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PubMed2001

Differential extrageniculostriate and amygdala responses to presentation of emotional faces in a cortically blind field.

Patient G.Y. is able to discriminate emotional facial expressions presented in his blind (right) hemifield despite an extensive lesion of the corresponding (left) striate cortex. One proposal is that this residual ability (affective "blindsight") depends on a subcortical visual pathway comprising the superior colliculus, posterior (extrageniculate) thalamus and amygdala. Here we report differential amygdala responses in G.Y. to presentation of fearful and fear-conditioned faces in his blind (right) hemifield. These amygdala responses exhibited condition-dependent covariation with neural activity in the posterior thalamus and superior colliculus. Our results provide further evidence that an extrageniculostriate (colliculo-thalamo-amygdala) neural pathway can process fear-related stimuli independently of both the striate cortex and normal phenomenal visual awareness.

Adult↗
PubMed1996

Organization of the visual reticular thalamic nucleus of the rat.

The visual sector of the reticular thalamic nucleus has come under some intense scrutiny over recent years, principally because of the key role that the nucleus plays in the processing of visual information. Despite this scrutiny, we know very little of how the connections between the reticular nucleus and the different areas of visual cortex and the different visual dorsal thalamic nuclei are organized. This study examines the patterns of reticular connections with the visual cortex and the dorsal thalamus in the rat, a species where the visual pathways have been well documented. Biotinylated dextran, an anterograde and retrograde tracer, was injected into different visual cortical areas [17; rostral 18a: presumed area AL: (anterolateral); caudal 18a: presumed area LM (lateromedial); rostral 18b: presumed area AM (anteromedial); caudal 18b: presumed area PM (posteromedial)] and into different visual dorsal thalamic nuclei (posterior thalamic, lateral geniculate nuclei), and the patterns of anterograde and retrograde labelling in the reticular nucleus were examined. From the cortical injections, we find that the visual sector of the reticular nucleus is divided into subsectors that each receive an input from a distinct visual cortical area, with little or no overlap. Further, the resulting pattern of cortical terminations in the reticular nucleus reflects largely the patterns of termination in the dorsal thalamus. That is, each cortical area projects to a largely distinct subsector of the reticular nucleus, as it does to a largely distinct dorsal thalamic nucleus. As with each of the visual cortical areas, each of the visual dorsal thalamic (lateral geniculate, lateral posterior, posterior thalamic) nuclei relate to a separate territory of the reticular nucleus, with little or no overlap. Each of these dorsal thalamic territories within the reticular nucleus receives inputs from one or more of the visual cortical areas. For instance, the region to the reticular nucleus that is labelled after an injection into the lateral geniculate nucleus encompasses the reticular regions which receive afferents from cortical areas 17, rostral 18b and caudal 18b. These results suggest that individual cortical areas may influence the activity of different dorsal thalamic nuclei through their reticular connections.

Animals↗
PubMed1996

Cerebral visual impairment in periventricular leukomalacia: MR correlation.

PURPOSE: To evaluate the involvement of central visual pathways in cases of periventricular leukomalacia, and to correlate the neuroradiologic findings with the degree of visual acuity. METHODS: The MR brain examinations of 27 preterm children affected by cerebral palsy resulting from periventricular leukomalacia and without significant ophthalmologic lesions were reviewed retrospectively to search for possible involvement of the optic radiations and/or of the calcarine cortex. The data were compared with the degree of visual acuity estimated by means of the Teller Acuity Cards test. RESULTS: Seventeen (63%) of the 27 patients had cerebral visual impairment, which correlated strongly with MR lesions. Quantitative reduction and signal hyperintensity of the peritrigonal white matter and atrophy of the calcarine cortex were present in the more severe cases. In two blind patients, an altered MR signal was detected in the lateral geniculate bodies. CONCLUSION: This study clearly establishes a relationship between specific MR findings and visual impairment in children with periventricular leukomalacia. The finding of hyperintensity in the lateral geniculate bodies was interpreted as an axonal reaction. MR imaging is useful for detecting potential visual impairment and for improving clinical diagnosis.

