Function Of Visual System Research Articles

MDMA use is associated with increased spatial BOLD fMRI visual cortex activation in human MDMA users

Previous animal studies have demonstrated that 3,4-methylenedioxymethamphetamine (MDMA) exposure causes serotonin axotomy that is greatest in occipital cortex (including primary visual cortex) where serotonergic axons innervate neurons and blood vessels. Human MDMA users have altered serotonergic function and reduced gray matter density in occipital cortex. The fMRI BOLD method is potentially sensitive to both the neuronal and vascular consequences of MDMA-induced serotonin toxicity. To test the hypothesis that MDMA users have altered visual system function, we used the fMRI BOLD technique to assay visual cortical activation after photic stimulation in a group of adult MDMA users. Because MDMA users worldwide are polydrug users and therefore difficult to match to comparison groups in terms of polydrug exposure, we conducted a primary within-group analysis examining the correlation between lifetime episodes of MDMA exposure and measures of visual cortical activation. The within-group correlational analysis in the MDMA user group revealed that the degree of prior MDMA exposure was significantly positively correlated with the number of activated pixels for photic stimulation ( r = 0.582, p = 0.007). A secondary between-group comparison of MDMA users with non-MDMA users found overall greater levels of polydrug exposure in the MDMA user cohort but no significant differences in visual cortical activation measures between the two groups. Additional research is needed to clarify the origin and significance of the current findings.

Microinjection of ritanserin into the CA1 region of hippocampus improves scopolamine-induced amnesia in adult male rats

The effect of ritanserin (5-HT 2 antagonist) on scopolamine (muscarinic cholinergic antagonist)-induced amnesia in Morris water maze (MWM) was investigated. Rats were divided into eight groups and bilaterally cannulated into CA1 region of the hippocampus. One week later, they received repeatedly vehicles (saline, DMSO, saline + DMSO), scopolamine (2 μg/0.5 μl saline/side; 30 min before training), ritanserin (2, 4 and 8 μg/0.5 μl DMSO/side; 20 min before training) and scopolamine (2 μg/0.5 μl; 30 min before ritanserin injection) + ritanserin (4 μg/0.5 μl DMSO) through cannulae each day. Animals were tested for four consecutive days (4 trial/day) in MWM during which the position of hidden platform was unchanged. In the fifth day, the platform was elevated above the water surface in another position to evaluate the function of motor, motivational and visual systems. The results showed a significant increase in escape latencies and traveled distances to find platform in scopolamine-treated group as compared to saline group. Ritanserin-treated rats (4 μg/0.5 μl/side) showed a significant decrease in the mentioned parameters as compared to DMSO-treated group. However, scopolamine and ritanserin co-administration resulted in a significant decrease in escape latencies and traveled distances as compared to the scopolamine-treated rats. Our findings show that microinjection of ritanserin into the CA1 region of the hippocampus improves the scopolamine-induced amnesia.

Visual functions and functional vision

ICF and ICIDH recognize various aspects of vision loss. Within this framework we use the term Visual functions to describe how well the eye and visual system function; functional vision is used to describe how well the person functions in vision-related activities. Optimizing the relationship between these two aspects is the goal of Vision Rehabilitation.Assessments to identify rehabilitation priorities are different from the usual clinical assessments to find underlying disorders. Most tests of visual functions were developed for clinical diagnostic use and thus are not optimized for assessment of functional vision. To base rehabilitation eligibility rules (e.g. “legal blindness”) exclusively on visual function measurement (e.g. 20/200 in the US, 3/60 in other countries) reflects a misunderstanding of this important distinction.This paper traces developments since the presentation of the “Guide for the Evaluation of Visual Impairment” at Vision-99 (ISLRR, 1999) to the implementation of its principles in the AMA “Guide to the Evaluation of Permanent Impairment” (AMA, 2001), to the report on “Visual Standards—Aspects and Ranges of Vision Loss” (International Council of Ophthalmology, 2002) and to the “Consultation on the Development of Standards for Characterization of Vision Loss and Visual Functioning” (WHO, 2003).

