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At least 1,207 records · Page 67Linked to original sources
PubMed2004

PRBS-determined temporal frequency characteristics of VEP in glaucoma.

The purpose of this study was to determine whether the PRBS-VEP-determined temporal frequency characteristics (TFCs) of the visual pathways are useful for evaluating the properties of the visual system of glaucomatous patients. The VEPs elicited by pseudorandom stimulation (PRBS) with red LEDs were recorded from 26 eyes with primary open angle glaucoma and 11 age-matched normal eyes. The glaucomatous patients were divided into 3 groups according to the size and shape of their visual field defect; early (9), moderate (9), and severe (8). The cross-spectrum between the PRBS and PRBS-VEPs was used as the TFCs. The TFC of each glaucomatous group were compared to those of the normal controls. A depression of the TFC was found for the middle and high frequencies (18-28 Hz) in the moderate group, and the depression spread to lower frequencies (4-16 Hz) in the severe groups. The TFC values at 14-20 Hz were depressed in the moderate group and most frequencies were depressed significantly with the progression of glaucoma (p < 0.05). The sensitivity of the TFC at 18-20 Hz was 56%, 89% and 100% in the early, moderate and severe glaucoma groups, respectively, and the specificity was 82%. We conclude that the PRBS-determined TFC is altered in glaucomatous eyes, and the frequencies depressed were related to the degree of glaucoma. These findings indicate that the PRBS-determined TFC can be useful for evaluating visual function of glaucomatous eyes.

Aged↗
PubMed1996

[Texture discrimination and visual search: development, learning and plasticity].

BACKGROUND: The abilities to segment coherent surfaces out of a discrepant background (texture segmentation) and to localize single objects out of a complex scene (visual search) are believed to occur at a peripheral level in the cortical visual pathway. AIM OF THE STUDY: Are texture segmentation and visual search innate or acquired? Can these abilities be improved in adulthood by learning? Does a disturbed early visual experience (strabismus, amblyopia) lead to a reduction of these abilities? RESULTS: Texture segmentation and visual search undergo a protracted development which continues well into adulthood. Even in adult observers, these abilities can be improved with practice. Learning is enduring, but not specific: It can be transferred from one task to another and between the two eyes of a given subject (including the two eyes of stereoblind subjects). DISCUSSION: We conclude that learning of texture segmentation and visual search does not involve the specific features of a visual scene, but rather reflects an improved strategy of localizing visual objects in a cluttered environment. This type of learning probably occurs at a very high level of cortical processing.

Adolescent↗
PubMed2000

Specificity of projections from wide-field and local motion-processing regions within the middle temporal visual area of the owl monkey.

The middle temporal visual area (MT) of the owl monkey is anatomically organized with respect to both preferred direction of motion and different types of center-surround interaction. The latter organization consists of clusters of neurons whose receptive fields have antagonistic surrounds that render them unresponsive to wide-field motion (local motion columns) interdigitated with groups of neurons whose receptive fields have additive surrounds and thus respond best to wide-field motion (wide-field motion columns). To learn whether the information in these regions remained segregated further along the visual pathways, we made injections of retrograde tracers into two visual areas to which MT projects [the medial superior temporal area (MST) and fundus of the superior temporal sulcus (FST)] and then labeled the wide-field and local organization using 2-deoxyglucose. In complementary experiments, we injected anterograde tracers into regions of MT that we had mapped using microelectrode recordings. Injections into both dorsal FST and ventral MST labeled clusters of cell bodies in MT that were concentrated within wide-field motion columns, whereas injections into dorsal MST labeled neurons predominantly within local motion columns. Results from the anterograde tracer experiments corroborated these findings. The high degree of specificity in the connections reinforces a model of functional organization for wide-field versus local motion processing within MT. Our data support the previously reported division of FST into separate dorsal and ventral areas, and they also suggest that MST of the owl monkey is, like MST of the macaque, functionally organized with respect to local versus wide-field motion processing.

Animals↗
PubMed2006

Regulation of GDNF and its receptor components GFR-alpha1, -alpha2 and Ret during development and in the mature retino-collicular pathway.

