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PubMed2004

The role of spared calcarine cortex and lateral occipital cortex in the responses of human hemianopes to visual motion.

Some patients, who are rendered perimetrically blind in one hemifield by cortical lesions, nevertheless exhibit residual visual capacities within their field defects. The neural mechanism that mediates the residual visual responses has remained the topic of considerable debate. One explanation posits the subcortical visual pathways that bypass the primary visual cortex and innervate the extrastriate visual areas as the substrate that underlies the residual vision. The other explanation is that small islands of the primary visual cortex remain intact and provide the signals for residual vision. We have performed behavioral and functional magnetic resonance imaging experiments to investigate the validity of the two explanations of residual vision. Our behavioral experiments indicated that of the seven hemianopes tested, two had the ability to discriminate the direction of a drifting grating. This residual visual response was shown with fMRI to be the result of spared islands of calcarine cortical activity in one of the hemianopes, whereas only lateral occipital activity was documented in the other patient. These results indicate that the underlying neural correlates of residual vision can vary between patients. Moreover, our study emphasizes the necessity of ruling out the presence of islands of preserved function and primary visual cortex before assigning residual visual capacities to the properties of visual pathways that bypass the primary visual cortex.

Adult↗
PubMed1993

Development of blobs in the visual cortex of macaques.

We have examined the area of the primary visual cortex and the number and size of blobs within it in 10 neonatal and 11 adult rhesus monkeys. The average area of the primary visual cortex (V1) increases from 919 mm2 in newborns to 1,069 mm2 in adult animals (16%). The number of blobs decreases per unit area from an average of 5.2/mm2 at birth to 4.3/mm2 in maturity (18%). As a consequence, the number of blobs remains approximately the same during maturation, at about 4,800/hemisphere. These observations correct a preliminary report on a subset of the animals studied here (Purves and LaMantia: Proc Natl Acad Sci 87:5765, '90), in which it appeared that blob number might increase between birth and maturity. As in other regions of the developing postnatal brain, we found no net loss of modular circuitry.

Animals↗
PubMed1999

Gaze direction controls response gain in primary visual-cortex neurons.

To localize objects in space, the brain needs to combine information about the position of the stimulus on the retinae with information about the location of the eyes in their orbits. Interaction between these two types of information occurs in several cortical areas, but the role of the primary visual cortex (area V1) in this process has remained unclear. Here we show that, for half the cells recorded in area V1 of behaving monkeys, the classically described visual responses are strongly modulated by gaze direction. Specifically, we find that selectivity for horizontal retinal disparity-the difference in the position of a stimulus on each retina which relates to relative object distance-and for stimulus orientation may be present at a given gaze direction, but be absent or poorly expressed at another direction. Shifts in preferred disparity also occurred in several neurons. These neural changes were most often present at the beginning of the visual response, suggesting a feedforward gain control by eye position signals. Cortical neural processes for encoding information about the three-dimensional position of a stimulus in space therefore start as early as area V1.

Animals↗
PubMed2004

Thinning of the cerebral cortex in aging.

The thickness of the cerebral cortex was measured in 106 non-demented participants ranging in age from 18 to 93 years. For each participant, multiple acquisitions of structural T1-weighted magnetic resonance imaging (MRI) scans were averaged to yield high-resolution, high-contrast data sets. Cortical thickness was estimated as the distance between the gray/white boundary and the outer cortical surface, resulting in a continuous estimate across the cortical mantle. Global thinning was apparent by middle age. Men and women showed a similar degree of global thinning, and did not differ in mean thickness in the younger or older groups. Age-associated differences were widespread but demonstrated a patchwork of regional atrophy and sparing. Examination of subsets of the data from independent samples produced highly similar age-associated patterns of atrophy, suggesting that the specific anatomic patterns within the maps were reliable. Certain results, including prominent atrophy of prefrontal cortex and relative sparing of temporal and parahippocampal cortex, converged with previous findings. Other results were unexpected, such as the finding of prominent atrophy in frontal cortex near primary motor cortex and calcarine cortex near primary visual cortex. These findings demonstrate that cortical thinning occurs by middle age and spans widespread cortical regions that include primary as well as association cortex.

Adolescent↗
PubMed1995

Receptive field expansion in adult visual cortex is linked to dynamic changes in strength of cortical connections.

