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At least 1,783 records · Page 99Linked to original sources
PubMed2006

Contrast-dependent, contextual response modulation in primary visual cortex and lateral geniculate nucleus of the cat.

In the primary visual cortex (V1), the responses of neurons to stimuli presented in their classical receptive fields (CRFs) are modulated by another stimulus concurrently presented in their surround (receptive field surround, SRF). We studied the nature of the modulatory effects of SRF stimulation with respect to stimulus contrast in cat V1. In 51 V1 neurons studied, large SRF stimuli (40 degreesx30 degrees ) induced only the suppression of responses to CRF stimulation and the suppressive effects became stronger as the contrast for SRF stimulation increased. The contrast sensitivity of SRF suppression did not correlate with that of CRF responses. By independently controlling contrast of CRF and SRF stimuli, we studied whether SRF effects vary with CRF response magnitude. Increasing contrast for CRF stimulation caused an upward shift of the range of effective contrasts for SRF stimulation, indicating that a high contrast for SRF stimulation is required for suppressing strong responses to CRF stimulation at high contrasts. To assess the possible origin of the suppressive SRF effect on V1 neurons, we also investigated the contrast dependency of SRF effects in 28 neurons from the lateral geniculate nucleus. Our results suggest that SRF effects obtained at the subcortical level strongly contribute to those in V1. Taken together, we conclude that along the thalamocortical projections, SRF modulation exhibits a gain-control mechanism that scales the suppressive SRF effect depending on the contrast for CRF stimulation. In addition, SRF effects can be facilitatory at low stimulus contrasts potentially due to the enlargement of the summation field.

Algorithms↗
PubMed2003

Postnatal changes in nerve growth factor and brain derived neurotrophic factor levels in the retina, visual cortex, and geniculate nucleus in rats with retinitis pigmentosa.

Royal College of Surgeons (RCS) rats are a well established animal model of inherited retinitis pigmentosa (RP). Using RCS rats we examined the distribution of nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) in the visual cortex, geniculate nucleus and retina at three different postnatal ages. It was found that the retina of rats with RP expresses low amounts of NGF and BDNF in young and adult life. Altered levels of these factors were found in the visual cortex and in the geniculate nucleus. Our findings indicate that NGF and BDNF are differentially affected in the visual system of developing and adult RCS rats, suggesting that neurotrophins may be implicated in the pathogenesis of inherited RP.

Age Factors↗
PubMed2003

Topographic organization of projections from the amygdala to the visual cortex in the macaque monkey.

The topography of amygdaloid projections to the visual cortices in the macaque monkey was examined by injecting the fluorescent tracers Fast Blue and Diamidino Yellow at different locations in the occipital and temporal lobes and mapping the distribution of retrogradely labeled cells in the amygdala. Injections involving regions from rostral area TE to caudal area V1 all resulted in labeled cells within the basal nucleus of the amygdala. Relatively few double-labeled cells were observed even when the two injections were separated by less than 3 mm. The projections were rostrocaudally organized such that projections to caudal visual areas originated from dorsal and caudal portions of the magnocellular division of the basal nucleus while projections to more rostrally situated visual areas originated in more rostral and ventral portions of the basal nucleus. When injections involved rostral and medial portions of area TE, retrogradely labeled cells were observed in the accessory basal and lateral nuclei in addition to the basal nucleus. These data confirm that the amygdala gives rise to feedback projections to all levels of the "ventral stream" visual pathway. The projections do not appear to be diffusely distributed since few double-labeled cells were observed. The largest cells of the basal nucleus, those located in the magnocellular division, project the farthest in the visual system and innervate all occipital and temporal levels. The smaller cells, in the intermediate and parvicellular regions, project to more rostral and medial portions of the visual cortex. These results suggest that the amygdala may have substantial modulatory control over sensory processing at all stages of the ventral-stream visual cortical hierarchy.

Amidines↗
PubMed1996

Functional MRI of the activated human visual cortex.

We explored the possibility of performing functional magnetic resonance imaging of visual evoked response in the primary visual cortex with a clinical MRI imager of 1.5 tesla. Gradient echo pulse sequence was chosen to acquire the functional image and binocular flash visual stimulation by light-proof goggles containing light emitting diodes was used as the source of cerebral activation. Data processing was performed utilizing direct subtraction for averaged images, correlation coefficient, statistical parametric mapping and matched filter. Of those methods, correlation coefficient and statistic parametric mapping proved to be the methods of choice in our study.