The development of the retino-tectal projection as part of the central visual pathway is accomplished around postnatal day (P) 10-14 in rodents, and trophic factors are important for topographic refinement of this projection. Emerging data indicate that GDNF may influence synaptic plasticity of this projection. To date, maturation-dependent kinetics of GDNF release and expression and biological function of single GDNF receptors along the retino-collicular pathway are ill-defined. Here, we examined mRNA and protein expression of GDNF and its multicomponent receptor complex in the retina and superior colliculus (SC) during postnatal development of the rat visual system, and after optic nerve (ON) injury by RT-PCR, immunoblotting and immunofluorescence. Stable mRNA transcription of GDNF and its receptors GFR-alpha1, -alpha2 and Ret was found in retina and SC throughout development into adulthood and after ON transection. Expression of GDNF protein increased during retinal development, declined in adulthood and was further reduced in injured retina. In the SC, GDNF peaked at P0, continuously declined with maturation, and was undetectable in the deafferentiated SC. GFR-alpha1 was abundant in retina and SC throughout, while GFR-alpha2 was not expressed. Since Ret was localized primarily to the vascular compartment, the receptor tyrosine kinase may play a minor role in neuronal GDNF signaling. In summary, we provide evidence for GDNF as survival and guidance factor during development of the retino-tectal projection with differential regulation in early and premature retina and SC. Postlesionally, midbrain targets do not induce GDNF, suggesting that retrograde GDNF is not essential for rescue of adult injured retinal ganglion cells (RGCs).

Animals↗
PubMed1993

Prenatal development of the retinohypothalamic pathway and the suprachiasmatic nucleus in the sheep.

Circadian rhythms are present during fetal life in several mammalian species. To characterize the ontogeny of the neural mechanisms that account for circadian rhythmicity in a precocious species, we studied the prenatal development of the retinohypothalamic pathway in lambs (gestation period of 147 days), using horseradish peroxidase and wheat germ agglutinin as anterograde tracers. The suprachiasmatic nucleus was present as early as embryonic day 52 (E52). After E58, the suprachiasmatic nucleus reached its full number of neurons, estimated by the disector method in about 160,000 cells per nucleus at E62. The retinohypothalamic axons invaded the suprachiasmatic nucleus from E58, while neuroblasts were still migrating to the nucleus. At E62, there was a strong retinal projection that evolved until E121, when the retinal afferents established their definitive pattern of distribution in the ventral and central regions of the suprachiasmatic nucleus, and adjacent hypothalamic structures. The development of the retinohypothalamic pathway was delayed by about a week relative to the innervation of other subcortical visual centers. The present findings demonstrated an early prenatal development of the visual pathways in lambs, including the retinohypothalamic pathway, suggesting that the mechanisms for the visual entrainment of circadian rhythms in lambs may be functioning several weeks before birth.

Animals↗
PubMed1989

Selective degeneration of the parvocellular-projecting retinal ganglion cells in a New World monkey, Saimiri sciureus.

Selective degeneration of retinal ganglion cells projecting to parvocellular layers of the dorsal lateral geniculate nucleus (LGN) was observed in squirrel monkeys (Saimiri sciureus) exposed to a range of doses of acrylamide monomer. Similar acrylamide-induced neuronal loss has previously been reported in parvocellular-projecting ganglion cells of macaques, but no such selective degeneration has been found in acrylamide-dosed rats, squirrels, rabbits or cats. The extent of ganglion cell loss observed in the present study suggests that in the squirrel monkey, as in the macaque, a majority of ganglion cells project to parvocellular layers of the LGN. The locus of optic tract degeneration suggests that the squirrel monkey parvocellular pathway passes in dorsolateral optic tract, as does that of the macaque. Patterns of decreases in cytochrome oxidase activity confirm that, in both of these primates, geniculocortical pathways driven by these vulnerable neurons project to cortical layers 4A and 4C beta. These results suggest close parallels in the neuroanatomical projections and toxic vulnerability of the parvocellular-projecting pathway in New and Old World monkeys. They indicate that acrylamide intoxication can be used to selectively damage this pathway in order to study the functional roles of parallel visual pathways in both New and Old World monkeys.

Animals↗
PubMed1985

Impaired transport of iron-dextran complex in myelinated central fibers.

The labeling of retinal ganglion cells by axonal transport of an iron-dextran complex (injected into the superior colliculus and the lateral geniculate nucleus) was compared to the previously described labeling of the dopaminergic neurons of the substantia nigra (after striatal injection), to test the validity of the method in myelinated and unmyelinated CNS pathways. It was shown that the retrograde labeling in the visual pathway was impaired, consecutive to the penetration of iron-dextran into the myelin sheaths and a subsequent myelin alteration along the optic nerve.

Animals↗
PubMed2000

Gamma knife radiosurgery as a primary treatment for prolactinomas.

