[Responses of rabbit visual cortex neurons to non-visual stimuli and their complexes with light].
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Animals↗
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Postnatal development of interhemispheric connections of the cat visual cortex.
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Animals↗
A critical period for long-term potentiation in the developing rat visual cortex.
The in vitro rodent visual cortical slice preparation demonstrates a critical period for long-term potentiation (LTP). Current source density (CSD) analysis reveals peak potentiation of both supra-(layers II-III) and infragranular (layers V) layers of visual cortex during the second postnatal week following stimulation of the subadjacent white matter. By day 30 both the supra- and infragranular CSD sinks show only minimal potentiation. In adults there is no change in supragranular response but infragranular layers reveal 177% potentiation. Therefore, we conclude that rodent visual cortex displays a critical period for maximum plasticity of both supra and infra-granular layers. Supragranular visual cortex plasticity ends by day 30 whereas infragranular layers retain plastic qualities into adulthood.
Aging↗
Heterotopic and homotopic callosal connections in rat visual cortex.
Heterotopic and homotopic callosal projections of rat visual cortex are evaluated. Callosal termination zones in visual cortex are identified with a degeneration technique following complete section of the corpus callosum. The zones which receive callosal afferents are the lateral one-third of area 17, an anteroposterior strip in dorsal and in ventral areas 18a, and 4 patches in area 18b. Following a large injection of lectin-bound horseradish peroxidase (WGA-HRP) into visual cortex, many retrogradely labeled neurons are found in the medial two-thirds of area 17 which does not receive callosal afferents, as well as in the lateral, callosal-recipient zone. These data suggest that heterotopic callosal pathways exist in visual cortex. Injections of tritiated amino acids into restricted parts of visual cortex show the following heterotopic connections: lateral area 17 projects to dorsal area 18a; medial area 17 projects to lateral area 17 and dorsal area 18a; area 18a projects to lateral area 17 and anteromedial area 18b; area 18b projects to lateral area 17, dorsal area 18a, heterotopic sites in area 18b, and to area 29d . Heterotopic connections are generally less dense than homotopic ones. In addition, heterotopic connections are generally less dense than homotopic ones. In addition, heterotopic projections terminate in the supragranular layers. This contrasts with the homotopic afferents of areas 17 and 18a which have additional strong projections to layer V. The distribution of label through the depth of the cortex in some of the callosal recipient zones has been quantified. Injections of WGA-HRP restricted to areas 17, 18a or 18b corroborate the presence of each of the heterotopic connections described above. Heterotopic afferents originate mostly from layer V neurons, whereas homotopic afferents arise from neurons primarily in layers II-V. Like the afferents, the numbers of callosal projection cells in heterotopic regions are substantially less than that in homotopic sites. heterotopic callosal connections may be one factor responsible for binocular vision and also may provide the basis for large, nonoriented receptive fields of units in layer V of rodent visual cortex.
Animals↗
Visual experience and deprivation bidirectionally modify the composition and function of NMDA receptors in visual cortex.
The receptive fields of visual cortical neurons are bidirectionally modified by sensory deprivation and experience, but the synaptic basis for these changes is unknown. Here we demonstrate bidirectional, experience-dependent regulation of the composition and function of synaptic NMDA receptors (NMDARs) in visual cortex layer 2/3 pyramidal cells of young rats. Visual experience decreases the proportion of NR2B-only receptors, shortens the duration of NMDAR-mediated synaptic currents, and reduces summation of synaptic NMDAR currents during bursts of high-frequency stimulation. Visual deprivation exerts an opposite effect. Although the effects of experience and deprivation are reversible, the rates of synaptic modification vary. Experience can induce a detectable change in synaptic transmission within hours, while deprivation-induced changes take days. We suggest that experience-dependent changes in NMDAR composition and function regulate the development of receptive field organization in visual cortex.
Animals↗
Influence of association cortex on penicillin discharges in the primary visual cortex.
