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At least 307 records · Page 17Linked to original sources
PubMed1989

Functional consequences of modification of callosal connections by perinatal enucleation in rat visual cortex.

The effects of neonatal monocular enucleation (right eye) on the callosal connections in the rat visual cortex were studied by physiological and morphological methods. Evoked activity was recorded in the left hemisphere, i.e. contralaterally to the enucleated eye. After enucleation, trans-callosally evoked responses were recorded in a widened stripe of the lateral visual cortex. Compared with the controls, the responsive area was expanded laterally and medially, i.e. into the lateral part of the primary visual area and within the secondary visual cortex (lateral part). Within about 0.5 mm of the expansion, the responses did not differ from those recorded in areas with "normal" callosal connections. Morphological evidence is presented suggesting that this expansion of evoked responses with high amplitudes and short latencies corresponds to an extension of callosal connections with a high density of axon terminals in layers two and three. Further medially within the primary visual cortex, callosally evoked responses with low amplitudes and longer latencies were recorded. The main types of unit responses and characteristic interactions between visually and callosally evoked responses are shown and discussed. These results suggest that following neonatal enucleation (1) the callosal connections expand and form functional synapses in the lateral part of the visual cortex, (2) these connections can activate cortical neurons either directly or by mediation of associational connections between the lateral secondary and primary visual cortex areas and (3) callosal connections can interact with visually evoked potentials and unit responses.

Animals↗
PubMed1989

Effect of early visual pattern deprivation on development and laminar distribution of cholinergic markers in rat visual cortex.

In order to evaluate the role of cholinergic cortical mechanisms in the shaping of visual cortical plasticity in more detail the present paper summarizes recent studies on the laminar distribution of muscarinic acetylcholine receptors, choline acetyltransferase, and sodium-dependent high-affinity choline uptake sites during postnatal ontogenesis of the visual cortex of monocularly derived rats using autoradiographic techniques as well as quantitative biochemical methods after separating the different cortical layers by a cryocut technique. The data are correlated to the laminar distribution of cholinergic fibers within the visual cortex as studied by the immunohistochemical visualization of choline acetyltransferase. The laminar distribution of cholinergic receptor binding in the visual cortex changes during ontogenesis. In adult rats, the highest muscarinic acetylcholine receptor density is found in layer I. The activity of the choline acetyltransferase is rather uniformly distributed in all cortical layers. Adult activity values are reached at the age of 25 days. In adult rats the enzyme activity is highest in layer V. In all visual cortical layers the highest 3H-hemicholinium-3 binding to choline uptake sites during the postnatal period studied is already detectable at the age of 10 days, then binding decreases sharply until day 25 at which age it nearly equals the value found in the adult brain. Binding sites exhibit highest density in layers I and IV of the adult rat visual cortex. Monocular deprivation resulted in significant changes in all three parameters studied with different cortical laminae preferentially affected. The data suggest that the normal laminar development of the modulatory function of cholinergic transmission in the rat visual cortex depends on the presence of physiological light stimulation.

Aging↗
PubMed1995

Cytochrome-oxidase blobs in cat primary visual cortex.

Cytochrome-oxidase blobs are central to two of the most influential ideas in contemporary visual neuroscience--cortical modularity and parallel processing pathways. In particular, the regular 2D array of cytochrome-oxidase-rich blobs in primate visual cortex is arguably the most compelling evidence for cortical modularity and has been hypothesized to mark a separate processing stream through the visual cortex. Although previously a variety of mammals have been studied, blobs have only been demonstrated in the visual cortex of primates, which has led to the conclusion that blobs represent a primate-specific feature of visual cortical organization. Here we demonstrate the presence of cytochrome-oxidase blobs in a nonprimate species. Throughout the full tangential extent of layers II-III in cat visual cortex the cytochrome-oxidase staining pattern is distinctly patchy, with the darkly stained blobs forming a regular 2D array. In addition, the blobs in cat visual cortex are functionally related to the underlying ocular dominance columns. The presence of cytochrome-oxidase blobs in the cat clearly demonstrates that they no longer can be considered a primate-specific feature of visual cortical organization.