OBJECT: The purpose of this study was to estimate the efficacy of gamma knife radiosurgery (GKS) in controlling tumor growth and endocrinopathy associated with prolactinomas. METHODS: Between 1993 and 1997, 164 of 469 patients with pituitary adenomas treated by GKS harbored prolactinomas. The dose to the tumor margin ranged from 9 to 35 Gy (mean 31.2 Gy), and the visual pathways were exposed to a dose of less than 10 Gy. The mean tumor diameter was 13.4 mm. The mean follow-up time for 128 cases was 33.2 months (range 6-72 months). Tumor control was observed in all but two patients who underwent surgery 18 and 36 months, respectively, after GKS. Clinical cure was achieved in 67 cases. Clinical improvement was noted with a decrease in the hyperprolactinemia after GKS. Nonetheless, in 31 (29%) of 108 patients who were followed for more than 2 years no improvement in serum prolactin levels was demonstrated, although this could be normalized by bromocriptine administration after treatment. Nine infertile women became pregnant 2 to 13 months after GKS and all gave birth to normal children. There was no visual deterioration related to GKS. Five women experienced premature menopause. In these patients there was subtotal disappearance of the tumor and an empty sella developed. CONCLUSIONS: Gamma knife radiosurgery as a primary treatment for prolactinomas can be safe and effective both for controlling tumor growth and for normalization of prolactin hypersecretion. A higher margin dose (> or = 30 Gy) seemed to be associated with a better clinical outcome. Gamma knife radiosurgery may make prolactinomas more sensitive to the bromocriptine.

Humans↗
PubMed1989

Consequences of visual deprivation in the absence of binocular competitive mechanisms in Siamese cat area 17.

The Siamese cat is a mutant with abnormally crossed visual pathways, which provides a model for studying the effects of visual deprivation in the absence of binocular competitive interactions. Siamese cats are known to be resistant to the effects of monocular eyelid suture. To further explore the nature of this resistance, the receptive field properties of neurons in area 17 of monocularly (MD) and binocularly (BD) deprived Siamese cats were studied. Neither condition produced a loss of cortical responsiveness, which is a characteristic result of binocular deprivation in normally pigmented cats. Somewhat more units in the deprived hemisphere of MD Siamese cats were orientation-selective, and many more units were direction-selective than in BD Siamese cats. This difference may be due to an effect of visual attentional mechanisms, which can function in MD but not BD Siamese cats. To test whether the resistance to the effects of visual deprivation in Siamese cats might be a more general phenomenon. Siamese and normally pigmented cats were raised in an 8-Hz stroboscopically illuminated environment. Both groups showed a severe loss of direction selectivity. Some of the normally pigmented cats also showed a loss of binocularity, which appeared to be secondary to strabismus.

Animals↗
PubMed2000

Degradation of stimulus selectivity of visual cortical cells in senescent rhesus monkeys.

Human visual function declines with age. Much of this decline is probably mediated by changes in the central visual pathways. We compared the stimulus selectivity of cells in primary visual cortex (striate cortex or V1) in young adult and very old macaque monkeys using single-neuron in vivo electrophysiology. Our results provide evidence for a significant degradation of orientation and direction selectivity in senescent animals. The decreased selectivity of cells in old animals was accompanied by increased responsiveness to all orientations and directions as well as an increase in spontaneous activity. The decreased selectivities and increased excitability of cells in old animals are consistent with an age-related degeneration of intracortical inhibition. The neural changes described here could underlie declines in visual function during senescence.

Aging↗
PubMed2005

Optic nerve axons: life and death before birth.

In the 2004 Bowman Lecture, I give a panegyric for Sir William Bowman, an estimate of the importance and the epidemiology of anterior visual pathway developmental disorders, followed by a history of the anterior visual system. I review the normal development of the optic nerve and chiasm and the main developmental disorders: Optic Nerve Aplasia, Optic Nerve Hypoplasia and Achiasmia.

Axons↗
PubMed1980

Clinical uses of the visually evoked potential.

The visually evoked potential is defined and described. The development of this test as a valuable clinical aid is traced. Its assistance in the diagnosis of Lesions of the Optic Nerve and Visual Pathways is shown, together with its use and assessment of visual acuity and visual fields both in infancy and adults. Future developments of this test are reviewed.

Aged↗
PubMed1999

Anatomic review and topographic diagnosis.