1. Receptive field (RF) sizes of neurons in adult primary visual cortex are dynamic, expanding and contracting in response to alternate stimulation outside and within the RF over periods ranging from seconds to minutes. The substrate for this dynamic expansion was shown to lie in cortex, as opposed to subcortical parts of the visual pathway. The present study was designed to examine changes in cortical connection strengths that could underlie this observed plasticity by measuring the changes in cross-correlation histograms between pairs of primary visual cortex neurons that are induced to dynamically change their RF sizes. 2. Visually driven neural activity was recorded from single units in the superficial layers of primary visual cortex in adult cats, with two independent electrodes separated by 0.1-5 mm at their tips, and cross-correlated on-line. The neurons were then conditioned by stimulation with an "artificial scotoma," a field of flashing random dots filling the region of visual space around a blank rectangle enclosing the RFs of the recorded neurons. The neuronal RFs were tested for expansion and their visually driven output again cross-correlated. After this, the neurons were stimulated vigorously through their RF centers to induce the field to collapse, and the visually driven output from the collapsed RFs was again cross-correlated. Cross-correlograms obtained before and after conditioning, and after RF collapse, were normalized by their flanks to control for changes in peak size due solely to fluctuations in spike rate. 3. A total of 37 pairs of neurons that showed distinct cross-correlogram peaks, and whose RF borders were clearly discernible both before and after conditioning, were used in the final analysis. Of these neuron pairs, conditioning led to a clear expansion of RF boundaries in 28 pairs, whereas in 9 pairs the RFs did not expand. RFs that did expand showed no significant shifts in their orientation preference, orientation selectivity, or ocularity. 4. When the RFs of a pair of neurons expanded with conditioning, the area of the associated flank-normalized cross-correlogram peaks also increased (by a factor ranging from 0.84 up to 3.5). Correlograms returned to their preconditioning values when RFs collapsed.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2002

Brain representation of hemifield stimulation in poststroke visual field defects.

BACKGROUND AND PURPOSE: Plasticity in extended, parallel, or reciprocal operating networks is well recognized. Changes in neuronal activity after lesions to distinct localized structures, such as the primary visual cortex, are less well characterized. We investigated the cortical reorganization in patients with poststroke visual field defects using blood oxygen level-dependent functional MRI. METHODS: Brain activation was measured in 7 patients with a single occipital cortical lesion and partially recovered hemianopia and in 7 age-matched control subjects. Differences in activation between rest and visual hemifield stimulation were assessed with statistical parametric mapping (SPM'99). RESULTS: In normal subjects, significant activation was found in the contralateral primary visual cortex and bilaterally in the extrastriate cortex. During hemifield stimulation of the unaffected side of stroke patients, a similar pattern was found compared with that seen in control subjects. During stimulation of the hemianopic side, bilateral activation was seen within the extrastriate cortex, stronger in the ipsilateral hemisphere. The primary visual cortex was not significantly activated in either hemisphere during stimulation of the hemianopic side. CONCLUSIONS: Visual field defects after stroke are associated with bilateral activation of the extrastriate visual cortex. This pattern of activation indicates altered neuronal activity in the visual system. Further investigation is necessary to determine the relationship between functional reorganization and recovery of lost visual function after poststroke hemianopia.

Adult↗
PubMed1999

Asymmetric suppression outside the classical receptive field of the visual cortex.

Areas beyond the classical receptive field (CRF) can modulate responses of the majority of cells in the primary visual cortex of the cat (). Although general characteristics of this phenomenon have been reported previously, little is known about the detailed spatial organization of the surrounds. Previous work suggests that the surrounds may be uniform regions that encircle the CRF or may be limited to the "ends" of the CRF. We have examined the spatial organization of surrounds of single-cell receptive fields in the primary visual cortex of anesthetized, paralyzed cats. The CRF was stimulated with an optimal drifting grating, whereas the surround was probed with a second small grating patch placed at discrete locations around the CRF. For most cells that exhibit suppression, the surrounds are spatially asymmetric, such that the suppression originates from a localized region. We find a variety of suppressive zone locations, but there is a slight bias for suppression to occur at the end zones of the CRF. The spatial pattern of suppression is independent of the parameters of the suppressive stimulus used, although the effect is clearest with iso-oriented surround stimuli. A subset of cells exhibit axially symmetric or uniform surround fields. These results demonstrate that the surrounds are more specific than previously realized, and this specialization has implications for the processing of visual information in the primary visual cortex. One possibility is that these localized surrounds may provide a substrate for figure-ground segmentation of visual scenes.