Brain Mapping↗
PubMed2005

Reduction of choline acetyltransferase activity in primary visual cortex in mild to moderate Alzheimer's disease.

BACKGROUND: Cholinergic deficits in the primary visual cortex (PVC) may underlie some of the abnormalities in visual processing and global cognitive performance in Alzheimer's disease (AD). OBJECTIVE: To correlate measures of general cognition (Mini-Mental State Examination and Global Cognitive Score) and visuospatial function with choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activities, and nerve growth factor protein levels in the PVC. DESIGN: The ChAT and AChE enzyme assays and a nerve growth factor protein enzyme-linked immunoabsorbent assay were performed on PVC tissue samples from subjects clinically diagnosed as having mild cognitive impairment (MCI), AD, or no cognitive impairment (NCI). SETTING AND PATIENTS: Nuns, priests and brothers enrolled in the Religious Order Study, with annual premortem records of neuropsychological testing. RESULTS: Significant differences in ChAT activity, but not in AChE activity or nerve growth factor protein levels, were found among diagnostic groups (P = .049). The ChAT activity was lower in AD than in MCI or NCI (P<.01); MCI was not different from NCI. The PVC ChAT activity correlated with measures of overall cognitive function (Mini-Mental State Examination and Global Cognitive Score), but less so with a composite measure of visuospatial function. CONCLUSIONS: The reduction in ChAT activity in the PVC of mild to moderate AD, but not in MCI, might serve to distinguish between clinical and preclinical forms of the disease. It appears that this change relates to generalized cognitive abnormalities but not specifically to visuospatial function.

Aged↗
PubMed1995

Regeneration of specific nerve cells in lesioned visual cortex of the human brain: an indirect evidence after constant stimulation with different spots of light.

The defective parts of the visual field of two brain-injured patients were stimulated with different spots of light. There is evidence for at least five independent visual functions which can be restored due to constant stimulation of the blind part of the visual field: (1) The constant stimulation of the blind part of the visual field with spots of white light leads to an increase of the visual field for the perception of white light only. (2) The constant stimulation with spots of light of different wavelengths leads to an increase of the visual field for different color perception. To enlarge the visual field for the perception of the color red, a light stimulus with the wavelength of 656 nanometers (nm) was used; for the visual field for the perception of the color green 525 nm; for yellow 578 nm; and for blue 450 nm. (3) The constant stimulation of the blind visual field with black and white light bars of different orientations and constellations leads to an increase of the foveal acuity and an improvement of form perception in the periphery of the visual field. The results suggest that the recovery of visual functions, different color perception and form perception, may depend upon neuronal regeneration in the human visual cortex; regeneration occurs with adequate and constant stimulation of its specific neurons.

Adolescent↗
PubMedDate not supplied

Comparison of functional MR and H2 15O positron emission tomography in stimulation of the primary visual cortex.

PURPOSE: To locate spoiled gradient-echo functional MR signal changes in relation to brain parenchyma. METHODS: The region of the primary visual cortex was evaluated using functional MR and H2 15O positron emission tomography in each of six male subjects who were being visually stimulated by means of red light-emitting diode flash goggles. RESULTS: The positron emission tomography technique demonstrated substantially greater relative signal change with visual stimulation than did the functional MR technique. Furthermore, the functional MR signal changes were concentrated in loci around the periphery of brain parenchyma exhibiting increased radiotracer activity, as opposed to being collocated. CONCLUSIONS: Signal changes found using functional MR based on gradient-echo techniques reflect primarily phenomena occurring within small veins and underrepresent activity intrinsic to brain parenchyma, thus introducing potential inaccuracies in locating regions of activated brain tissue. Positron emission tomography, however, directly measures changes in metabolically related activity within the parenchyma.

Adult↗
PubMed1998

Neuronal correlates of amblyopia in the visual cortex of macaque monkeys with experimental strabismus and anisometropia.

