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PubMed1983

The organization of the fibers in the optic nerve of normal and tectum-less Rana pipiens.

We have examined the detailed order of retinal ganglion cell (RGC) axons in the optic nerve and tract of the frog, Rana pipiens. By using horseradish peroxidase (HRP) injections into small regions of the retina, the tectum, and at various points along the visual pathway, it has been possible to follow labelled fibers throughout their course in the nerve and tract. Several surprising features in the order of fibers in the visual pathway were discovered in our investigation. The fascicular pattern of RGC axons in the retina is similar to that described in other vertebrates; however, immediately central to their entry into the optic nerve head, approximately half of the fibers from the nasal or temporal retina cross over to the opposite side of the nerve. Although the axons from the dorsal and ventral regions of the retina generally remain in the dorsal and ventral regions of the nerve, some fiber crossing occurs in those axons as well. The result of this seemingly complex rearrangement is that the optic nerve of Rana pipiens contains mirror symmetric representations of the retinal surface on either side of the dorsal ventral midline of the nerve. The fibers in each of these representations are arranged as semicircles representing the full circumference of the retina. This precise fiber order is preserved in the nerve until immediately peripheral to the optic chiasm, at which point age-related axons from both sides of the nerve bundle together. Consequently, when a small pellet of HRP is placed in the chiasmic region of the nerve, an annulus of retinal ganglion cells and a corresponding annulus of RGC terminals in the tectum are labelled. As the age-related bundles of fibers emerge from the chiasm they split to form a medial bundle and a lateral bundle, which grow in the medial and lateral branches of the optic tract, respectively. Although the course followed by RGC axons in the visual pathway is complex, we propose a model in which the organization of fibers in the nerve and tract can arise from a few rules of axon guidance. To determine whether the optic tecta, the primary retinal targets, play a role in the development and organization of the optic nerve and tract, we removed the tectal primordia in Rana embryos and examined the order in the nerve when the animals had reached larval stages. We found that the order in the nerve and tract was well preserved in tectumless frogs. Therefore, we propose that guidance factors independent of the target direct axon growth in the frog visual system.

Animals↗
PubMed1995

Surgical management of lesions affecting the anterior optic pathways.

Although benign neoplasms are the most common lesions of the anterior visual pathways that lend themselves to a combined neurosurgical and ophthalmic surgical treatment, malignant neoplasms, infections (especially fungal), and vascular lesions are also sometimes treated via a combined approach. Recent advances in the field of skull base surgery have made it possible to treat complicated lesions in precarious locations with increasing margins of safety and decreasing morbidity. The role of the neurosurgeon in managing selected patients with lesions of the anterior a visual pathway should not be underestimated.

Adult↗
PubMed1988

Transient asymmetry in the projections of the rostral thalamus to the visual hyperstriatum of the chicken, and reversal of its direction by light exposure.

Asymmetry in the visual pathways from the rostral thalamus to the hyperstriatum of the chicken has been found after injecting the retrograde tracer, True Blue (TB), into either the left or right hyperstriatum on day 2 or 12, post-hatching. There are ipsilateral connections from the ventromedial region of the left dorsolateral thalamus, lateral part (DLL) to the left hyperstriatum, and contralateral connections from the left dorsolateral thalamus, rostrolateral part (DLAlr) and the dorsolateral thalamus, dorsal part (DLLd) to the right hyperstriatum. On the right side of the thalamus, the ipsilateral connections from DLL to the right hyperstriatum are present, but there are only very few contralateral connections to the left hyperstriatum. No asymmetry in these pathways is seen in animals injected with TB on day 21. By this age the contralateral connections from the right thalamus to the left hyperstriatum have developed. Thus, the structural asymmetry in these visual pathways is transient, a finding which explains a controversy between two papers published recently in this journal, and which adds considerably to our understanding of the behavioural asymmetries known to occur in the chicken's response to stimuli presented to either the left or right eye. The direction of the asymmetry in visual pathways depends on asymmetrical light input to the eyes of the embryo. Normally the head of the embryo is oriented such that the left eye is occluded. If the head is withdrawn from the egg so that the right eye can be occluded and the left eye exposed to light, the direction of asymmetry in the thalamo-hyperstriatal pathways is reversed.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1980

Optic atrophy in experimental vitamin B12 deficiency in monkeys.