Animals↗
PubMed2000

Comparison of the hemodynamic response to different visual stimuli in single-event and block stimulation fMRI experiments.

Experiments with three different types of basic visual stimulation were performed to compare cortical activation in single-event and block trials. Independent of the stimulation paradigm, the single-event presentation leads to highly consistent signal responses regarding both the activated cortical areas and the dynamics of the signal time course. In contrast, signal time courses during block paradigms depend on the stimulus applied and are a complex and nonlinear function of the single-event responses. Additionally, the initial dip during the first 2 seconds after stimulus onset is consistently observed. However, the small amplitude change (-0.1% to -0.3%) requires signal averaging to establish statistical significance of the effect. Furthermore, different patterns of activation were observed within the primary visual cortex. In an anterior part of the primary visual cortex, activation was only observed at the onset and at the cessation of stimulation involving luminance changes.

Adult↗
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↗
PubMedDate not supplied

The role of ipsilateral and contralateral inputs from primary cortex in responses of area 21a neurons in cats.

Neuronal responses in cat visual area 21a were analyzed when the primary visual cortex (areas 17 and 18) was deactivated by cooling. Ipsilateral and contralateral cortices were deactivated separately. Results established that (1) cooling the ipsilateral primary cortex diminished the activity of all area 21a cells and, in 30%, blocked responsiveness altogether, and (2) cooling the contralateral primary cortex initially increased activity in area 21a cells but, with further cooling, reduced it to below the original level although only 9% of cells ceased responding. These findings were then compared to earlier results in which bilateral deactivation of the primary cortex greatly reduced and, in most cases, blocked the activity of area 21a cells (Michalski et al., 1993). Despite the response attenuation following cooling of the primary visual cortex (either ipsilateral or contralateral), neurons of area 21a retained their original orientation specificity and sharpness of tuning (measured as the half-width at half-height of the orientation tuning curve). Direction selectivity also tended to remain unchanged. We concluded that for area 21a cells (1) the ipsilateral primary cortex provides the main excitatory input; (2) the contralateral primary cortex supplies a large inhibitory input; and (3) the nature of orientation specificity, sharpness of orientation tuning, and direction selectivity are largely unaffected by removal of the ipsilateral hemisphere excitatory input or the contralateral hemisphere inhibitory input.

Animals↗
PubMed1985

Localization of glutaminase-like and aspartate aminotransferase-like immunoreactivity in neurons of cerebral neocortex.

The distribution of glutaminase (GLNase)- and aspartate aminotransferase (AATase)-immunoreactive cells was examined in the cerebral neocortex of rat and guinea pig and in the somatic sensorimotor and primary visual cortex of the Macaca fascicularis monkey. These enzymes are involved in the metabolism of glutamate and aspartate, two amino acids thought to be excitatory amino acid transmitters for cortical neurons. In each of the species examined a large percentage of layer V and VI pyramidal neurons have pronounced glutaminase-like immunoreactivity (GLNase IR). In contrast, neurons in layers I, II, and IV show little GLNase IR. Layer III in the rat and guinea pig contains only a few, densely labeled GLNase-like-immunoreactive (GLNase-Ir) pyramidal neurons, whereas in the monkey the number of GLNase-Ir cells in layer III varies between cytoarchitectonic fields. Area 3b of the primary somatic sensory cortex and area 17 (primary visual cortex) contain few GLNase-Ir cells in layer III. However, layer III contains moderate numbers of GLNase IR in cells in areas 3a, 1, 2, 5, and in the primary motor cortex. Within the motor cortex the largest pyramidal ("Betz") cells are not labeled. In marked contrast to the results with antibody to GLNase, antibody to AATase labels cells that appear nonpyramidal in form, and these cells are in all cortical layers in each of the species examined. This distribution is roughly similar throughout all areas of rodent neocortex, but in monkey visual cortex AATase-immunoreactive neurons are more numerous in layers II-III, IVc, and VI. When combined with the findings of other studies, our results suggest that GLNase IR marks pyramidal neurons that use an excitatory amino acid transmitter. Antibody to AATase appears to mark intrinsic cortical neurons. The AATase immunoreactivity of these cells could indicate that they use an excitatory amino acid transmitter. However, their form and distribution in cortex suggest that this antibody labels GABAergic neurons.

Animals↗
PubMed2000

A functional MRI case study of acquired cerebral dyschromatopsia.