Atrophy↗
PubMed2000

Monocular lustre from flicker.

A spot that flickers at 16 Hz between two luminance levels (on a grey surround) has an appearance of metallic lustre, which we call 'monocular lustre'. Binocular and monocular lustre were measured in comparable conditions by a rating procedure, and both were reported only when the light and dark values of the flickering (or binocularly fused) spot straddled the surround luminance, so that the spot was alternately brighter and darker than the surround. We attribute lustre to competition between ON and OFF visual pathways.

Flicker Fusion↗
PubMed1990

The effect of local luminance contrast on induced motion.

Recently it has been suggested that the magnocellular, as opposed to the parvocellular, subsystem of the primary visual pathway of primates subserves motion perception. This suggestion is partly based on the observation that both the visual responses of magnocellular neural units and certain motion perception phenomena have high contrast sensitivity and are only dependent on luminance contrast for a narrow range of low contrasts. Parvocellular units have low contrast sensitivity and are dependent on contrast for a wide range of values. In the present experiment, the effect of local luminance contrast on induced motion was measured using a nulling procedure to quantify the magnitude, of illusory motion perceived in a centre grating which was viewed against a moving surround grafting. Centre grating contrast was either matched to the surround or maintained at a low (2.5%) or high (60%) value. Surround contrast ranged from 2.5% to 60%. It was found that (1) centre contrast had no observable effect on the magnitude of the illusion, (2) induced motion was marginal or absent with low contrast but detectable surrounds, and (3) induced motion increased as contrast in the surround increased for the range of contrasts tested. This contrast response function is more similar to that of parvocellular than magnocellular units and therefore suggests that the parvocellular stream may play a role in some aspects of motion processing.

Adult↗
PubMed1984

Changes in the cortical components of the visual evoked potential with age in man.

Pattern visual evoked potentials (PVEPs) were recorded from 177 normal subjects aged 11-87 years. The purpose was to determine age-dependent changes in the latency of the individual components of the waveform. The initial components of the PVEP, which are thought to reflect activity in the primary visual (striate) cortex, showed no change in latency from 11-50 years followed by an abrupt increase occurring during the sixth decade. It is probable that this delay in the PVEP is mediated largely by pathophysiological changes occurring either in the eye or visual pathways, or both. The secondary components of the PVEP, which are presumed to be generated in the visual association areas of the extrastriate cortex, showed a progressive although initially very limited increase in latency starting after adolescence. It is suggested that those delays occurring between 21 and 50 years in the secondary components are mediated largely by intracortical factors. The more pronounced delays occurring for the older age groups (over 50 years) appear to be caused by a combination of cortical, subcortical and ocular changes in visual function. The prolongation of the later components in the age group 11-20 years seems to reflect maturational changes almost entirely confined to intracortical processes. The results are discussed in terms of the many conflicting reports of the effects of age on the PVEP.

Adolescent↗
PubMed1999

High spatial resolution functional magnetic resonance imaging at very-high-magnetic field.

Although neuroimaging methods have been used successfully to map large-scale neurocognitive networks distributed across the human cortex, functional mapping and differentiation of localized brain organization within a small structure has been limited by inadequate sensitivity for high spatial resolution imaging. Functional magnetic resonance imaging (fMRI) technique based on blood oxygenation level-dependent (BOLD) contrast has become one of the most useful neuroimaging techniques. It has been used extensively to study human brain function from sensory perception to cognitive performance. However, the majority of these studies used a relatively low spatial resolution (typically with a voxel size of 3.1 x 3.1 x 5.0 mm3), which is incapable of mapping on the millimeter and submillimeter spatial scale. In this article, we review the technical aspects of the high-resolution fMRI technique and the sensitivity and spatial specificity of BOLD-based fMRI. We demonstrate applications of high-resolution fMRI in studying the human visual pathway from the lateral geniculate nucleus in the thalamus to the ocular dominance columns in the primary visual cortex. Most results were obtained at very-high-magnetic fields (3.0 and 4.0 Tesla). They reveal that high-resolution fMRI at very-high-magnetic field is promising for functional mapping of brain organization from large cortical networks, small nuclei, and even to cellular layer structures.

