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At least 739 records · Page 41Linked to original sources
PubMed1987

Receptor plasticity in the human brain: some autoradiographic studies.

Receptor modifications in human postmortem material were studied by quantitative autoradiography. Alterations of several neurotransmitter receptors in neurodegenerative diseases such as senile dementia and Huntington's chorea, in lesions of specific brain pathways, like the visual pathway or after drug treatments, were examined. In all these situations alterions of the density or localization of receptors were seen using autoradiography. The results suggest that several mechanisms of receptor adaptation operate in the human brain. These mechanisms include: compensatory changes in receptor density as a consequence of cell loss, in some cases preceding the neuropathological changes; differential alterations in receptors depending on their location in a given pathway, for example in the visual pathway or selective homologous or heterologous modification of receptors after drug treatment.

Antidepressive Agents↗
PubMedDate not supplied

A neural model of predictive recognition in form pathway of visual cortex.

We present a functional model of form pathway in visual cortex based on predictive coding scheme, in which the prediction is compared with feedforward signals filtered by two kinds of spatial resolution maps, broad and fine resolution map. We propose here the functional role of the prediction and of the two kinds of resolution maps in perception of object form in visual system. The prediction is represented based on memory of dynamical attractors in temporal cortex, categorized by an elemental figure in posterior temporal cortex. The prediction is generated by the feedforward signals of main neurons in broad resolution maps of V(1) and V(4), and then is compared with the feedforward signals of main neurons in fine resolution map of V(1) and V(4).

Animals↗
PubMed1991

Slow and fast pathways in the human rod visual system: electrophysiology and psychophysics.

Under most conditions, increasing the intensity of a flickering light makes the flicker more conspicuous. For a light flickering at 15 times per second, however, increasing the intensity can cause the flicker to disappear before reappearing again at higher intensities [Vision Res. 29, 1539 (1989)]. This flicker disappearance or null is also evident in human electrophysiological recordings at the same intensity levels. These results point to a duality within the rod visual pathway, in which flicker signals travel through a slow and a fast pathway and then recombine at a later stage. At 15 Hz the slow rod flicker signals are delayed by half a cycle relative to the fast signals. Thus, when the two signals are recombined, they destructively interfere and diminish the perception of flicker. The dual-pathway interpretation is supported by both electroretinographic and psychophysical evidence showing a phase difference of half a cycle between 15-Hz rod signals just below and just above the null region. These effects are apparent not only in the normal observer but also in an achromat observer who lacks functioning cone vision.

Electrophysiology↗
PubMed1992

Perception of optical flow in cortical blindness: a case report.

Motion perception was studied in a subject with bilateral lesion of the visual cortex, involving severe damage to cortical areas V1 and V4, but with no apparent damage to visual associative areas situated in occipito-parietal and lateral occipito-temporal (presumably V5) zones. He was able to perceive optical flow motions simulating motion in depth in "blind" parts of his visual field, provided that the stimulus-onset was temporally dissociated from its motion. Moreover, he was able to discriminate between different velocities and directions of motion. The results suggest that perimetrically "blind" parts of the visual field in this patient have true capacities to process visual motion. They are discussed in reference to the subject's ability to move freely in his environment and in reference to the role of extrastriate visual pathways in visual motion processing.

Acceleration↗
PubMed2004

Combination of event-related fMRI and diffusion tensor imaging in an infant with perinatal stroke.

Focal ischemic brain injury, or stroke, is an important cause of later handicap in children. Early assessment of structure-function relationships after such injury will provide insight into clinico-anatomic correlation and potentially guide early intervention strategies. We used combined functional MRI (fMRI) with diffusion tensor imaging (DTI) in a 3-month-old infant to explore the structure-function relationship after unilateral perinatal stroke that involved the visual pathways. With visual stimuli, fMRI showed a negative BOLD activation in the visual cortex of the intact right hemisphere, principally in the anterior part, and no activation in the injured hemisphere. The functional activation in the intact hemisphere correlated clearly with the fiber tract of the optic radiation visualized with DTI. DTI confirmed the absence of the optic radiation in the damaged left hemisphere. In addition, event-related fMRI (ER-fMRI) experiments were performed to define the characteristics of the BOLD response. The shape is that of an inverted gamma function (similar to a negative mirror image of the known positive adult BOLD response). The maximum decrease was reached at 5-7 s with signal changes of -1.7 +/- 0.4%.Thus, this report describes for the first time the combined use of DTI and event-related fMRI in an infant and provides insight into the localization of the fMRI visual response in the young infant and the characteristics of the BOLD response.