The influence of visual association cortex on the pattern of neuronal activity in the lateral geniculate body, during penicillin-induced discharges in visula cortex, was studied. In a majority of those geniculate cells which exhibited a pre-penicillin discharge increment in activity, that increment was abolished or significantly reduced during focal hypothermia of the visual association cortex. The frequency of penicillin discharges in visual cortex was, in most cases, reduced during cooling of association cortex. These findings are briefly discussed in terms of corticothalamic interactions.
Animals↗
Postnatal dendritic development in the rabbit visual cortex.
Golgi preparations of rabbit visual cortex aged 1-25 days, as well as similar tissues from adults, were examined for the growth of the dendritic arbor, and in particular the development of dendritic spines. The layer 5 pyramidal neurons and layer 4 stellate neuron were chosen as representatives of larger classes of neurons in the visual cortex. It was determined that the growth of the dendritic arbor, determined by counts of total number of dendritic and total dendritic length, is quite similar for pyramidal and stellate neurons. Dendritic spine development, however, is more rapid in pyramidal neurons than in stellate. This disparity in the rate of dendritic spine development is discussed in the light of physiologic studies on the development of receptive field properties in the rabbit visual cortex.
Age Factors↗
Human primary visual cortex and lateral geniculate nucleus activation during visual imagery.
The functional magnetic resonance (fMRI) technique can be robustly used to map functional activation of the visual pathway including the primary visual cortex (V1), the lateral geniculate nucleus (LGN), and other nuclei of humans during visual perception stimulation. One of the major controversies in visual neuroscience is whether lower-order visual areas involve the visual imagery process. This issue was examined using fMRI at high magnetic field. It was demonstrated for the first time that the LGN was activated during visual imagery process in the human brain together with V1 and other activation. There was a tight coupling of the activation between V1 and the LGN during visual imagery.
Adult↗
Brain SPECT evaluation of the visual cortex in amblyopia.
UNLABELLED: In amblyopia, the number of visual cortical neurons are reduced and abnormal or absent sensitivity to retinal light stimulation of the amblyopic eye is demonstrated. Ten amblyopic patients were studied to evaluate the response of the visual cortex to visual stimulation. METHODS: All patients with unilateral amblyopia received 500-550 MBq 99mTc-HMPAO during visual stimulation. Strobe light flashing was used as the stimulus for five patients and a checkerboard pattern reversal was used in the other five patients, closing one eye. For both groups a 2-Hz frequency was used. One week later, the same procedure was repeated with the opposite eye closed. SPECT images were reconstructed with prefiltering techniques and sliced along the orbitomeatal line. RESULTS: For all patients, the amblyopic eye demonstrated less radioactivity in the visual cortex than in the normal eye. The mean cerebral-to-cerebellar ratios were 0.95 +/- 0.05 and 1.09 +/- 0.07 for amblyopic and normal eyes, respectively (p < 0.0001). CONCLUSION: Visual cortex response of the amblyopic eye to light stimulation was severely reduced when compared to the normal eye.
Adolescent↗
[The afferent and efferent connections of field 7 in the parietal cortex with field 18a of the visual cortex in the rat].
By means of administration of horseradish peroxidase to the field 7 in the parietal cortex of rat, studies have been made on antero- and retrograde axonal transport of the enzyme to reveal bilateral connections of this field with the field 18a of the visual cortex. It was shown that the field 18a is a source of afferent fibers which reach the field 7, more evident projections being observed at the ipsilateral side. Labeled neurones in the field 18a were found mostly in layer III, and to a lesser extent-in layer V. Single labeled neurones were also found in layers II and VI. Labeled terminals of efferent fibers originating from cells in the field 7 were located in layers I, II-III and V of the field 18a. Maximum density of the terminals was registered in layer II-III, less one in layer V, and very negligible-in layer I. It should be noted that labeled terminals were revealed in those cortical regions, which contained labeled neurones which gave origin to afferent fibers to the field 7.
Afferent Pathways↗
Modification of single neurons in the kitten's visual cortex after brief periods of monocular visual experience.