Neuro-ophthalmologic disorders affecting the afferent and efferent visual systems are frequently encountered by neurosurgeons in clinical practice. This article focuses on aspects of neuroanatomy that are most important to the diagnosis of afferent and efferent visual pathway lesions. The use of optic disc and fundus appearance, visual fields, and ocular motility and pupillary findings are emphasized in the discussions of topographic diagnosis.

Eye↗
PubMed2000

Cortical cell orientation selectivity fails to develop in the absence of ON-center retinal ganglion cell activity.

Neuronal activity is necessary for the normal development of visual cortical cell receptive fields. When neuronal activity is blocked, cortical cells fail to develop normal ocular dominance and orientation selectivity. Patterned activity has been shown to play an instructive, rather than merely permissive, role in the segregation of geniculocortical afferents into ocular dominance columns. To test whether normal patterns of activity are necessary to instruct the development of cortical orientation selectivity, we studied ferrets raised without ON-center retinal ganglion cell activity. The ON-center blockade was produced by daily intravitreal injections of DL-2-amino-4-phosphonobutyric acid (APB). Effects of this treatment on the development of orientation selectivity in primary visual cortex were assessed using extracellular electrode recordings and optical imaging. In animals raised with an ON-center blockade starting after visual cortical cells are visually driven but still poorly tuned for orientation, cortical cell responsivity was maintained, but no maturation of orientation selectivity was seen. No recovery of orientation tuning was seen in animals treated with APB during the normal period of orientation development and then allowed several months of development without treatment. These results suggest that patterns of neuronal activity carried in the separate ON- and OFF-center visual pathways are necessary for the development of orientation selectivity in visual cortical neurons of the ferret and that there is a critical period for this development.

Aminobutyrates↗
PubMed2001

Anatomic review and topographic diagnosis.

Neuro-ophthalmologic disorders, affecting the afferent and efferent visual systems, are frequently encountered by ophthalmologists in clinical practice. This article focuses on aspects of ocular, orbital, and neuroanatomy that are most important to the diagnosis of afferent and efferent visual pathway lesions. The use of optic disc and fundus appearance, visual fields, and ocular motility and pupillary findings are emphasized in the discussions of topographical diagnosis.

Humans↗
PubMed1984

[Commissural mechanisms of restoring visual functions].

The perimetric studies were carried out in adult cats with reduced classical and commissural visual pathways. The data obtained indicate substantial recovery of the temporal visual fields in cats with unilateral optic tract transection during one year after operation. The animals with combined optic tract and cerebral commissural transection did not demonstrate any remarkable restitution of visual fields. These findings allow the conclusion about the primary role of cerebral commissures in the compensatory mechanisms of visual functions after visual tract injury.

Animals↗
PubMed1994

Topographic mapping of the visual evoked potential after source derivation.

Visual Evoked Potentials (VEP) have been considered an important complementary diagnostic method for the study of the physiology of the visual pathways and cortex. It is suggested that topographic maps of Visual Evoked Potentials (VEP) generate information related to the anatomy and physiology of the visual cortex. The source derivation technique applied to electroencephalographic signals has been shown to improve the sensitivity and spatial selectivity of the results. Mapping VEP after source derivation was implemented in order to facilitate the interpretation of the VEP signals and as an aid in identifying the distribution of the sources generating them. The signals were obtained from normal individuals stimulated with full-field and half-field pattern reversal checker-boards. The electrode system used (16 channels) was that proposed by Bodis-Wollner; this system concentrates the electrodes in the scalp areas related to the visual cortex. Signals were sampled simultaneously with a 10-bit A/D converter, saved and, thereafter, processed in an IBM/PC compatible microcomputer. After coherently averaging the responses, source derivation considering inter-electrode distances was applied. It was observed that the spatial spreading of the signals was reduced, leading to an improved spatial selectivity, locating the VEP sources in the expected brain regions and suggesting the existence of electrical dipoles.

Adult↗
PubMed1982

The biological and clinical behaviour of pilocytic astrocytomas of the optic pathways.

The clinical and pathological findings in thirty patients with pilocytic astrocytomas of the anterior visual pathways were analysed. Our cases and those in the literature did not show positive correlation between the extent of surgical resection or irradiation or the presence of residual tumour, and the length of survival or degree of residual disability. Most of these tumours may be considered hamartoma-like growths, akin to developmental malformations, among which aggressive behaviour is exceptional. Their inherently benign nature, associated with long periods of dormancy and spontaneous regression, indicate that treatment should be conservative, and that a surprisingly favourable outcome can often be anticipated.

Adolescent↗