The clinical findings and pathological changes of the visual pathway of vitamin B12 deficient monkeys have been described. The cellular morphology and counts of the peripheral blood and bone marrow remained normal during the study (nine deficient, three controls: one still alive in each group). Visual impairment was noted in all seven of the deficient animals that were evaluated by clinical observations. Ophthalmoscopic examination disclosed optic atrophy in six of the seven deficient monkeys. Degeneration of the visual pathway was demonstrated by pathological examination in eight of the deficient group of nine (one is still alive). Loss of ganglion cells was noted in the maculae of two of three deficient animals with completed studies. In the three control animals there were neither visual disturbances nor ophthalmoscopic changes and in two animals autopsies disclosed no lesions in the visual pathways. The third animal is still alive and well.

Animals↗
PubMed1999

Inflammatory response and retinal ganglion cell degeneration following intraocular injection of ME7.

Scrapie is a prion disease which occurs naturally in sheep and which can be transmitted experimentally to rodents. After intracerebral injection of ME7 into mouse, an atypical inflammatory response, characterized by T-lymphocytes and activated microglia is present early in the course of the disease. In the present work, we have investigated the relationship between this inflammatory response, astrocytosis and neuronal loss along the visual pathway after intraocular injection (intraocular) of ME7 in C57BL/6J mice. We have demonstrated that microglia activation and T-lymphocyte recruitment accompanies the spread of prion pathology along the visual pathway and in the early stages of the disease is restricted to the subcortical visual pathway. Inflammation was also present in non-visual areas in association with PrPsc deposition at late stages of the disease, possibily indicating that diffusion of the scrapie agent also contributes to the spread of the disease. After intraocular injection of the prion agent, the disease is believed to be transported into the brain via axons of retinal ganglion cells (RGCs). Despite the high levels of infectivity reported to be present in the retina early in the disease after intraocular injection of ME7, retinal pathology has not been extensively investigated. We have studied the RGCs response in whole mount retinas after intraocular injection of ME7. We have shown that RGCs degenerate after intraocular injection of ME7 whereas amacrine cells, retinal interneurones, are more resistant. Our results suggest that two distinct population of neurones, exposed in vivo at the same time to the same agent scrapie strain, show different susceptibility to the toxic effects of PrPsc.

Animals↗
PubMed2005

Two visual processing pathways are targeted by gonadotropin-releasing hormone in the retina.

In fish the terminal nerve is comprised of a group of cells with somata adjacent to the olfactory bulb and processes that extend both anteriorly to the olfactory mucosa and posteriorly to the telencephalon. In teleost fish an additional group of axons extends along the optic tract and delivers putative neuromodulators to the retina. One peptide - gonadotropin-releasing hormone (GnRH) - has been implicated as a prime candidate neuromodulator based on electrophysiological evidence that exogenous application influences neural activity. Here we describe the expression patterns of two GnRH receptor subtypes in the retina of a teleost fish, Astatotilapia (Haplochromis) burtoni. The type 1 GnRH receptor (GnRH-R1) was expressed in cells of the amacrine cell layer - where lateral inputs affect the flow of visual information from photoreceptors to the brain - and in a distribution and location pattern similar to dopaminergic interplexiform cells. Immunohistochemical labeling of GnRH fibers revealed varicosities along terminal nerve axons near the amacrine cell layer and near cells immunoreactive for tyrosine hydroxylase, a dopaminergic cell marker. This finding supports an existing model that the terminal nerve forms synapses with dopaminergic interplexiform cells. Surprisingly, the type 2 GnRH receptor (GnRH-R2) was abundantly expressed in ganglion cells, which lie along the direct pathway of visual information to the brain. These data suggest that GnRH from the TN could broadly influence processing of retinal signals both in lateral processing circuits through GnRH-R1 and in the vertical throughput pathway through GnRH-R2.

Animals↗
PubMed2004

Beyond sensory images: Object-based representation in the human ventral pathway.

We investigated whether the topographically organized, category-related patterns of neural response in the ventral visual pathway are a representation of sensory images or a more abstract representation of object form that is not dependent on sensory modality. We used functional MRI to measure patterns of response evoked during visual and tactile recognition of faces and manmade objects in sighted subjects and during tactile recognition in blind subjects. Results showed that visual and tactile recognition evoked category-related patterns of response in a ventral extrastriate visual area in the inferior temporal gyrus that were correlated across modality for manmade objects. Blind subjects also demonstrated category-related patterns of response in this "visual" area, and in more ventral cortical regions in the fusiform gyrus, indicating that these patterns are not due to visual imagery and, furthermore, that visual experience is not necessary for category-related representations to develop in these cortices. These results demonstrate that the representation of objects in the ventral visual pathway is not simply a representation of visual images but, rather, is a representation of more abstract features of object form.

Adult↗
PubMed2000

Short latency visual evoked potentials in occupational exposure to organic solvents.