Kittens were deprived of form vision by suturing the lids of both eyes, except for a brief period (1, 6 or 20 hours) on the 29th day when the right eye was opened. 6 space and 20 hours of monocular vision produced a distinct shift in the ocular dominance of visual cortical neurons towards the experienced eye, and an increase in the proportion of cells with obvious orientation selectivity. These modifications in the visual cortex were enhanced by a period of "consolidation": they were somewhat less obvious if recordings were taken immediately after the exposure but were complete 2 days later. Although remarkably little visual experience was needed for these changes, the results contrast with the effects of rearing in an environment of vertical stripes, where only 1 hour of exposure produces much more striking effects. A normal visual environment may have a less powerful organizing influence on cortical neurons than such an environment containing only one orientation.
Animals↗
Estimating linear cortical magnification in human primary visual cortex via dynamic programming.
Human primary visual cortex is organized retinotopically, with adjacent locations in cortex representing adjacent locations on the retina. The spatial sampling in cortex is highly nonuniform: the amount of cortex devoted to a unit area of retina decreases with increasing retinal eccentricity. This sampling property can be quantified by the linear cortical magnification factor, which is expressed in terms of millimeters of cortex per degree of visual angle. In this paper, we present a new method using dynamic programming and fMRI retinotopic eccentricity mapping to estimate the linear cortical magnification factor in human primary visual cortex (V1). We localized cortical activity while subjects viewed each of seven stationary contrast- reversing radial checkerboard rings of equal thickness that tiled the visual field from 1.62 to 12.96 degrees of eccentricity. Imaging data from all epochs of each ring were contrasted with data from fixation epochs on a subject-by-subject basis. The resulting t statistic maps were then superimposed on a local coordinate system constructed from the gray/white matter boundary surface of each individual subject's occipital lobe, separately for each ring. Smoothed maps of functional activity on the cortical surface were constructed using orthonormal bases of the Laplace-Beltrami operator that incorporate the geometry of the cortical surface. This allowed us to stably track the ridge of maximum activation due to each ring via dynamic programming optimization over all possible paths on the cortical surface. We estimated the linear cortical magnification factor by calculating geodesic distances between activation ridges on the cortical surface in a population of five normal subjects. The reliability of these estimates was assessed by comparing results based on data from one quadrant to those based on data from the full hemifield along with a split-half reliability analysis.
Brain Mapping↗
Age-related changes in the visual cortex: a review.
The main age related changes in visual cortex are reviewed. The visual cortex (occipital cortex, areas 17-19) undergoes a variety of anatomical, biochemical and functional changes with aging. From a morphological point of view the visual cortex loses nerve cells mainly in the last period of life. From a biochemical point of view cholinergic, serotonergic, and GABAergic neurotransmissions seem to be the most remarkably affected. In terms of functional correlates, a decline of several activities of the visual cortex has been documented in the elderly. Due to the importance of visual cortex in the realization of visual function, the influence of aging on this cerebrocortical area requires a more detailed analysis.
Journal Article↗
System-wide repercussions and adaptive plasticity: the sequelae of immature visual cortex damage.
Damage of primary visual cortex in mature mammals severely disrupts vision by disconnecting much of the cognitive processing machinery of extrastriate cortex from its source of visual signals in the retina. However, equivalent lesions incurred early in postnatal life unmask a substantial latent flexibility of the brain to minimize the disruption by specific and ordered pathway expansions that bypass the lesion. The expansions shape pathways from retina through thalamus to extrastriate cortex and onto the midbrain into new, useful forms that are retained into adult life. These useful modifications support relatively normal signal processing in a variety of structures and the sparing of certain visually guided behaviors, such as aspects of complex-pattern vision and localizing objects introduced into the visual field. Thus, both the brain and the individual are optimized, in the absence of primary visual cortex, by adaptations for useful interactions with the environment. So far, the repercussions of early visual cortex lesions have been most thoroughly documented in cats, although it is likely on the basis of known repercussions and similarity of visual system organization and developmental sequence, that broadly equivalent repercussions and adaptations occur in monkeys and humans following early lesions of primary visual cortex. The knowledge gained has implications for devising therapeutic strategies to attenuate defects in vision induced by cortical lesions.