Amblyopia is a developmental disorder of pattern vision. After surgical creation of esotropic strabismus in the first weeks of life or after wearing -10 diopter contact lenses in one eye to simulate anisometropia during the first months of life, macaques often develop amblyopia. We studied the response properties of visual cortex neurons in six amblyopic macaques; three monkeys were anisometropic, and three were strabismic. In all monkeys, cortical binocularity was reduced. In anisometropes, the amblyopic eye influenced a relatively small proportion of cortical neurons; in strabismics, the influence of the two eyes was more nearly equal. The severity of amblyopia was related to the relative strength of the input of the amblyopic eye to the cortex only for the more seriously affected amblyopes. Measurements of the spatial frequency tuning and contrast sensitivity of cortical neurons showed few differences between the eyes for the three less severe amblyopes (two strabismic and one anisometropic). In the three more severely affected animals (one strabismic and two anisometropic), the optimal spatial frequency and spatial resolution of cortical neurons driven by the amblyopic eye were substantially and significantly lower than for neurons driven by the nonamblyopic eye. There were no reliable differences in neuronal contrast sensitivity between the eyes. A sample of neurons recorded from cortex representing the peripheral visual field showed no interocular differences, suggesting that the effects of amblyopia were more pronounced in portions of the cortex subserving foveal vision. Qualitatively, abnormalities in both the eye dominance and spatial properties of visual cortex neurons were related on a case-by-case basis to the depth of amblyopia. Quantitative analysis suggests, however, that these abnormalities alone do not explain the full range of visual deficits in amblyopia. Studies of extrastriate cortical areas may uncover further abnormalities that explain these deficits.

Amblyopia↗
PubMedDate not supplied

A nonlinear model of the behavior of simple cells in visual cortex.

Despite their structured receptive fields (RFs) and the strong linear components in their responses, most simple cells in mammalian visual cortex exhibit nonlinear behaviors. Besides the contrast-response function, nonlinearities are evident in various types of failure at superposition tasks, in the disagreement between direction indices computed from drifting and counterphase flickering gratings, in various forms of response suppression (including end- and side-stopping, spatial-frequency-specific inhibition and cross-orientation inhibition), in the advance of phase with increasing contrast, and in phase-insensitive and frequency-doubled responses to counterphase flickering gratings. These behaviors suggest that nonlinearities are involved in the operation of simple cells, but current models fail to explain them. A quantitative model is presented here that purports to describe basic and common principles of operation for all visual cortical cells. Simple cells are described as receiving afferents from multiple subunits that differ in their individual RFs and temporal impulse responses (TIRs). Subunits are independent and perform a spatial integration across their RFs followed by halfwave rectification and temporal convolution with their TIRs. This parallel operation yields a set of temporal functions representing each subunit's contribution to the membrane potential of the host cell, whose final form is given by the weighted sum of all subunits' contributions. By varying the number of subunits and their particular characteristics, different instances of the model are obtained each of which displays a different set of behaviors. Extensive simulation results are presented that illustrate how all of the reported nonlinear behaviors of simple cells arise from these multi-subunit organizations.

Animals↗
PubMed1999

Metabolic activity patterns in the monkey visual cortex as revealed by spectral analysis.

The metabolic activity pattern of the monkey visual cortex was mapped quantitatively with [14C]-2-deoxyglucose during the performance of a visually guided reaching task. After bandpass filtering of the reconstructed two-dimensional metabolic maps of areas V1 and V2, alternating bands of high and low metabolic activity were apparent in control and experimental hemispheres. The spatial arrangement of active bands was studied with two-dimensional spectral analysis, and bands were found to be more organized in the experimental monkey. In area V1 of the control monkey the spectral amplitude was spread over a wider range of directions and frequencies than in the experimental subject. The finding that layer IV is characterized by more complex spectra than layers I through III suggests the coexistence of more than one active columnar system in the geniculorecipient layer. In area V2, stripes running almost perpendicular to the V1/V2 border were found along with superimposed patches of enhanced metabolic activity. In the experimental hemispheres, the corresponding spectra were extremely sharp yielding a constant periodicity. It is suggested that the well-organized columnar arrangement within areas V1 and V2 of the experimental hemispheres emerges from the diffusely organized background network of activity patterns in the control state.

Algorithms↗
PubMed1999

Genetic and epigenetic regulation of NMDA receptor expression in the rat visual cortex.