Brain↗
PubMed2001

Visual field loss associated with vigabatrin: pathological correlations.

Pathological changes are reported in the anterior visual pathways of a 41 year old man with complex partial seizures treated with vigabatrin who developed bilateral visual field constriction. There was peripheral retinal atrophy with loss of ganglion cells and loss of nerve fibres in the optic nerves, chiasm, and tracts. No evidence of intramyelinic oedema was seen. These findings suggest that the primary site of injury lies within the ganglion cells in the retina. The degree of atrophy seen would suggest that the visual field loss is irreversible.

Adult↗
PubMed1999

Synchronous activity in the visual system.

Synchronous activity among ensembles of neurons is a robust phenomenon observed in many regions of the brain. With the increased use of multielectrode recording techniques, synchronous firing of ensembles of neurons has been found at all levels in the mammalian visual pathway, from the retina to the extrastriate cortex. Here we distinguish three categories of synchrony in the visual system, (a) synchrony from anatomical divergence, (b) stimulus-dependent synchrony, and (c) emergent synchrony (oscillations). Although all three categories have been well documented, their functional significance remains uncertain. We discuss several lines of evidence both for and against a role for synchrony in visual processing: the perceptual consequences of synchronous activity, its ability to carry information, and the transmission of synchronous neural events to subsequent stages of processing.

Animals↗
PubMed2006

Homonymous hemianopias: clinical-anatomic correlations in 904 cases.

OBJECTIVE: To describe the clinical characteristics and clinical-anatomic correlations of homonymous hemianopia (HH). BACKGROUND: Homonymous hemianopia impairs visual function and frequently precludes driving. Most knowledge of HH is based on relatively few cases with clinical-anatomic correlations. METHODS: The authors reviewed medical records of all patients with HH seen in their service between 1989 and 2004. Demographic characteristics, characteristics of visual field defects, causes of visual field defects, neuroradiologic definition of lesion location, and associated neurologic deficits were recorded. RESULTS: A total of 904 HH were found in 852 patients. A total of 340 HH (37.6%) were complete and 564 HH (62.4%) were incomplete. Homonymous quadrantanopia (264 HH, 29%) was the most common type of incomplete HH, followed by homonymous scotomatous defects (116 HH, 13.5%), partial HH (114 HH, 13%), and HH with macular sparing (66 HH, 7%). A total of 407 HH (45.0%) were isolated. Causes of HH included stroke (629 HH, 69.6%), trauma (123, 13.6%), tumor (102, 11.3%), brain surgery (22, 2.4%), demyelination (13, 1.4%), other rare causes (13, 1.4%), and unknown etiology (2, 0.2%). The lesions were most commonly located in the occipital lobes (45%) and the optic radiations (32.2%). Every type of HH, except for unilateral loss of temporal crescent and homonymous sectoranopia, was found in all lesion locations along the retrochiasmal visual pathways. CONCLUSION: Homonymous hemianopia is usually secondary to stroke, head trauma, and tumors. Although the characteristics of visual field defects can be helpful in lesion location, specific visual field defects do not always indicate specific brain locations.

Adolescent↗
PubMed1975

The pulvinar nucleus of Galago senegalensis.

