grating (far-out jerk effect). The findings showed that there were no significant differences in focal contrast sensitivity between the control and positive-symptom group with a blank field and stationary grating surround. However, a 5.0Hz drifting grating surround resulted in a significant reduction in contrast sensitivity at 0.5, 1.0 and 2.0 c/degrees in the positive-symptom group. In comparison with the control group the negative-symptom group showed a generalised reduction in focal contrast sensitivity, a significantly smaller far-out jerk effect, and a significant reduction in contrast sensitivity at 0.5 c/degrees with a stationary grating surround. The finding that both stationary and moving peripheral surrounds have an inhibitory effect on focal contrast sensitivity suggests that there is a dispersion in the visual demarcation between stationary and temporal events in the perception of visual motion in the negative-symptom group.

Spatial frequency discrimination in schizophrenia

Pathways within the visual system can be distinguished on the basis of selectivity for low or high spatial frequencies. Spatial frequency discrimination was evaluated in 17 medicated male patients with schizophrenia and 19 male control subjects. Subjects were required to discriminate whether pairs of high contrast, sinusoidally modulated gratings were the same or different in spatial frequency. Accuracy performance was compared at high, medium, and low spatial frequencies on tasks matched for control performance. Patients showed a greater performance decrement of 12% on low as compared with 4% on high spatial frequencies. These findings suggest a disturbance of right hemisphere mechanisms involved in spatial perception and attention in schizophrenia.

Spatial frequency discrimination in schizophrenia

Pathways within the visual system can be distinguished on the basis of selectivity for low or high spatial frequencies. Spatial frequency discrimination was evaluated in 17 medicated male patients with schizophrenia and 19 male control subjects. Subjects were required to discriminate whether pairs of high contrast, sinusoidally modulated gratings were the same or different in spatial frequency. Accuracy performance was compared at high, medium, and low spatial frequencies on tasks matched for control performance. Patients showed a greater performance decrement of 12% on low as compared with 4% on high spatial frequencies. These findings suggest a disturbance of right hemisphere mechanisms involved in spatial perception and attention in schizophrenia.

Amblyopia

Over the past thirty years, much has been learned about the physiological basis for amblyopia. For many years amblyopia was considered to be a retinal disorder; it has now been well established through animal studies that amblyopia represents functional and morphological effects of visual deprivation on the visual cortex and the lateral geniculate nucleus. With this knowledge has come the recognition of a "sensitive period" of development of the visual system, during which time visual deprivation causes amblyopia. The best approach to managing amblyopia is to detect amblyogenic factors before the age of two years and prevent it through eliminating the causes of visual deprivation. When amblyopia exists, it can be cured if adequately treated in children less than 6-7 years of age. Even in older patients, visual improvement can be achieved with therapy. Current research is aimed at developing substances and delivery modes that will allow the sensitive period of visual development to be manipulated, increasing the period during which it can develop and enhancing preventative and therapeutic measures. In this review selected literature contributing to current understanding of causes, prevention and treatment of amblyopia is discussed. Although many new treatment modalities have been tried, occlusion still seems to be the most successful therapy.

Visual processing, lateralization and syndromes of schizotypy

There is an association between positive features of schizotypy and dyslexia, largely involving distortions of visual and auditory perception and associated cognitive anomalies. In dyslexia, there is strong evidence for a disorder of subcortical magnocellular pathways, which may underlie these perceptual aberrations. Here we investigated visual processing in relation to three syndromes of schizotypy-Active, Withdrawn and Unreality-in normal adults, using a "dot localization" test of visual direction sense on which dyslexics perform poorly, and which should be sensitive to magnocellular dysfunction. Results showed that increased error scores were associated with both the positive syndromes, Active and Unreality, which in this study were highly correlated. There were also interactions between each syndrome and the direction of errors, such that high scorers on the Unreality and Active syndromes made more errors on leftward trials, while high Withdrawn subjects made fewer left and more right errors. Mixed handers also showed poorer performance than consistent handers. The evidence of poor visual direction sense in relation to positive syndromes of schizotypy and mixed handedness is consistent with the previous results for dyslexics, who also show these features. The opposite lateralization of errors in the positive v negative syndromes of schizotypy is in keeping with the model of hemisphere imbalance and patterns of cognitive asymmetry which distinguish Active from Withdrawn syndromes. These results are considered in relation to the proposal that a disorder of magnocellular function may underlie the perceptual distortions which are common to dyslexia and positive syndromes of schizotypy.

Dopaminergic modulation of visual sensitivity in man

A large body of experimental evidence supports the hypothesis that dopamine is a functional neuromodulator at many levels of the visual system. Intrinsic dopaminergic neurons were characterized in most mammalian retina, including man. These neurons give rise to a dendritic plexus covering the retina. Thus, dopamine seems to be involved in the organization of the ganglion cell and the bipolar cell receptive fields and modulates physiological activity of photoreceptors, both processes which underlie sensitivity and spatial selectivity of visual processing in the early stage of the visual system. Moreover, few data are now available concerning the functional significance of dopaminergic modulation of visual sensitivity in man. Parkinson's disease is a specific disorder of central dopaminergic systems. Abnormalities in the pattern-evoked potentials and electroretinogram have been found in parkinsonian patients. Contrast sensitivity, a useful tool for measuring visual spatio-temporal sensitivity in man, has also been shown to be modified due to this affection. Dynamic contrast sensitivity is primarily decreased in these patients, distinguishing them from the normal aging process. Because these modifications in shape of the contrast sensitivity function are reversed by L-Dopa, and that neuroleptic administration could reproduce them in schizophrenia patients, it was suggested that dopamine might tune the contrast sensitivity function in man. We have recently shown that subcutaneous apomorphine induces changes in contrast sensitivity in healthy volunteers, which preferentially affect motion sensitivity. These dopaminergic sensitive modifications in the shape of the contrast sensitivity function might reflect a change in the range of sensitivity of the visual system, both in dynamic and spatial properties. This could be explained by a modification in the spatial and dynamic properties of the ganglion cell responses in the retina. Moreover, we suggest both from our results and from the review of the literature that human psychophysical data confirm the hypothesis that dopamine may be involved in light retinal adaptation, as light-induced and dopamine-induced modifications in the shape in the contrast sensitivity function are quite similar.

Specific effects of the benzodiazepine midazolam on visual receptive fields in light and dark adapted human subjects

Psychophysical experiments in humans have revealed similar characteristics of visual receptive fields as were found in cats and monkeys from retinal ganglion cell recordings. In addition, in some retinal ganglion cells of cats the GABA antagonist bicuculline decreases the activity of the inhibitory surround. These findings led to two predicitions: 1) benzodiazepines will selectively increase the inhibitory surround of human visual receptive fields, 2) after dark adaptation, no free GABA will be available in the synapses and benzodiazepines will have no effect on the visual system. Characteristics of human receptive fields were determined by subthreshold summation: the contrast threshold of a vertical line was measured dependent on the distance of two parallel flanking lines whose contrast was below threshold. Both hypotheses were confirmed: the threshold in the inhibitory region of receptive fields was specifically increased in a dose-dependent manner by midazolam PO (7.5 mg: P < 0.05; 15 mg: P < 0.01). In dark-adapted subjects no effect of midazolam was found. Control experiments with atropine (1 mg IV), sulpiride (100 mg IM), and levodopa (100 mg PO) showed no specific effect. The visual system may be a model to bridge the gap between animal and human psychopharmacology.