Identification of Zebrafish Insertional Mutants With Defects in Visual System Development and Function

Genetic analysis in zebrafish has been instrumental in identifying genes necessary for visual system development and function. Recently, a large-scale retroviral insertional mutagenesis screen, in which 315 different genes were mutated, that resulted in obvious phenotypic defects by 5 days postfertilization was completed. That the disrupted gene has been identified in each of these mutants provides unique resource through which the formation, function, or physiology of individual organ systems can be studied. To that end, a screen for visual system mutants was performed on 250 of the mutants in this collection, examining each of them histologically for morphological defects in the eye and behaviorally for overall visual system function. Forty loci whose disruption resulted in defects in eye development and/or visual function were identified. The mutants have been divided into the following phenotypic classes that show defects in: (1) morphogenesis, (2) growth and central retinal development, (3) the peripheral marginal zone, (4) retinal lamination, (5) the photoreceptor cell layer, (6) the retinal pigment epithelium, (7) the lens, (8) retinal containment, and (9) behavior. The affected genes in these mutants highlight a diverse set of proteins necessary for the development, maintenance, and function of the vertebrate visual system.

The Heparan Sulfate Proteoglycans Dally-like and Syndecan Have Distinct Functions in Axon Guidance and Visual-System Assembly in Drosophila

Heparan sulfate proteoglycans (HSPGs), a class of glycosaminoglycan-modified proteins, control diverse patterning events via their regulation of growth-factor signaling and morphogen distribution. In C. elegans, zebrafish, and the mouse, heparan sulfate (HS) biosynthesis is required for normal axon guidance, and mutations affecting Syndecan (Sdc), a transmembrane HSPG, disrupt axon guidance in Drosophila embryos. Glypicans, a family of glycosylphosphatidylinositol (GPI)-linked HSPGs, are expressed on axons and growth cones in vertebrates, but their role in axon guidance has not been determined. We demonstrate here that the Drosophila glypican Dally-like protein (Dlp) is required for proper axon guidance and visual-system function. Mosaic studies revealed that Dlp is necessary in both the retina and the brain for different aspects of visual-system assembly. Sdc mutants also showed axon guidance and visual-system defects, some that overlap with dlp and others that are unique. dlp+ transgenes were able to rescue some sdc visual-system phenotypes, but sdc+ transgenes were ineffective in rescuing dlp abnormalities. Together, these findings suggest that in some contexts HS chains provide the biologically critical component, whereas in others the structure of the protein core is also essential.

Visual Warning Signals Optimized for Human Perception: What the Eye Sees Fastest

This study aimed to answer the question of how to design a visual warning signal that is most easily seen and produces the quickest reaction time. This is a classic problem of bionic optimization—if one knows the properties of the receiver one can most easily find a suitable solution. Because the peak of the spatio-temporal contrast sensitivity function of the human visual system occurs at non-zero spatial and temporal frequencies, it is likely that movement enhances the detectability of threshold visual signals. Earlier studies employing extended drifting sinewave gratings bear out this prediction. We have studied the ability of human observers to detect threshold visual signals for both moving and stationary stimuli. We used discrete, localized signals such as might be employed in aerospace or automotive warning signal displays. Moving stimuli show a superior detectability to non-moving stimuli of the same integrated energy. Moving stimuli at threshold detectability are seen faster than non-moving threshold stimuli. Under some conditions the speed advantage is over 0.25 seconds. Similar advantages have also been shown to occur for suprathreshold signals.

The Improved Binary Tree Vector Quantization Using Spatial Sensitivity of HVS

In this paper, we proposed an improved binary tree vector quantization in special consideration of the area of spatial sensitivity which is an important characteristic of the human visual system. We regarded spatial sensitivity as a function of the human visual system, which works using variations of the three primary colors in blocks of input images. In addition, we applied the weight derived from HVS spatial sensitivity to the process of splitting nodes using eigenvectors in binary tree vector quantization. The test results showed that the proposed method provided better visual quality and greater PSNR than conventional methods.