OBJECTIVES: Short latency visual evoked potentials (SVEP), in response to high-intensity flashes from light emitting diodes (LED), were used to detect subclinical effects along the visual pathway in four groups of subjects with different levels of exposure to gasoline, all within legally acceptable limits. METHODS: Potentials and exposure levels were obtained from 31 subjects with different occupational exposure levels to gasoline fumes, as well as from 17 non-exposed control subjects. SVEP were recorded from four electrode sites (infra-orbital, Cz, Pz, Oz), in response to flashes presented to each eye in turn from goggle-mounted LEDs. SVEP components were defined after digital filtering, which eliminated the high-frequency oscillatory potentials and accentuated five major components: a periocular P30, attributed to the retina; a fronto-central N50, attributed to the optic nerve; centro-parietal P65 and N85, attributed to the optic tracts and radiation; and an occipital, cortical P105. RESULTS: The latencies of successive SVEP components of the exposed subjects showed a significant latency prolongation compared to controls, beginning with activity attributed to the optic nerve and increasing cumulatively with the later components. Retinal components were not affected by the exposure to organic solvents. Among the exposed groups, differences in latency prolongation corresponded to occupational exposure. CONCLUSION: The low-frequency components of SVEP were reliably measured and proved to be sensitive to subclinical effects of organic solvents on conduction along the visual pathway. These components are likely to be sensitive to other subcortical visual pathway lesions, but their clinical promise needs further verification.

Adult↗
PubMed1990

Selective disconnection of specific visual association pathways in cases of Alzheimer's disease presenting with Balint's syndrome.

During a recent clinical and neuropathological evaluation of a large autopsy population of brains our attention was drawn to a subset of patients with Alzheimer's disease (AD) presenting with a major impairment of visuospatial skills referred to as Balint's syndrome. In this subset a shift in the distribution of certain pathological profiles had occurred in that the visual areas of the occipital and posterior parietal regions had an increased number of lesions, whereas the prefrontal cortex had fewer lesions than usually observed in AD. Previous quantitative analyses have shown that generally in AD, primary sensory cortical areas are less damaged than association areas of the frontal and temporal lobes, as demonstrated by the laminar and regional distribution of two neuropathological features of the disease, neurofibrillary tangles and neuritic (senile) plaques. The distribution of pathological lesions in the AD cases with Balint's syndrome revealed that specific visual association pathways were disrupted, which are normally spared in AD. These data suggest that in some cases of AD, the particular psychological and neurological symptomatology may be caused by the selective loss of specific corticocortical systems, as reflected in the differential distribution of the neuropathological markers of the disease.

Aged↗
PubMed2004

Spatial and temporal properties of visual responses in the thalamus of the developing ferret.

Spatiotemporal patterning of neural activity is thought to influence the development of connections in the visual pathway. This patterning can arise spontaneously or through sensory experience. Here, we use a combination of natural and simple stimuli to investigate which elements of the visual environment modulate the earliest responses in the primary visual pathway of developing ferrets. Recordings were made during the first 2 weeks of visual responsiveness, which, in the ferret, overlaps with the period that the eyelids have not yet opened. Even when the eyelids are closed, both thalamic and cortical activity was found to be temporally modulated under conditions of natural visual stimulation. The modulations correlated with temporal changes in stimulus contrast but also reflected spatial structure in the visual scene. Simple stimuli were used to show that early responses to naturalistic stimuli are influenced by the localization and structure of through-the-eyelid receptive fields. The early visual responses were also characterized by substantial variability in the ability of the cells to detect stimuli of different duration and different intensity, in a temporally precise manner. These temporal and spatial properties should constrain how plasticity mechanisms interpret naturally patterned activity.

Action Potentials↗
PubMed2001

Dysfunction of early-stage visual processing in schizophrenia.

OBJECTIVE: Schizophrenia is associated with deficits in higher-order processing of visual information. This study evaluated the integrity of early visual processing in order to evaluate the overall pattern of visual dysfunction in schizophrenia. METHOD: Steady-state visual-evoked potential responses were recorded over the occipital cortex in patients with schizophrenia and in age- and sex-matched comparison volunteers. Visual-evoked potentials were obtained for stimuli composed of isolated squares that were modulated sinusoidally in luminance contrast, number of squares, or chromatic contrast in order to emphasize magnocellular or parvocellular visual pathway activity. RESULTS: Responses of patients to magnocellular-biased stimuli were significantly lower than those of comparison volunteers. These lower response levels were observed in conditions using both low luminance contrast and large squares that biased processing toward the magnocellular pathway. In contrast, responses to stimuli that biased processing toward the parvocellular pathway were not significantly different between schizophrenia patients and comparison volunteers. A significant interaction of group and stimulus type was observed in the condition using low luminance contrast. CONCLUSIONS: These findings suggest a dysfunction of lower-level visual pathways, which was more prominent for magnocellular than parvocellular biased stimuli. The magnocellular pathway helps in orienting toward salient stimuli. A magnocellular pathway deficit could contribute to higher-level visual cognitive deficits in schizophrenia.