The present study was undertaken to analyze the connections of the pulvinar nucleus in a prosimian. The experiments, which rely on the Fink-Heimer ('67) method for staining degenerating axons and their terminals, fall into two parts: first, the tracing of ascending tectal projections to the caudal thalamus and second, the tracing of projections from this thalamic target to the cortex. Large lesions of the superior colliculus resulted in dense degeneration in the caudal half of the inferior subdivision of the pulvinar complex. This pathway could be identified when the lesion was restricted to the superficial layers of the superior colliculus, signifying that it is a visual pathway. In general, the projections of the deep and superficial layers of the superior colliculus were distinct and in this respect Galago resembles Tupaia. The inferior pulvinar nucleus in turn projects to area MT, a conspicuous subdivision of the temporal cortex. The superior division of the pulvinar, in contrast to the inferior division, is not a major target of ascending projections from the superior colliculus and projects to the areas of the occipital and temporal lobe intercalated between areas MT and 17. When these results are compared with similar studies in nonprimates, notably studies of Tupaia, a striking difference in organization emerges. In Tupaia, and in distantly related mammals such as the squirrel, the target of the tecto-pulvinar system includes area 18 adjacent to area 17. This feature is important since the two parallel projection systems seem to be related to each other in terms of the way in which the zero vertical meridian is spatially represented. However, in Galago the subdivision of the pulvinar receiving projections arising from the superior colliculus does not project to area 18. Area 18 is indeed the target of pulvinar projections, but these projections arise from that portion of the pulvinar which is not a recipient of ascending tectal projections. It is not easy to see how this primate organization, if indeed the Galago is representative of primates, evolved from the organization reflected in Tupais.

Animals↗
PubMed1972

The outer disinhibitory surround of the retinal ganglion cell receptive field.

1. There is an outer disinhibitory zone surrounding the classical inhibitory surround of the retinal ganglion cell receptive field.2. The disinhibitory surround is strong and narrow in ;sustained' cells but weak and laterally spread in ;transient' cells.3. The disinhibitory surround can be demonstrated using a black spot as a probing stimulus as well as by a white spot, and is therefore not an artifact of scattered light.4. Stimulation with a light spot in the disinhibitory zone gives an increase in firing to ;stimulus on' in on-centre cells and to ;stimulus off' in off-centre cells.5. The disinhibitory surround may be revealed by plotting the latency of the first spike discharge following stimulation against position in the receptive field. The disinhibitory zone shows a decrease in latency to the centre-type stimulus.6. The disinhibitory surround may be revealed by plotting the threshold intensity of a spot against position in the receptive field. It is thus a feature of the sensitivity gradients of both ;transient' and ;sustained' cells.7. Using two spots, one at the centre of the receptive field and the other at varying distances from the receptive field centre, dynamic interactions between the centre, inhibitory and disinhibitory zones are demonstrated. A spot presented in the disinhibitory zone causes an enhancement of the centre response when flashing in phase with the centre spot, while it causes inhibition of the centre response when presented 180 degrees out of phase.8. A scheme for the anatomical basis of the disinhibitory surround is proposed, and the relation of disinhibition to the spatial transfer characteristics of the visual pathways is discussed.

Action Potentials↗
PubMed1991

Fibre divergence in the distal optic radiation: possible basis of functional plasticity in adult primate visual cortex.

The precision of retinotopy in primate visual cortex is commonly thought to result from highly ordered arrangement of fibres in the visual pathways. However, rigid point-to-point representation is hardly compatible with findings of a substantial reorganization of visual cortical maps after peripheral and central lesions. Such observations could be accounted for by divergence in the optic radiation. To explore the hypothesis of fibre divergence, we made small knife cuts in the distal optic radiation of macaca fascicularis. After subsequent axonal tracing by injecting WGA-HRP into lateral geniculate nucleus, we studied the course of distal fibres in white matter. The amount of divergence was assessed by measuring, relative to the prevailing fibre course, length and orientation of labelled fibres between lesion and entry into cortex. Lesion sizes between 1 mm to 3 mm did not result in any detectable diminution of terminal labelling in layer IVC of striate cortex. Individual labelled fibres were found to diverge symmetrically from both sides into the gap distal to the lesion. Divergence starts at a distance of about 3 mm before cortex. At the white matter boundary, less than 10% of all fibres still retain the original direction, with the remaining fibres taking any other orientation without preference. We estimate that this corresponds to a divergence of visual afferents encompassing about 6-10 mm of cortical distance, if intracortical arborization of terminal fibres is taken into account. Possible consequences for functional plasticity in the adult primate visual cortex are discussed.