Asphyxia Neonatorum↗
PubMed2005

Unsupervised identification of white matter tracts in a mouse brain using a directional correlation-based region growing (DCRG) algorithm.

The inner product of the major eigenvectors of adjacent pixels, known as the directional correlation (DC), has been used previously as a quantitative index to investigate directional similarity in white matter (WM) tracts. A high degree of directional similarity (i.e., high DC) among pixels within individual WM tracts was observed. Based on this observation, a region growing algorithm was employed to propagate an area from a seed point as a function of the DC threshold (DCt) to manually identify WM tracts in two-dimensional (2D) slices from diffusion tensor imaging (DTI). In the present study, an improved unsupervised DC based region growing (DCRG) method was implemented to reduce the operator-dependent variance and to improve the ease of use of the technique. By employing improved method, a multi-slice DTI data set of an in vivo mouse brain was used to identify the external capsule, visual pathway, and corpus callosum. The resultant WM tracts are computer rendered in three-dimensional (3D) images with anatomical images as structural references. In addition, three sets of ex vivo mouse brain data were used to examine the effects of different slice thickness and the signal-to-noise ratio (SNR) to the outcome of DCRG.

Algorithms↗
PubMed1992

Transfer of contrast sensitivity in linear visual networks.

Contrast sensitivity is a useful measure of the ability of an observer to distinguish contrast signals from noise. Although usually applied to human observers, contrast sensitivity can also be defined operationally for individual visual neurons. In a model linear neuron consisting of a filter and noise source, this operational measure is a function of filter gain, noise power spectrum, signal duration, and a performance criterion. This definition allows one to relate the sensitivities of linear neurons at different levels in the visual pathway. Mathematical formulae describing these relationships are derived, and the general model is applied to the specific problem of relating the sensitivities of parvocellular LGN neurons and cortical simple cells in the primate.

Animals↗
PubMed1989

[Predominance of crossed optic nerve fibers is a characteristic of albinism, but not of dissociated vertical deviation].

We examined eight patients with dissociated vertical deviation (DVD) for evidence of misrouting in the visual pathway, using visually evoked potentials. Full-field monocular pattern-onset chequerboard stimulation was employed. The visually evoked potentials were recorded from both occipital lobes. Their differential activity during stimulation of the right eye was compared with that obtained during stimulation of the left eye. In contrast to an earlier report, a predominance of the crossed projection was not found in any of the DVD cases. The results in eight normal control subjects were similar, unlike our findings in eight albino patients (predominance of crossed projection: a relative positivity over the contralateral hemisphere about 100 ms after pattern-onset). Possible artifacts are discussed that may have led to the earlier assumption of misrouting in DVD patients.

Adolescent↗
PubMed2005

Structure and function of parallel pathways in the primate early visual system.

Parallel processing streams in the primate visual system originate from more than a dozen anatomically and functionally distinct types of retinal ganglion cells (RGCs). A central problem in determining how visual information is processed is understanding how each of these RGC types connects to more central structures, including the lateral geniculate nucleus (LGN) of the thalamus and (via the LGN) the primary visual cortex. Nevertheless, the available functional and anatomical evidence linking together specific cell types across these structures is surprisingly indirect. This review evaluates the available evidence and assesses the strength of the many inferences that can be made from these observations. There is strong evidence that parasol RGCs are the provenance of the magnocellular (M) visual pathway and that midget RGCs give rise to the parvocellular (P) pathway. Furthermore, the M and P pathways remain segregated up to the input layer of primary visual cortex. The relationships between the numerous other RGC types and cell types in the LGN remain less certain. and there remains ambiguity about how best to define additional pathways, such as the koniocellular (K) pathway, which probably arise from these other, less common, RGC types.

Action Potentials↗
PubMedDate not supplied

Stato-kinetic dissociation in subjects with normal and abnormal visual fields.

PURPOSE: The aim of the study was to assess the presence and the importance of stato-kinetic dissociation (SKD) in subjects with normal and pathological visual fields (VF). METHODS: A "customized" perimetric test designed for the assessment of SKD was carried out in seven homogeneous samples of subjects (normal, glaucomatous, ocular hypertensive, retinopathic, cataract, anterior visual pathways neuropathic and posterior visual pathways neuropathic). The results were statistically compared (Anova). RESULTS AND CONCLUSIONS: SKD is a physiological phenomenon, more evident in the central, paracentral and superior VF, influenced by age and partially by sex. SKD increases in the VF periphery when a posterior visual pathway disorder or retinopathy is present, and decreases in the centro-paracentral VF of glaucomatous and ocular hypertensive eyes. This SKD behaviour should be useful for early diagnosis of glaucoma.