The susceptibility of cortical networks to use-dependent modifications declines with age (critical period) and this decline of neuronal plasticity during development is paralleled by the shortening of NMDA receptor EPSCs. We showed previously in the somatosensory cortex that the shortening of NMDA receptor kinetics correlates with a developmentally-regulated increase in the NR2A subunit expression. Here we examine whether this developmental regulation of NR2A expression is related to the duration of critical periods and whether it is influenced by experience. Functional NMDA receptors and their molecular characteristics are studied in identified layer IV neurons of rat visual cortex. In this structure the time course of the critical period differs from that in the somatosensory cortex and can be changed by sensory deprivation, thus permitting examination of correlations between the time course of receptor expression and the duration of the critical period. We find that the developmental expression of the NR2A subunit is delayed compared with the somatosensory cortex, in agreement with the prolonged critical period in the visual cortex. Moreover, sensory deprivation further delays the developmental change in the NMDA receptor subunit composition, demonstrating the activity dependence of this process and strengthening the correlation between changes in subunit composition and the time course of the critical period.

Aging↗
PubMedDate not supplied

Visual cortex neurons in monkeys and cats: detection, discrimination, and identification.

A descriptive function method was used to measure the detection, discrimination, and identification performance of a large population of single neurons recorded from within the primary visual cortex of the monkey and the cat, along six stimulus dimensions: contrast, spatial position, orientation, spatial frequency, temporal frequency, and direction of motion. First, the responses of single neurons were measured along each stimulus dimension, using analysis intervals comparable to a normal fixation interval (200 ms). Second, the measured responses of each neuron were fitted with simple descriptive functions, containing a few free parameters, for each stimulus dimension. These functions were found to account for approximately 90% of the variance in the measured response means and response standard deviations. (A detailed analysis of the relationship between the mean and the variance showed that the variance is proportional to the mean.) Third, the parameters of the best-fitting descriptive functions were utilized in conjunction with Bayesian (optimal) decision theory to determine the detection, discrimination, and identification performance for each neuron, along each stimulus dimension. For some of the cells in monkey, discrimination performance was comparable to behavioral performance; for most of the cells in cat, discrimination performance was better than behavioral performance. The behavioral contrast and spatial-frequency discrimination functions were similar in shape to the envelope of the most sensitive cells; they were also similar to the discrimination functions obtained by optimal pooling of the entire population of cells. The statistics which summarize the parameters of the descriptive functions were used to estimate the response of the visual cortex as a whole to a complex natural image. The analysis suggests that individual cortical neurons can reliably signal precise information about the location, size, and orientation of local image features.

Animals↗
PubMed2004

Popout modulates focal attention in the primary visual cortex.

The influence of context-dependent interactions on attention-related neural activity was studied in the human primary visual cortex (V1) with event-related fMRI. Retinotopic field-sign mapping was used to determine the localization of V1 with respect to adjacent retinotopic areas. Observers reported the orientation of a Gabor patch at pre-cued extrafoveal locations when it was salient among distractor Gabors and when it was not. Saliency was caused by local orientation contrast between Gabors-a mechanism that is thought to arise from context-dependent interactions in the V1 proper. A comparison of the attention-related BOLD response for salient and non-salient stimuli in V1 revealed that salient Gabors caused a significantly smaller BOLD response than non-salient Gabors. This differential effect was not observed in higher-order visual areas (V3/V3A, MT+/LO, IPS). When attention was not focused onto the target, the size of the BOLD response was generally reduced in all visual areas, and no difference was seen in V1 for salient and non-salient Gabors. These findings suggest that contextual interactions underlying saliency influence attentional modulations in V1 and support the view that perceptual and attentional mechanisms share neural circuits at this early stage of visual processing.

Adult↗
PubMed1993

Cortical hierarchy reflected in the organization of intrinsic connections in macaque monkey visual cortex.

Neuronal response properties vary markedly at increasing levels of the cortical hierarchy. At present it is unclear how these variations are reflected in the organization of the intrinsic cortical circuitry. Here we analyze patterns of intrinsic horizontal connections at different hierarchical levels in the visual cortex of the macaque monkey. The connections were studied in tangential sections of flattened cortices, which were injected with the anterograde tracer biocytin. We directly compared the organization of connections in four cortical areas representing four different levels in the cortical hierarchy. The areas were visual areas 1, 2, 4 and Brodman's area 7a (V1, V2, V4 and 7a, respectively). In all areas studied, injections labeled numerous horizontally coursing axons that formed dense halos around the injection sites. Further away, the fibers tended to form separate clusters. Many fibers could be traced along the way from the injection sites to the target clusters. At progressively higher order areas, there was a striking increase in the spread of intrinsic connections: from a measured distance of 2.1 mm in area V1 to 9.0 mm in area 7a. Average interpatch distance also increased from 0.61 mm in area V1 to 1.56 mm in area 7a. In contrast, patch size changed far less at higher order areas, from an average width of 230 micron(s) in area V1 to 310 micron(s) in area 7a. Analysis of synaptic bouton distribution along axons revealed that average interbouton distance remained constant at 6.4 micron(s) (median) in and out of the clusters and in the different cortical areas. Larger injections resulted in a marked increase in the number of labeled patches but only a minor increase in the spread of connections or in patch size. Thus, in line with the more global computational roles proposed for the higher order visual areas, the spread of intrinsic connections is increased with the hierarchy level. On the other hand, the clustered organization of the connections is preserved at higher order areas. These clusters may reflect the existence of cortical modules having blob-like dimensions throughout macaque monkey visual cortex.