Cognition Disorders↗
PubMed1983

Visual mechanisms for the analysis of spatial pattern.

After a brief outline of the structure and electrophysiology of the normal visual pathways, the responses, as revealed by psychophysical studies, of the visual system to spatially and temporally varying stimuli are reviewed. An appropriate network model, involving two sequentially organized classes of visual channel, is presented. Examples are given in which the psychophysical methods developed to analyse the normal visual pathways are applied to cases in which these pathways are defective (amblyopia, albinism, hemianopia, parietal cortical lesion, inhibitory central colour vision defect). These cases not only illustrate the value of psychophysical techniques in analysing disturbances of visual function but are also suggestive of the mechanisms involved in higher processing of the retinal image.

Adolescent↗
PubMed1999

Spontaneous partial regression of low-grade glioma in children with neurofibromatosis-1: a real possibility.

At the age of 41 and 31 months, respectively, a boy and a girl affected by neurofibromatosis-1 were diagnosed with a visual pathway glioma during surveillance contrast-enhanced head magnetic resonance imaging (MRI). In the first child, the initial MRI showed that the entire optic chiasm, the intracranial tract of the left optic nerve, and hypothalamus were grossly enlarged and enhanced in the post-gadolinium T1-weighted images. Ten months later, the hypothalamic component of the lesion had regressed markedly and there were no more areas of contrast enhancement. In the second child, the initial MRI showed that the optic chiasm, the right optic tract, and geniculate body were enlarged and enhanced after gadolinium injection. At 6-month follow-up, the MRI showed that the right optic tract and the anterior aspect of the optic chiasm decreased in size and the contrast enhancement of the entire lesion was reduced dramatically. These findings, as indicated by other similar reports, confirm that spontaneous regression of visual pathway glioma is a rare but real possibility in children with neurofibromatosis-1. Therefore, clinicians need to be aware of visual pathway glioma's erratic behavior in children with neurofibromatosis-1 with special attention given to the importance of a very conservative attitude toward any type of treatment for such patients.

Child, Preschool↗
PubMed2004

Neural underpinnings of dyslexia as a disorder of visuo-spatial attention.

For nearly 100 years, the underlying cause of dyslexia has been a matter of much debate, with widely varying viewpoints that have ranged from considering dyslexia as largely a learning disability to claims that it is essentially a perceptual defect occurring early along the visual pathway. This paper reviews some of this literature with particular reference to the studies that have implicated a defect in the afferent visual pathways in the aetiology of the disorder, then goes on to outline a neural theory of how functionally distinct parallel pathways in vision interact with each other in the process of reading and suggests how a defect in these pathways can lead to reading difficulties. Central to the proposed scheme is the suggestion that a fast-track pathway, arising from the magnocellular cells in the retina and acting through an attentional mechanism, has a gating function in spotlighting the individual letters of a text in a sequential fashion. That such gating occurs at the level of the primary visual cortex is supported by recent physiological evidence concerning attentional mechanisms.

Attention↗
PubMed1997

Radiation optic neuropathy after stereotactic radiosurgery.

PURPOSE: The purpose of the study is to report the occurrence of optic neuropathy after stereotactic radiosurgery for perichiasmal tumors. METHODS: Records of four patients with visual deterioration after stereotactic radiosurgery were reviewed, including clinical findings, neuroimaging results, and treatment methods. RESULTS: Optic neuropathy developed 7 to 30 months after gamma knife radiosurgery. All patients experienced an abrupt change in visual function. Clinical findings indicated anterior visual pathway involvement. Patterns of field loss included nerve fiber bundle and homonymous hemianopic defects. Gadolinium-enhanced magnetic resonance imaging (MRI) showed swelling and enhancement of the affected portion of the visual apparatus in three patients. Systemic corticosteroids were administered in all patients and one partially recovered. One patient also received hyperbaric oxygen without improvement. CONCLUSIONS: Although rare, optic neuropathy may follow radiosurgery to lesions near the visual pathways. Careful dose planning guided by MRI with restriction of the maximal dose to the visual pathways to less than 8 Gy will likely reduce the incidence of this complication.

Adenoma↗