Evidence from imaging studies suggests that primary visual cortex and multiple areas in ventral occipitotemporal cortex subserve color perception in humans. To learn more about the organization of these areas, we used structural and functional MRI (fMRI) to examine a patient with damage to ventral cortex. An art professor, KG, suffered a cerebrovascular accident during heart surgery that impaired his ability to perceive color. The Farnsworth-Munsell 100-Hue test was used to assess the extent of his deficit. When tested 12 months after the lesion, KG performed worse than 95% of age-matched normals on the 100-Hue test, but well above chance. Structural and functional MRI studies were conducted 3 years after the lesion to investigate the neuroanatomical correlates of KG'ss remaining color ability. Structural MRI revealed bilateral damage to ventral occipitotemporal cortex. In young and age-matched normal controls, an fMRI version of the 100-Hue reliably activated bilateral, color-selective regions in primary visual cortex and anterior and posterior ventral cortex. In subject KG, color-selective cortex was found in bilateral primary visual cortex. In ventral cortex, no color-selective activity was observed in right ventral cortex, and only a small area of activity was observed in left anterior ventral cortex. However, significant color-selective activity was observed in posterior left ventral cortex spared by the lesion. This posterior left ventral activation was similar in extent, position, and degree of color-selectivity to the posterior left posterior activation observed in normal controls, suggesting that this focus may be the cortical substrate underlying KG's remaining color perception.

Adult↗
PubMed2003

Androgen receptor expression in the developing male and female rat visual and prefrontal cortex.

Gonadal steroid hormones are known to influence the development of the cerebral cortex of mammals. Steroid hormone action involves hormone binding to cytoplasmic or nuclear receptors, followed by DNA binding and gene transcription. The goals of the present study were twofold: to determine whether androgen receptors are present during development in two known androgen sensitive regions of the rat cerebral cortex, the primary visual cortex (Oc1) and the anterior cingulate/frontal cortex (Cg1/Fr2); and to determine whether androgen receptor (AR) expression in these regions differs between developing males and females. We used immunocytochemistry to detect AR protein on postnatal days 0, 4, and 10, and in situ hybridization to detect AR mRNA on postnatal day 10 in male and female rats. The level of AR expression was specific to the cortical region, with higher AR immunoreactive cell density and more AR mRNA in Oc1 than in Cg1/Fr2. AR immunoreactive cell density increased with age in both regions. Finally, on postnatal day 10, males had a higher AR immunoreactive cell density and more AR mRNA in Oc1 than did females. Thus, the presence of ARs may allow androgens to directly influence the development the cerebral cortex.

Animals↗
PubMed1993

Development of orientation selectivity in ferret visual cortex and effects of deprivation.

The orientation selectivity of cells in ferret primary visual cortex was studied during normal development and in animals deprived of vision or of visual cortical activity. In normal animals from the age when visual responses were first recorded (postnatal day 23) through postnatal week 5, only about 25% of cells showed orientation-selective responses. By postnatal week 7, cortical responses had matured to an adult-like state, with approximately 75% of cells clearly selective for orientation. This development of orientation selectivity was not merely a reflection of the development of cortical cell responsiveness: at all ages studied, there was no correlation between responsiveness and selectivity. Infusion of TTX into visual cortex to silence neuronal activity completely blocked the maturation of orientation selectivity. Visual deprivation by bilateral lid suture impaired but did not completely block the normal development of orientation selectivity. We conclude that the maturation of orientation-selective responses in ferret primary visual cortex requires cortical neuronal activity, and that normal development requires visually driven activity.

Animals↗
PubMedDate not supplied

Basic information processing and higher cognition: does the mammalian cerebral cortex deal with them at different hierarchical levels?

1. The primary visual cortex has been generally considered as a feature processing area where the neurons code for basic stimulus attributes, such as orientation, colour and direction of movement. Higher cognitive functions, such as attention and memory, were believed to be mediated by extrastriate regions at higher levels in a hierarchy of visual areas. However, the experiments reported here suggest that the function of the primary visual cortex is more than basic information processing in at least two respects. 2. First, significant modulation of visual responses occur in the primary visual cortex (V1) of the monkey depending upon attentional factors. There appears to be a feedback to V1 from extrastriate regions that facilitates neuronal responses in areas of attention and suppresses responses in other regions of the visual field. 3. Second, one would expect a region that is concerned with coding basic stimulus features to be rather resistant to plastic changes, as cells need to be reliable feature detectors for meaningful behaviour by the organism. However, the experiments on anaesthetized cats reported here show that a subset of neurons in visual area 17 of the cat have a potential for synaptic plasticity. Their response properties can undergo long-term changes under certain circumstances. To induce these changes, specific visual stimuli were combined with iontophoretic administration of NMDA and the GABA antagonist, bicuculline, which are believed to provide the molecular milieu for long-term potential and long-term depression.