Animals↗
PubMed2001

The contribution of sensory experience to the maturation of orientation selectivity in ferret visual cortex.

Sensory experience begins when neural circuits in the cerebral cortex are still immature; however, the contribution of experience to cortical maturation remains unclear. In the visual cortex, the selectivity of neurons for oriented stimuli at the time of eye opening is poor and increases dramatically after the onset of visual experience. Here we investigate whether visual experience has a significant role in the maturation of orientation selectivity and underlying cortical circuits using two forms of deprivation: dark rearing, which completely eliminates experience, and binocular lid suture, which alters the pattern of sensory driven activity. Orientation maps were present in dark-reared ferrets, but fully mature levels of tuning were never attained. In contrast, only rudimentary levels of orientation selectivity were observed in lid-sutured ferrets. Despite these differences, horizontal connections in both groups were less extensive and less clustered than normal, suggesting that long-range cortical processing is not essential for the expression of orientation selectivity, but may be needed for the full maturation of tuning. Thus, experience is beneficial or highly detrimental to cortical maturation, depending on the pattern of sensory driven activity.

Animals↗
PubMed1995

Relation between oscillatory activity and long-range synchronization in cat visual cortex.

Recent theoretical studies have suggested that oscillatory firing patterns with frequencies in the gamma band (30-70 Hz) may be instrumental for the establishment of synchrony among widely distributed neurons if synchrony is to be achieved by reciprocal connections. We have now investigated the relationship between synchrony and oscillations in cat visual cortex. Our results show that when synchronization of neuronal activity occurs over distances of > 2 mm in primary visual cortex, or occurs between the two hemispheres, it is almost always associated with oscillatory firing patterns, whereas synchronization over short distances occurs also in the absence of oscillations. Furthermore, our results indicate that short-range interactions affect both the firing rate of the respective neurons and the timing of their discharges, whereas only the latter is influenced by long-range interactions. These data support the hypothesis that oscillatory activity can contribute to the establishment of long-range synchrony in a network of reciprocally coupled neurons.

Animals↗
PubMed2004

Search for color 'center(s)' in macaque visual cortex.

It is often stated that color is selectively processed in cortical area V4, in both macaques and humans. However most recent data suggests that color is instead processed in region(s) antero-ventral to V4. Here we tested these two hypotheses in macaque visual cortex, where 'V4' was originally defined, and first described as color selective. Activity produced by equiluminant color-varying (versus luminance-varying) gratings was measured using double-label deoxyglucose in awake fixating macaques, in multiple areas of flattened visual cortex. Much of cortex was activated near-equally by both color- and luminance-varying stimuli. In remaining cortical regions, discrete color-biased columns were found in many cortical visual areas, whereas luminance-biased activity was found in only a few specific regions (V1 layer 4B and area MT). Consistent with a recent hypothesis, V4 was not uniquely specialized for color processing, but areas located antero-ventral to V4 (in/near TEO and anterior TE) showed more color-biased activity.

Animals↗
PubMed1996

Expression of neurotrophin-3 mRNA in the rat visual cortex and hippocampus is influenced by environmental conditions.

Environmental enrichment results in structural changes in the brain. Recent findings indicate involvement of neurotrophins in neuronal plasticity. This study examined the effect of environmental complexity on the levels of neurotrophin-3 (NT-3) mRNA in the rat visual cortex and hippocampus, studied by in situ hybridization. Rats housed in groups in a complex, stimulating environment had significantly higher levels of NT-3 mRNA in the visual cortex and the hippocampus than rats housed in individual cages without stimulating objects. These results indicate a possible role for NT-3 in synaptic plasticity.

Animals↗