Disturbed visual system function in methionine synthase deficiency

Isolated functional methionine synthase deficiency occurs in the cblE and cblG defects of methylcobalamin metabolism and is one of a number of causes of severely elevated plasma homocysteine. Clinical features are predominantly of a neurological nature but also include functional restriction of the visual system manifesting as loss of visual acuity and nystagmus. As yet, the origin and pathogenesis of impaired vision have not been explained. We investigated a patient who was proven by complementation analysis in cultured fibroblasts to belong to the cblG complementation group. Ganzfeld electroretinograms (ERG) and flash visual evoked potentials (VEP) were recorded over a period of 4 years. Amplitudes of all International Society for Clinical Electrophysiology of Vision (ISCEV) standard responses were below normal. The greatest reductions were of rod response to 24 microV, of standard combined response (SC) b-wave to 120 microV, of oscillatory potentials (OP) to 5 microV, of cone response b-wave to 35 microV, and of 30 Hz flicker response to 8 microV. Except for SC and cone a-waves at age 2.5 and 3.5 years, as well as cone b-wave at 3.5 years, amplitudes remained at a subnormal level at follow-up examinations. Implicit times were slightly prolonged (SC b-wave 6 ms, OPs 2 ms, cone b-wave 2 ms, 30 Hz flicker 4 ms) or fell within the normal range. Responses of the flash VEP were severely deformed but reproducible. This is the first report of detailed investigations of the visual system in a patient with isolated methionine synthase deficiency. Reduced oscillatory potentials suggest microvascular damage to the retina through homocysteine. Decreased photoreceptor function as well as ganglion cell loss as indicated by pathological flash VEPs may reflect a cytotoxic impact of homocysteine on neurons of the visual pathway.

Lesion-induced pseudo-dominance at functional magnetic resonance imaging: implications for preoperative assessments.

To illustrate how lesion-induced neurovascular uncoupling at functional magnetic resonance imaging (fMRI) can mimic hemispheric dominance opposite the side of a lesion preoperatively. We retrospectively reviewed preoperative fMRI mapping data from 50 patients with focal brain abnormalities to establish patterns of hemispheric dominance of language, speech, visual, or motor system functions. Abnormalities included gliomas (31 patients), arteriovenous malformations (AVMs) (11 patients), other congenital lesions (4 patients), encephalomalacia (3 patients), and tumefactive encephalitis (1 patient). A laterality ratio of fMRI hemispheric dominance was compared with actual hemispheric dominance as verified by electrocortical stimulation, Wada testing, postoperative and posttreatment deficits, and/or lesion-induced deficits. fMRI activation maps were generated with cross-correlation (P < 0.001) or t test (P < 0.001) analysis. In 50 patients, a total of 85 functional areas were within 5 mm of the edge of a potentially resectable lesion. In 23 of these areas (27%), reduced fMRI signal in perilesional eloquent cortex in conjunction with preserved or increased signal in homologous contralateral brain areas revealed functional dominance opposite the side of the lesion. This suggested possible lesion-induced transhemispheric cortical reorganization to homologous brain regions (homotopic reorganization). In seven patients, however, the fMRI data were inconsistent with other methods of functional localization. In two patients with left inferior frontal gyrus gliomas and in one patient with focal tumefactive meningoencephalitis, fMRI incorrectly suggested strong right hemispheric speech dominance. In two patients with lateral precentral gyrus region gliomas and one patient with a left central sulcus AVM, the fMRI pattern incorrectly suggested primary corticobulbar motor dominance contralateral to the side of the lesion. In a patient with a right superior frontal gyrus AVM, fMRI revealed pronounced left dominant supplementary motor area activity in response to a bilateral complex motor task, but right superior frontal gyrus perilesional hemorrhage and edema subsequently caused left upper-extremity plegia. Pathophysiological factors that might have caused neurovascular uncoupling and facilitated pseudo-dominance at fMRI in these patients included direct tumor infiltration, neovascularity, cerebrovascular inflammation, and AVM-induced hemodynamic effects. Sixteen patients had proven (1 patient), probable (2 patients), or possible (13 patients) but unproven lesion-induced homotopic cortical reorganization. Lesion-induced neurovascular uncoupling causing reduced fMRI signal in perilesional eloquent cortex, in conjunction with normal or increased activity in homologous brain regions, may simulate hemispheric dominance and lesion-induced homotopic cortical reorganization.

Efficacy and safety of memantine treatment for reduction of changes associated with experimental glaucoma in monkey, I: Functional measures.