Animals↗
PubMed1996

The calcarine sulcus as an estimate of the total volume of human striate cortex: a morphometric study of reliability and intersubject variability.

The human primary visual cortex consists of a region buried in the calcarine sulcus and a region outside this sulcus on the free surface of the occipital lobe. Since the depth of the calcarine sulcus can be easily estimated in magnetic resonance images of the living human brain, in vivo morphometry of the human primary visual cortex would be feasible for studying development, intersubject variability and interhemispheric asymmetry if the sulcal depth or a correlated measure such as the intracalcarine surface area would be a precise and reliable estimate of the total volume of the human primary visual cortex. The correlations between total volume of the striate cortex and its intra- and extra-calcarine surface areas were therefore tested in the present observations. The total volume of the striate cortex and the surface areas of its intra-and extracalcarine portions were measured in Nissl-stained serial sections through 20 adult human hemispheres. The intra- and extracalcarine portions of the striate area are not significantly correlated with each other, but correlated with the total volume of the striate cortex. The intracalcarine surface area or the depth of the calcarine sulcus are thus useful parameters for in vivo estimates of the total size of the striate cortex.

Adolescent↗
PubMed1988

The effects of ablation of visual cortex in neonatal rabbits on the organization of retinothalamic and retinopretectal projections.

Primary visual cortex was ablated unilaterally in neonatal rabbits. Following a survival of 2-4 months, retrograde degeneration of the dorsal lateral geniculate nucleus (LGd) was assessed, and reorganization of retinofugal pathways was studied using methods of anretrograde transport of [3H]proline or of horseradish peroxidase. A complete lesion of primary visual cortex resulted in complete retrograde degeneration of the LGd with no sparing of any class of neurons. The terminations of retinofugal axons in the pretectum and thalamus were compared with those observed in normal animals. No major reorganization of ipsilateral retinofugal projections was observed in either the thalamus and pretectum ipsilateral to the ablated cortex, or in the thalamus and pretectum contralateral to the ablated cortex. However, contralateral retinofugal projections to the thalamus and to the pretectum ipsilateral to the ablated cortex were significantly different from normal. In the thalamus, the projections to the lateral posterior nucleus were expanded in area and increased in density. In the pretectum, the projections to the rostral pretectal areas were greatly increased in area, especially in the region of the olivary pretectal nucleus and posterior pretectal nucleus. However, the density of these projections was not increased relative to normal. Consideration of these results in relation to other published data on the anatomical consequences of neonatal visual cortex lesions, both in mammals which show behavioral sparing following neonatal visual cortex lesions and in mammals which, like the rabbit, show no behavioral sparing, suggests that: (1) behavioral sparing may correlate with patterns of survival or death of neurons in the thalamus and retina; and (2) reorganization of retinofugal pathways is not necessarily associated with behavioral sparing.

Animals↗
PubMed1996

Age-related changes in oligodendrocytes in monkey cerebral cortex.

Compared with those in young monkeys (5-12 years of age), oligodendrocytes in area 46 of frontal cortex and primary visual cortex of monkeys over 25 years of age develop bulbous swellings along their processes. Such swellings are filled with characteristic inclusions that resemble age pigment, and other accumulations of these inclusions occur within the cell bodies of the oligodendrocytes in old monkeys. In addition, whereas the oligodendrocytes in young monkeys most commonly occur singly, in old monkeys it is common to find oligodendrocytes in groups or rows. These aggregates are often situated close to capillaries, and in some instances it is found that the cell bodies of the oligodendrocytes abut the basal lamina surrounding the capillary, so that the normally intervening astrocytic glial limiting membrane is absent. In these groups and rows, the perikarya of the oligodendrocytes are squashed close together, and it is common to find tight junctions formed between them. The cortices of the old monkeys also show extensive degeneration of myelin, and it is supposed that this is linked to the changes in the oligodendrocytes. It is hypothesized that the alterations in the oligodendrocyte-myelin system lead to changes in the rates of conduction along fibers whose myelin sheaths are affected, and this may be one of the causes of the behavioral changes associated with aging in primates.

Aging↗