Journal Article↗
Dysfunction of visual cortex contributes to disturbed processing of visual information in Alzheimer's disease.
The relation between visual impairment and regional cerebral metabolic rate of glucose (rCMRGl) was studied in 31 patients with probable Alzheimer's disease (AD) by using positron emission tomography with 18F-2-fluoro-2-deoxy-D-glucose. To exclude any precortical cause of visual dysfunction only patients were included who had amplitudes and latencies of visually evoked potentials (flash and pattern reversal) within the normal range. Visual information processing was evaluated psychometrically by a fragmented picture test (FPT), which is a combined perception (identification score) and memory (reidentification score) task and refers to Gollin's incomplete pictures. The identification and reidentification scores were significantly worse than in normals. Reductions of the rCMRGl in the primary visual fields, and in the secondary visual fields were found. A significant partial correlation with adjustment for age between the reidentification score of the FPT and the rCMRGl of the secondary visual fields (r = -.39, p < .05) in AD patients was found, indicating involvement of the secondary visual cortex in the pathological changes in AD.
Aged↗
Pathological and experimentally induced blindness induces auditory activity in the cat primary visual cortex.
Early blindness in humans and experimental visual deprivation in animal models are known to induce compensatory somatosensory and/or auditory activation of the visual cortex. An abnormal hydrocephalic cat with extreme malformation of the visual system, born in our breeding colony, rendered a good model system for investigating possible cross-modal compensation in such a pathological case. For comparison, we used normal and neonatally enucleated cats. When introduced to a novel environment, the abnormal cat behaved as if it was completely blind, yet it responded normally to auditory stimuli. As anticipated, single cells in the visual cortex of normal cats responded to visual, but not to auditory stimuli. In the visual cortex of enucleated cats, flashes of light did not elicit field-evoked potentials or single-unit responses. However, several cells did respond to various auditory stimuli. In the remnant visual cortex of the abnormal cat, auditory stimuli evoked field potentials and single-cell responses. Unexpectedly, however, unlike the enucleated cats, in the abnormal cat, flashes of light also elicited field-evoked potentials. Judging by its behavior, it is very likely that this deformed cat had completely lost its ability to perceive images, but had probably retained some sensitivity to light.
Abnormalities, Multiple↗
Ipsilateral visual field represented in the cat's visual cortex.
Responses to visual stimulation were recorded in cells from the border region between cortical areas 17 and 18 in anaesthetized cats. There was found to be a band of ipsilateral representation, the cells of which had rather large receptive fields centered up to 8-12 degrees into the ipsilateral hemifield. The edges of the receptive fields extended 2-3 degrees still further ipsilaterally. The cortical region involved is very small, however, stretching 0.2-1.0 mm mediolaterally, and possibly restricted largely to layers II and III. Hence, in cats, as in sheep and hamsters, there is a band about 20 degrees wide of visual field along the naso-temporal division that is represented in the visual cortices of both hemispheres.
Animals↗
Transient attention enhances perceptual performance and FMRI response in human visual cortex.
When a visual stimulus suddenly appears, it captures attention, producing a transient improvement of performance on basic visual tasks. We investigate the effect of transient attention on stimulus representations in early visual areas using rapid event-related fMRI. Participants discriminated the orientation of one of two gratings preceded or followed by a nonpredictive peripheral cue. Compared to control conditions, precueing the target location improved performance and produced a larger fMRI response in corresponding retinotopic areas. This enhancement progressively increased from striate to extrastriate areas. Control conditions indicated that the enhanced fMRI response was not due to sensory summation of cue and target signals. Thus, an uninformative precue increases both perceptual performance and the concomitant stimulus-evoked activity in early visual areas. These results provide evidence regarding the retinotopically specific neural correlate for the effects of transient attention on early vision.
Adult↗