Animals↗
PubMed1997

Retinotopic organization in human visual cortex and the spatial precision of functional MRI.

A method of using functional magnetic resonance imaging (fMRI) to measure retinotopic organization within human cortex is described. The method is based on a visual stimulus that creates a traveling wave of neural activity within retinotopically organized visual areas. We measured the fMRI signal caused by this stimulus in visual cortex and represented the results on images of the flattened cortical sheet. We used the method to locate visual areas and to evaluate the spatial precision of fMRI. Specifically, we: (i) identified the borders between several retinotopically organized visual areas in the posterior occipital lobe; (ii) measured the function relating cortical position to visual field eccentricity within area V1; (iii) localized activity to within 1.1 mm of visual cortex; and (iv) estimated the spatial resolution of the fMRI signal and found that signal amplitude falls to 60% at a spatial frequency of 1 cycle per 9 mm of visual cortex. This spatial resolution is consistent with a linespread whose full width at half maximum spreads across 3.5 mm of visual cortex.

Brain Mapping↗
PubMed2001

Mapping visual cortex in monkeys and humans using surface-based atlases.

We have used surface-based atlases of the cerebral cortex to analyze the functional organization of visual cortex in humans and macaque monkeys. The macaque atlas contains multiple partitioning schemes for visual cortex, including a probabilistic atlas of visual areas derived from a recent architectonic study, plus summary schemes that reflect a combination of physiological and anatomical evidence. The human atlas includes a probabilistic map of eight topographically organized visual areas recently mapped using functional MRI. To facilitate comparisons between species, we used surface-based warping to bring functional and geographic landmarks on the macaque map into register with corresponding landmarks on the human map. The results suggest that extrastriate visual cortex outside the known topographically organized areas is dramatically expanded in human compared to macaque cortex, particularly in the parietal lobe.

Animals↗
PubMedDate not supplied

Cysteamine-induced depletion of somatostatinergic systems alters potentials evoked from the rat visual cortex.

This study was performed in order to establish whether selective depletion of somatostatin (SS) in the rat primary visual cortex obtained by cysteamine (CSH) administration results in changes of visual evoked potentials (VEPs). VEPs in response to a contrast reversal (0.5 Hz) of an optimal sinusoidal grating (0.1 cycle/deg, contrast 90%, mean luminance 15 cd/m2) were recorded from different layers of the binocular portion of the primary visual cortex of anesthetized rats with saline injection as well as before and after CSH treatment (90 mg/kg, s.c.). VEPs of CSH treated rats, as compared to those obtained either in saline-injected animals or before drug administration, are reduced in amplitude at intermediate cortical layers whereas they are increased at deeper layers. VEP changes depend on CSH treatment and not on the extended anesthesia since no alterations in the VEP profile can be observed in saline-injected animals maintained in the same experimental condition. Forty-eight hours following CSH treatment, the VEP profile is comparable to that of saline-injected animals. Immunocytochemical analysis of the visual cortex of rats recorded 7 h after CSH treatment shows a 20-30% reduction in the number of SS-containing cortical cells. The highest reduction can be observed in cortical layer 5 although a significant decrease is also found in layers 2-3. In contrast, the pattern of SS immunoreactivity of the visual cortex of rats recorded 48 h after CSH administration is similar to that obtained in control conditions. These results indicate that a selective toxin for somatostatinergic systems induces a transient decrease of SS-containing cell number in selected cortical layers. Accordingly, CSH can serve as a useful pharmacological tool for the study of somatostatinergic function in the rat visual cortex since changes in VEPs can be related to a reduction of somatostatinergic neurons associated to CSH treatment. In particular, the present results suggest that one of the possible actions of somatostatinergic neurons in the rat visual cortex is to modulate the excitatory-inhibitory balance.

Animals↗
PubMed1984

Direct connections of rat visual cortex with sensory, motor, and association cortices.