To determine, using electrophysiological measures of visual system function, whether oral daily dosing of memantine is both safe and effective to reduce the injury associated with experimental glaucoma in primates. Argon laser treatment of the anterior chamber angle was used to induce chronic ocular hypertension (COHT) in the right eye of 18 macaque monkeys. Nine animals were orally dosed daily with 4 mg/kg memantine while the other nine animals received an oral dose of vehicle only. Using both conventional and multifocal methods, recordings of the electroretinogram (ERG) were made at approximately 3, 5, and 16 months after elevation of the intraocular pressure (IOP). Recordings of the visually-evoked cortical potential (VECP) were also made at the 16-month time point. Chronic ocular hypertension was associated with a reduction in the amplitude of components of the multifocal ERG response and visually-evoked cortical potential. Memantine-treated animals suffered less amplitude reduction for these measures than did vehicle-treated animals, though this treatment effect on the ERG measures was observed only at the early time points (3 and 5 months post IOP elevation). Memantine treatment was not associated with an effect on either the kinetics or amplitude of ERG or VECP response measures obtained from the normotensive eyes. Systemic treatment with memantine, a compound which does not lower intraocular pressure, was both safe and effective for reduction of functional loss associated with experimental glaucoma.

Aberrant brain dynamics in schizophrenia: delayed buildup and prolonged decay of the visual steady-state response

In schizophrenia, aberrant brain activity has been reported both during stimulus processing and at rest. Evoked response amplitude is a function of both the number and synchronization of neurons firing in relation to a stimulus. It is at present unclear whether schizophrenia patients have normal synchronization of neural activity in relation to stimulus processing, and whether the amount and time course of synchronization is related to their evoked response amplitudes. EEG brain dynamics in response to visual steady-state stimulation were assessed in 12 schizophrenia and 12 healthy subjects at three stimulation durations (2, 4, and 6 s). Group differences in the visual evoked potential, the visual steady-state response, and the local coherence of the visual steady-state response were evaluated over time. Schizophrenia patients had smaller and delayed event-related potentials. Moreover, they had a slower buildup of steady-state amplitude following stimulation onset and a prolonged decrease after stimulation offset. Groups did not differ during mid-segments of steady-state stimulation. Increase in coherence to stimulation onset did not differ between-groups, but coherence decay of the visual steady-state response following stimulus offset was delayed in schizophrenia patients. The initial response to visual stimulation among schizophrenia subjects, therefore, may be reduced in amplitude due to weak signal strength, not poor coordination between distant cortical regions. The prolonged recovery function of schizophrenia patients' visual system may indicate abnormal nonlinearity in neural response. These findings have implications understanding the nature of evoked response differences between schizophrenia and normal groups especially in repetitive stimulus paradigms.

Neuronal nicotinic receptors: insights gained from gene knockout and knockin mutant mice.

Neuronal nicotinic acetylcholine receptors are ligand-gated ion channels that subserve a range of functions in the brain and peripheral nervous system. They are pentamers variously composed of alpha (alpha2-alpha10) and beta subunits (beta2-beta4). Pharmacological and ligand-binding studies have shown that the different subunits vary in their distribution and channel properties, but precise delineation of the in vivo function of individual subunits has been hampered by lack of subunit-specific antagonists. The development of transgenic mice with targeted deletions of specific subunits (knockout mice) or mutations in critical receptor domains (knockin mice) has extended understanding of nicotinic receptors, revealing that some subunits are necessary for viability, whereas others mediate modulatory effects on learning and memory, locomotion, anxiety, nociception, dopaminergic neurotransmission, seizure threshold, development of the visual system and autonomic function. In some cases, studies of transgenic mice have confirmed expectations derived from pharmacological and expression studies, but in other cases, compensation by related subunits has revealed a degree of functional redundancy not predicted by previous approaches.

Olfactory input increases visual sensitivity in zebrafish: a possible function for the terminal nerve and dopaminergic interplexiform cells.