Adolescent↗
PubMed1991

Reappearance of the visual percept after intentional blinking in a patient with Balint's syndrome.

We present a patient with Balint's syndrome who complained of fading of the scenes under visual fixation. When he intentionally blinked, the faded visual percept reappeared. The disappearance of the visual percept may be explained as the result of either unstable visual fixation or of saturation of the visual pathways. The role of blinking in reviving the visual percept may be explained accordingly as causing a refixation of the target under visual fixation or as resetting the visual pathways for visual processing.

Aged↗
PubMed2005

Visual-perceptual dysfunctions are possible endophenotypes of schizophrenia: evidence from the psychophysical investigation of magnocellular and parvocellular pathways.

Visual information processing is impaired in schizophrenia patients and their biological relatives. The authors measured vernier thresholds in 72 schizophrenia patients, their 86 siblings, and 60 healthy control subjects. Subjects were asked to detect the direction of the horizontal displacement of 2 stimuli (left or right). During magnocellular (M) pathway tests, stimuli were dots with low contrast (5%) or counterphase-modulated gratings (25 Hz). For parvocellular (P) pathway tests, isoluminant blue-red dots with yellow-green background were used. Results revealed that patients with schizophrenia and their siblings were more impaired in M pathway conditions than in P pathway conditions. There was no color-specific impairment. The patients and their siblings displayed lower performances on tests of executive functions, psychomotor speed, and verbal memory compared with the controls. Visual-perceptual and neuropsychological data did not correlate. In conclusion, M pathway dysfunction is a potential endophenotype of schizophrenia.

Analysis of Variance↗
PubMed1995

Aging and pattern visual evoked potentials.

Visual abilities decline during senescence. Age-related declines cannot be attributed entirely to changes in optical factors (senile miosis and opacification of the ocular media) and must therefore be caused by changes in the visual pathways. Because pattern visual evoked potentials (VEPs) provide information regarding the sensory function and integrity of the visual system, recent research on age-related changes in VEPs is reviewed. The results of this review show that stimulus parameters of VEPs (luminance, contrast, spatial and temporal frequencies) modify the age-related changes in VEPs. The visual system consists of multiple, parallel channels which process different information. Therefore, it is hypothesized that aging may affect the specific functional subdivisions in the visual pathways differentially.

Adolescent↗
PubMed1976

An aberrant crossed visual corticotectal pathway in albino rats.

This study investigates the dynamic nature of the developing corticotectal pathway arising in the visual cortex. Special attention is given to the interaction occurring between the corticotectal pathways of each side of the brain and between corticotectal and retinotectal terminations. Normally the visual cortex of rats projects only to the ipsilateral superior colliculus. If one visual cortex is removed at birth, the remaining visual cortex subsequently shows a bilateral projection to the superior colliculus. The aberrant corticotectal pathway is heavier if the cortical ablation is accompanied by eye removal at birth but eye enucleation alone is not a sufficient stimulus for production of a crossed corticotectal projection. The aberrant crossed pathway shows a topographic order which appears to correspond to that of the normal ipsilateral corticotectal pathway. The pathway differs from the aberrant projections from the retina in that lesions done as late as 20 days postnatal still result in an aberrant crossed corticotectal pathway. This is similar to the aberrant crossed cortical projections from sensorimotor cortex. The pathway would appear to arise as a result of lack of competition from corticotectal axons normally present contralaterally or from attraction of denervated corticotectal sites. While denervated retinotectal sites stimulate sprouting of the corticotectal axons once in the deafferented colliculus, they do not stimulate crossing of the corticotectal projection.

Age Factors↗
PubMed1982

Correlation of evoked potentials (SEP and VEP), EEG and CT in the diagnosis of brain tumors and cerebrovascular diseases.