Each division of rat visual cortex, areas 17, 18a, and 18b, has connections with sensory, motor, and association cortices. These corticocortical connections were sampled using anterograde autoradiographic and retrograde horseradish peroxidase labeling techniques. Area 17 is connected via reciprocal pathways with each division of visual cortex, the posterior one-third of motor area 8, association area 7, and posteroventral area 36 of temporal cortex. It also receives projections from perirhinal areas 13 and 35. Area 18a has reciprocal connections with areas 17 and 18b, a patch in posterior somatosensory area 3, and dorsal auditory area 41. Like area 17, area 18a receives afferents from and projects to the posterior one-third of motor area 8. The connections of area 18a with association cortices are extensive; these regions include parietal areas 7, 39, 40, and 14, posteroventral and dorsal area 36, and perirhinal cortex. Area 18b is connected with areas 17 and 18a, a patch in medial area 3, and dorsal area 41. There are reciprocal projections between area 18b and posterior area 8. As for association cortex, area 18b projects to frontal area 11, area 7, posteroventral and dorsal area 36, and perirhinal cortex. In addition, area 18b receives input from and projects efferents to the dorsal claustrum. Most of the interconnections among areas 17, 18a, and 18b originate from neurons in layers II, III, and V and end in terminal fields in layers I-III and V. In contrast, projections of other sensory, motor, and association cortices to visual cortex originate mainly from neurons in layer V and to a lesser extent from layer II. The reciprocal pathways from visual cortex terminate predominantly in the supragranular layers. In conclusion, these corticocortical pathways provide the basis for cortical visuosensory and visuomotor integration that may aid the rat in the coordination of visually guided behaviors.

Animals↗
PubMed1985

Reinstatement of binocular depth perception by amphetamine and visual experience after visual cortex ablation.

In adult cats with bilateral visual cortex ablation the complete deficit in binocular depth perception, as measured on a visual cliff, was reversed by 4 doses of amphetamine. The amphetamine-induced recovery endured after the amphetamine treatment was discontinued. This enduring recovery of function was not obtained if the animals were housed in the dark during drug intoxication. Therefore, both amphetamine intoxication and visual experience are simultaneously required for recovery of binocular depth perception after visual cortex ablation.

Animals↗
PubMed2001

Suppression of perception in migraine: evidence for reduced inhibition in the visual cortex.

BACKGROUND: Results from transcranial magnetic stimulation (TMS) studies of visual cortex have confirmed visual cortical hyperexcitability in patients with migraine. It has been speculated that this may be due to deficient intracortical inhibitory tone. However, the TMS induction of phosphenes relies on the reporting of a subjective experience, and may thus be subject to bias. METHODS: Seven migraineurs with visual aura and seven sex- and age-matched controls were studied. Fifty-four different three-letter combinations were briefly displayed and followed by a magnetic pulse at 40, 70, 100, 130, 160, and 190 msec. Subjects were required to report as many letters as they thought they had recognized. RESULTS: In the migraine group, the mean proportion of correctly identified letters was significantly higher at 100 msec, as was the proportion of trials with two or three letters correctly reported. The time window in which perceptual suppression could be introduced was narrower in migraineurs compared to controls. CONCLUSION: These findings suggest that inhibitory systems are activated to a lesser extent by TMS pulses in patients. This observation is in agreement with the hypothesized deficiency of intracortical inhibition of the visual cortex, at least in migraineurs with aura.

Adolescent↗
PubMed1986

Cortical activity blockade prevents ocular dominance plasticity in the kitten visual cortex.

Recordings from single units in kitten primary visual cortex show that a reversible blockade of the discharge activities of cortical neurons and geniculocortical afferent terminals by intracortical infusion of the sodium channel blocker tetrodotoxin (TTX) completely prevented the ocular dominance shift that would normally be seen after monocular deprivation. The blockade of cortical plasticity, like the blockade of discharge activity, was reversible, and plasticity was restored following recovery from the effects of TTX. These results extend previous work suggesting involvement of electrical activity at the level of the cortex in the phenomenon of cortical plasticity by demonstrating an absolute requirement for discharge activities in the primary visual cortex.

Action Potentials↗
PubMed1992

Effect of dark rearing on the volume of visual cortex (areas 17 and 18) and number of visual cortical cells in young kittens.