Centrifugal innervation of the neural retina has been documented in many species. In zebrafish Danio rerio, the only so-far described centrifugal pathway originates from terminal nerve (TN) cell bodies that are located in the olfactory bulb. Most of the TN axons terminate in the forebrain and midbrain, but some project via the optic nerve to the neural retina, where they synapse onto dopaminergic interplexiform cells (DA-IPCs). While the anatomical pathway between the olfactory and visual organs has been described, it is unknown if and how olfactory signals influence visual system functions. We demonstrate here that olfactory input is involved in the modulation of visual sensitivity in zebrafish. As determined by a behavioral assay and by electroretinographic (ERG) recording, zebrafish visual sensitivity was increased upon presentation of amino acids as olfactory stimuli. This effect, however, was observed only in the early morning hours when zebrafish are least sensitive to light. The effect of olfactory input on vision was eliminated after lesion of the olfactory bulbs or after the destruction of DA-IPCs. Intraocular injections of a dopamine D(2) but not a D(1) receptor antagonist blocked the effect of olfactory input on visual sensitivity. Although we cannot exclude the involvement of other anatomical pathways, our data suggest that the TN and DA-IPCs are the prime candidates for olfactory modulation of visual sensitivity.

Assessing visual function in clinical practice

Examination of the afferent visual system involves a structural assessment of the eye, the associated neurological structures, and the ocular adnexa, in addition to an evaluation of afferent visual function. The most common measure of visual function is Snellen visual acuity, which provides a rapid and useful measure of some aspects of central visual function but is a subjective parameter that conveys limited information regarding the overall function of the visual system. A wide variety of psychophysical and electrophysiological tests have been developed to capture various aspects of visual function more thoroughly. The main goal of this issue of the Ophthalmology Clinics of North America is to provide a comprehensive overview of tests that are currently available to practicing clinicians and promising new technologies that may eventually provide useful clinical information. This issue begins with a summary of techniques for evaluating the afferent pupillary light response. This often-underemphasized aspect of the afferent visual examination provides the only objective information regarding visual function that is readily available to a clinician during a routine eye exam. The articles by Kniestedt and Stamper, Swanson and Cohen, and Anderson review the evaluation of visual acuity, color vision, and standard perimetry, which, along with pupillary assessment, form the backbone of the clinical evaluation of visual function in standard practice. Owsley reviews contrast sensitivity, which is not used as often as it should be in clinical practice but nevertheless provides a more comprehensive measure of central function. Specialized tests of peripheral function are summarized by Anderson and Johnson and Racette and Sample for both short-wavelength automated perimetry and frequency doubling technology perimetry, which are designed to emphasize specific ganglion cell subpopulations and improve the sensitivity of visual field testing. Finally, Holopigian and Hood provide a thorough overview of the broad topic of electrophysiology; this article reviews tests that are useful in elucidating causes of occult visual loss (eg, electroretinography, electro-oculography, and visual-evoked potentials) as well as more recent developments in this area, including multifocal electrophysiologic techniques. Robbins and colleagues and Schneck and Haegerstrom-Portnoy tackle the difficult problems associated with visual function assessment in pediatric and elderly populations. These articles provide readers with practical tips and guidelines for the examination of these more challenging patients. Finally, Ball brings the assessment of visual function out of the clinic and laboratory in her summary of visual function in the real world. This

Behavioral genetic approaches to visual system development and function in zebrafish.

The zebrafish is a recent vertebrate model system that shows great potential for a genetic analysis of behavior. Early development is extraordinarily rapid, so that larvae already display a range of behaviors 5 days after fertilization. In particular the visual system develops precociously, supporting a number of visually mediated behaviors in the larva. This provides the opportunity to use these visually mediated behaviors to screen chemically mutagenized strains for defects in vision. Larval optokinetic and optomotor responses have already been successfully employed to screen for mutant strains with defects in the visual system. In the adult zebrafish a visually mediated escape response has proved useful for screening for dominant mutations of the visual system. Here, I summarize visually mediated behaviors of both larval and adult zebrafish and their applicability for genetic screens, and present, the approaches and results of visual behavior carried out to date.