In studying evoked potentials, EEG and CT in 29 patients with cerebral lesions (25 brain tumors and 4 with cerebrovascular diseases), a pathological EEG was found in 24 cases, pathological CT in 26 cases, pathological VEP in 10 of the 25 cases, pathological SEP in 10 cases out of 19. - Patients with pituitary tumors (chiasmatic tumors) showed pathological VEP and normal SEP due to compression of the visual pathway, and this correlated with the clinical site of the lesion, EEG and CT. - One patient with thalamic tumor showed pathological SEP and VEP due to affection of the somatic relay nucleus and the lateral geniculate body, this correlated with EEG and CT. - Patients with supratentorial tumors showed pathological SEP (absent cortical response), when there was cortical sensory loss and delayed conduction between N14-N20 when the posteior columns was affected. - Cases with infratentorial tumors showed pathological SEP only when there was affection of the ascending sensory pathway or the visual pathway. These cases showed pathological EEG and CT. - Cases with cerebrovascular disease showed pathological SEP when the lesion is at the thalamic level or affecting the thalamo-cortical pathway, evidenced clinically by cortical sensory loss or posterior column affection.

Adolescent↗
PubMedDate not supplied

Optic nerve enlargement and chronic visual loss.

We present four patients with sarcoidosis of the anterior visual pathways. The first patient presented with unilateral visual loss, a mass lesion at the optic nerve head, and an enlarged orbital optic nerve. The second patient presented with bilateral progressive painless visual loss, associated with optic nerve pallor and visual field loss. In these two patients, optic nerve biopsy was diagnostic of sarcoidosis. The third patient developed optic nerve and chiasmal involvement after sarcoidosis was established by lacrimal gland biopsy. The fourth patient had optic nerve, pulmonary, and lymph node involvement with sarcoidosis. A conjunctival and lung biopsy were diagnostic. Computed tomography and magnetic resonance imaging have greatly facilitated diagnosis of sarcoidosis of the anterior visual pathways. Sarcoidosis of the anterior visual pathways may occur alone or in association with other ocular or systemic manifestations. A conjunctival or lacrimal gland biopsy may be preferable as the initial diagnostic approach. Treatment of patients with this condition may require systemic immunosuppression, in addition to corticosteroids, to prevent permanent visual loss.

Adult↗
PubMed1994

Differential expression of keratins in goldfish optic nerve during regeneration.

The goldfish visual pathway, unlike the visual pathway of higher vertebrates, retains continuous growth and development throughout life and is capable of functional regeneration. The structure and expression of proteins that support the physiological attributes of this system are of interest. Glial cells in this pathway express keratins as the predominant intermediate filament proteins rather than the expected glial fibrillary acidic protein. Previously we identified and characterized cDNA clones representing two type I keratins from the goldfish optic nerve, GK48 and GK49. The GK48 protein is the type I keratin partner to the type II keratin ON3, while the GK49 protein is expressed in a different cell type. Here, we extend our studies on the expression of mRNA for the GK48, GK49, and ON3 proteins at the early stages of optic nerve regeneration. RNase protection assays show that at 10 days post-crush, there is no overall change in levels of mRNA for these proteins as compared to uncrushed control nerves and nerves from unoperated fish. In addition, we show by in situ hybridization that the GK49 protein shows no changes in its distribution of mRNA in the optic nerve after crush. In contrast, the levels of GK48 and ON3 mRNA are greatly reduced within the crush zone. However, these two mRNAs are differentially expressed at different time points during regeneration, with GK48 mRNA appearing in the crush zone before ON3. These results indicate that the mRNA for the GK48 and ON3 proteins are differentially regulated during regeneration and that these two proteins are expressed in a different cell type from the GK49 protein.

Animals↗
PubMed1982

Interactions between components of the avian visual system.

Pigeons were trained to perform in a psychophysical procedure for determining luminance difference thresholds. After the psychometric data had stabilized, lesions were made in specific cell populations of the tectofugal visual pathway. In one group of pigeons, the lesions were confined to nucleus rotundus (Rt). In a second group, the lesions included not only Rt, but also the pars ventralis of the lateral geniculate nucleus (GLv). In the third group, the lesions included Rt, GLv and nucleus subpretectalis (SP). The postoperative performance of the birds in the Rt group showed elevations in threshold that represented substantial losses in sensory capacity. In contrast, the birds with lesions of Rt + GLv showed little or no change in their postoperative thresholds. Finally, the birds in the Rt + GLv + SP group were as impaired postoperatively as the birds with lesions of Rt alone. The results are interpreted in the context of the interaction between components of parallel routes within the tectofugal pathway and parallel pathways within the visual system.

Animals↗