The surface area, total volume, and total number of neurons of areas 17 and 18 in one hemisphere of dark-reared (DR), dark-reared and light-exposed (DRL), and normally reared (NR) kittens were studied at the age of 6 weeks. The thickness of the visual cortex was lower by 13% and 11% (area 17) and by 17% and 16% (area 18) in DR and DRL groups, respectively, when compared with similar cortical areas in NR kittens. The surface area values of area 17 were nearly the same in DR and DRL kittens, both being, however, 37% smaller than in NR animals. The surface area of area 18 was significantly smaller than that of area 17 in each group, and was also lower in DR (by 27%) and DRL (by 21%) groups when compared with the NR group. As a consequence of dark rearing, the numerical density of cortical neurons in area 17 amounted to about double of the value observed in normally reared kittens and was also significantly higher in area 18. The numerical density of nerve cells of DRL kittens fell between the DR and NR groups. The total cortical volume of area 17 was similar in DR and DRL groups but it was by 46% (DR) and by 44% (DRL) smaller than in NR kittens. In each experimental group, the total volume of area 18 was significantly smaller than that of area 17. The cortical volume of area 18 was also smaller than in the NR group by 39% and 34% in DR and DRL groups, respectively. In DR and NR kittens, the total numbers of neurons in areas 17 (DR = 26.4 million, NR = 25.7 million) and 18 (DR = 8.5 million, NR = 9.0 million) were essentially similar. In the DRL groups a significantly smaller number of cortical neurons was found both in area 17 (21.5 million) and in area 18 (6.8 million). It is concluded that, in spite of considerable differences in the cortical thickness, surface area, numerical density, and total cortical volume, the absolute numbers of neurons in area 17 and 18 of visually deprived (DR) and NR kittens do not differ at 6 weeks of age. The main deficit in cortical organization following dark rearing, therefore, appears to be confined mainly to the neuropil, as a result of an underdevelopment of neuronal processes and of depressed synaptic organization.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1997

Blockade of serotonin-2C receptors by mesulergine reduces ocular dominance plasticity in kitten visual cortex.

We have investigated the role of serotonin-2C (5-HT2C) receptors in modulation of ocular dominance plasticity in kitten visual cortex. A small quantity of the 5-HT2C receptor blocker, mesulergine, was infused into the visual cortex of one hemisphere of 5- to 7-week-old kittens using osmotic minipumps, while the control hemisphere received vehicle solution. At the same time, one eyelid of the experimental animals was sutured shut. The ocular dominance distributions in the visual cortex (area 17) were assessed using extracellular recording methods after 1 week of combined mesulergine infusion and monocular deprivation. We found that the majority of the neurons remained binocularly responsive in the mesulergine-treated hemisphere, while most of the neurons recorded were either unresponsive or only weakly responsive to the deprived eye in the control hemisphere. Local infusion of mesulergine into the kitten visual cortex thus reduced the shift of ocular dominance that normally occurs in animals of these ages following monocular deprivation. The blocking effect seems to be distance-dependent and therefore dose-dependent: the farther away the recording sites were from the injection site, the fewer binocularly responsive cells were found. These results are relevant to previous findings indicating transient overexpression of 5-HT2C receptor in visual cortex of kittens at these ages. The data suggest that the 5-HT2C receptor system may be involved in the formation and modification of ocular dominance columns in the developing visual cortex.

Animals↗
PubMed1986

Why spatial frequency processing in the visual cortex?

The nature of redundancy in visual images is discussed and the methods of removing statistical redundancies are considered. It is demonstrated that local spatial frequency analysis is a highly successful method and it is thus argued that this determines the choice of representation of the visual image by the simple cells of the visual cortex.

Humans↗
PubMed1999

Monocular deprivation decreases brain-derived neurotrophic factor immunoreactivity in the rat visual cortex.