Spatio-temporal frequency characteristics of the optomotor response in zebrafish

The optomotor response (OMR) is a simple experimental paradigm that is widely used in the study of visual system functions. In the current paper we investigated how spatial and temporal properties of repetitive stimuli determine the OMR in zebrafish. The experiments showed that the OMR has the temporal characteristic of a low-pass filter when the spatial frequencies are low and of a band-pass filter when the spatial frequencies are high. These findings are discussed on the basis of inherent sampling constraints of any motion detector. We found some indications that the strength and direction of the OMR vary with the spatio-temporal frequency of the stimulus pattern as has previously been described for other species.

Performance evaluation of wavelet based on human visual system

We have constructed a compactly supported biorthogonal wavelet that approximates the modulation transfer function (MTF) of human visual system in the frequency domain. In this paper, we evaluate performance of the constructed wavelet, and compare it with the widely used Daubechies 9-7, Daubechies 9-3 and GBCW-9-7 wavelets. The result shows that coding performance of the constructed wavelet is better than Daubechies 9-3, and is competitive with Daubechies 9-7 and GBCW-9-7 wavelets. Like Daubechies 9-3 wavelet, the filter coefficients of the constructed wavelet are all dyadic fractions, and the tap is less than Daubechies 9-7 and GBCW-9-7. It has an attractive feature in the realization of discrete wavelet transform.

Compelling classroom demonstrations that link visual system anatomy, physiology, and behaviour.

One of our approaches to teaching a course in anatomy and physiology is to stress the fundamental, systems-level concepts. One successful strategy we use is to continually highlight the relationships among anatomy, physiology, and behavior. In this article, we describe a set of classroom demonstrations that stress these links while fostering critical thinking. These demonstrations, on the topic of sensory system structure and function, rely on two perceptual consequences of neural adaptation in the visual system: afterimages and aftereffects. Viewing specific visual stimuli under binocular or monocular conditions with interocular transfer permits several concepts to be observed and discussed, including neural adaptation, anatomical and functional segregation of visual system pathways, and the relationship among visual system structure, function, and perception. This article discusses how to produce and present the required visual stimuli, suggests a set of questions to stimulate critical thinking, and presents student evaluation of this activity.

Control of Visuotemporal Attention by Inferior Parietal and Superior Temporal Cortex

The human cortical visual system is organized into two major pathways: a dorsal stream projecting to the superior parietal lobe (SPL), considered to be critical for visuospatial perception or on-line control of visually guided movements, and a ventral stream leading to the inferotemporal cortex, mediating object perception [1–3]. Between these structures lies a large region, consisting of the inferior parietal lobe (IPL) and superior temporal gyrus (STG), the function of which is controversial. Lesions here can lead to spatial neglect [4, 5], a condition associated with abnormal visuospatial perception [6, 7] as well as impaired visually guided movements [8, 9], suggesting that the IPL+STG may have largely a “dorsal” role. Here, we use a nonspatial task [10] to examine the deployment of visuotemporal attention in focal lesion patients, with or without spatial neglect. We show that, regardless of the presence of neglect, damage to the IPL+STG leads to a more prolonged deployment of visuotemporal attention compared to lesions of the SPL. Our findings suggest that the human IPL+STG makes an important contribution to nonspatial perception, and this is consistent with a role that is neither strictly “dorsal” nor “ventral” [11]. We propose instead that the IPL+STG has a top-down control role, contributing to the functions of both dorsal and ventral visual systems.

Clock mechanisms in zebrafish.

Recent research on the circadian system of the zebrafish is reviewed. This teleost has become an attractive model system because of its advantages for genetic analyses. Circadian rhythms of zebrafish behavior, visual system function, and pineal melatonin synthesis have been described, and behavioral and pineal rhythms are being used to identify and characterize clock mutants. Zebrafish heart, kidney, and embryonic cell lines contain circadian oscillators and phototransduction mechanisms for entrainment, suggesting that circadian pacemaking functions may be distributed throughout the animal. Studies of circadian system development in zebrafish have found that a molecular circadian oscillation in unfertilized oocytes persists through embryonic development with its phase intact, but that the pacemakers that drive rhythms of melatonin synthesis and behavior require environmental entraining signals late in development for initial synchronization. Zebrafish homologs of several of the core clock genes identified in other animals have been cloned. Transcripts for most of these are rhythmically expressed in multiple tissues. The interactions of clock gene products are for the most part similar to their interactions in mammals, although there are some potentially interesting differences.