Animals↗
PubMedDate not supplied

[Effect of diazepam on visual potentials evoked by reversible checkerboard pattern].

Visual potentials evoked by means of reversible checkerboard pattern were studied in 12 healthy subjects, 5 patients with probable multiple sclerosis and 3 patients with photosensitive epilepsy before and after intravenous injection of 10 mg of Valium. The most characteristic feature was lowering of the amplitude of the evoked visual potentials (in 75%) which appeared as a rule between 30 seconds and 4 minutes after Valium injection. Prolongation of Pmax latency was observed less frequently (in 65%) and this change was of low grade. In nearly half the cases sings of synchronization appeared with development of late multiphasic components of the visual evoked potential. No significant difference was noted in the effect of Valium on the latency and amplitude of the visual evoked potentials between healthy subjects and patients. The possible mechanisms of diazepam action on the conduction in visual pathways are discussed.

Diazepam↗
PubMed1978

Changes in synaptic function induced by blockage of axonal transport in the rabbit optic pathway.

This study was undertaken to elucidate the physiological significance of material involved in the rapid axonal transport. The effects of colchicine-induced inhibition of axonal transport in the retinal ganglion cells on the electrophysiological properties of the retrobulbar visual pathways were investigated in Albino rabbits. An impaired signal transmission to the contralateral visual cortex, superior colliculus and lateral geniculate body following flash light stimulation as well as direct optic nerve stimulation appeared 4--6 days after an intravitreous injection of 10--25 microgram colchicine. It was concluded that inhibition of the fast axonal transport within the retinal ganglion cells interferes with transsynaptic signal transmission from optic nerve terminals in the subcortical nuclei. This indicates a functional relationship between material supplied via the rapid phase of axonal transport and an unimpaired transsynaptic signal transmission, previously not revealed in the central nervous system of mammals.

Animals↗
PubMedDate not supplied

A cooperation and competition based simple cell receptive field model and study of feed-forward linear and nonlinear contributions to orientation selectivity.

We present a model for development of orientation selectivity in layer IV simple cells. Receptive field (RF) development in the model, is determined by diffusive cooperation and resource limited competition guided axonal growth and retraction in geniculocortical pathway. The simulated cortical RFs resemble experimental RFs. The receptive field model is incorporated in a three-layer visual pathway model consisting of retina, LGN and cortex. We have studied the effect of activity dependent synaptic scaling on orientation tuning of cortical cells. The mean value of hwhh (half width at half the height of maximum response) in simulated cortical cells is 58 degrees when we consider only the linear excitatory contribution from LGN. We observe a mean improvement of 22.8 degrees in tuning response due to the non-linear spiking mechanisms that include effects of threshold voltage and synaptic scaling factor.

Animals↗
PubMed1996

Functional mapping of color processing by magnetic resonance imaging of responses to selective P- and M-pathway stimulation.

Magnetic resonance imaging sensitized to activity-related changes in cerebral blood oxygenation was performed to map responses to selective stimulation of the parvo- and magnocellular visual pathways in calcarine and adjacent ventral occipital cortex of human subjects. In a repetitive stimulation protocol isoluminant chromatic or isochromatic luminance modulation was alternated with steady light of the same mean chromaticity and luminance as a reference condition. While no significant effects were observed for diffuse luminance modulation, two consistent cortical foci responded to isoluminant chromatic stimulation. A strong response was obtained in calcarine cortex at both 2 and 10 Hz, and even for selective S-cone stimulation. A second weaker color-sensitive response was seen bilaterally in the collateral sulcus. Thus, the data not only confirm color-sensitive activation in the collateral sulcus elicited in previous studies by selective cognitive tasks, but additionally demonstrate color-sensitive activation in primary visual cortex. With stimuli defined according to electrophysiological response properties of early visual processing stages, this study complements phenomenological or cognitive approaches in functional mapping of the human visual system.

Adult↗