Neurotrophins play a crucial role in the development and activity-dependent plasticity of the visual cortex [Berardi N. et al. (1994) Proc. natn. Acad. Sci. U.S.A. 91, 684-688; Bonhoeffer T. (1996) Curr. Opin. Neurobiol. 6, 119-126; Cellerino A. and Maffei L. (1996) Prog. Neurobiol. 49, 53-71; Domenici L. et al. (1994) NeuroReport 5, 2041-2044; Galuske R. A. W. et al (1996) Eur. J. Neurosci. 8, 1554-1559; Katz L. C. and Shatz C. J. (1996) Science 274, 1133-1138; Maffei L. et al. (1992) J. Neurosci. 12, 4651-4662; Pizzorusso T. and Maffei L. (1996) Curr. Opin. Neurol. 9, 122-125; Thoenen H. (1995) Science 270, 593-598]. As a possible mechanism of action, it has been postulated that the activity-dependent expression of neurotrophins by cortical cells could regulate synapse stabilization during the first period of postnatal life (critical period). Indeed, brain-derived neurotrophic factor messenger RNA expression in the visual cortex is regulated by neuronal activity as well as during development [Castrén E. et al. (1992) Proc. natn. Acad. Sci. U.S.A. 89, 9444-9448]. Moreover, we showed that monocular deprivation decreases brain-derived neurotrophic factor messenger RNA levels in the visual cortex receiving input from the deprived eye [Bozzi Y. et al. (1995) Neuroscience 69, 1133-1144]. What is missing, however, is the demonstration that brain-derived neurotrophic factor protein expression follows that of brain-derived neurotrophic factor messenger RNA. The aim of the present study is to fill this important gap in order to support the hypothesis that brain-derived neurotrophic factor is fundamental in the plasticity of the visual cortex. We found that brain-derived neurotrophic factor immunoreactivity peaks during the critical period and that it is preferentially localized in layers II-III and V-VI. We also demonstrated that monocular deprivation determines a decrease of brain-derived neurotrophic factor immunoreactivity exclusively in the visual cortex contralateral to the deprived eye. Our results support the proposed role for brain-derived neurotrophic factor in the development and activity-dependent plasticity of the visual cortex [Cabelli R. J. et al. (1995) Science 267, 1662-1666].

Aging↗
PubMed1994

Summation and division by neurons in primate visual cortex.

Recordings from monkey primary visual cortex (V1) were used to test a model for the visually driven responses of simple cells. According to the model, simple cells compute a linear sum of the responses of lateral geniculate nucleus (LGN) neurons. In addition, each simple cell's linear response is divided by the pooled activity of a large number of other simple cells. The cell membrane performs both operations; synaptic currents are summed and then divided by the total membrane conductance. Current and conductance are decoupled (by a complementary arrangement of excitation and inhibition) so that current depends only on the LGN inputs and conductance depends only on the cortical inputs. Closed form expressions were derived for fitting and interpreting physiological data. The model accurately predicted responses to drifting grating stimuli of various contrasts, orientations, and spatiotemporal frequencies.

Animals↗
PubMed1975

Laminar thermocoagulation of the visual cortex in the rat. II. Visual pattern discrimination.

Hooded rats were trained on a series of four visual discrimination tasks in a Y-maze, and subjected to a variety of posterior cortical lesions. In 17 animals this consisted of an extensive aspiration lesion contralateral to a more superficial lesion made by laminar thermocoagulation and centered over the striate area. After operation the animals were tested on the same problem series. The behavioral deficit in this group varied with the extent and depth of the thermal lesion, and six animals with very superficial thermal lesions displayed an isolated difficulty in solving an encircled triangle problem. This deficit seemed to be referable to widespread involvement of supragranular cortex, and specifically of layer I of area striata which receives an input from the nonspecific thalamocortical afferents. The possible influence of various interlaminar projections upon underlying vertically-oriented cell columns as a mechanism for the mediation of 'selective attention' was discussed.

Animals↗
PubMed1995

Interocular transfer of receptive field expansion in cat visual cortex.

Receptive fields in primary visual cortex have been shown to be capable of rapid expansion and contraction when exposed to an artificial scotoma, a masked segment of the visual field. To distinguish cortical from thalamic contributions to receptive field mutability, we tested interocular transfer of the effect in binocular cortical receptive fields, presenting the conditioning stimulus to the field in one eye and measuring size changes in the receptive field of the other eye. The expansion of the receptive fields in the non-conditioned eye was comparable to that in the conditioned eye. This result suggests that the expansion is due to mechanisms intrinsic to the cortex.

Animals↗