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PubMed1995

Modulation of GABAA receptor binding in human brain by neuroactive steroids: species and brain regional differences.

Allosteric modulation by neuroactive steroids of radioligand binding sites on the GABAA receptor complex was demonstrated by autoradiography in vitro in several regions of human brain and the effects compared to those in rat brain. Comparing human and rat, two steroids known to be active in enhancing GABA-mediated postsynaptic inhibition, 5 alpha-pregnane-3 alpha,21-diol-20-one (tetrahydro-deoxycorticosterone, THDOC) and alphaxalone (5 alpha-pregnane-3 alpha-hydroxy-11,20-dione), allosterically inhibited [35S]T-BPS binding to the picrotoxin/convulsant site in both species in several regions including the hippocampus. Unlike rat, human brain binding of [3H]flunitrazepam to the benzodiazepine site was not enhanced by alphaxalone (at any concentration), but was unaffected in many regions and inhibited in others. Binding of [3H]muscimol to high and low affinity GABA sites were enhanced by both steroids in all tested regions of rat brain, although to varying degrees. However, several lobes of human cortex showed no modulation of muscimol binding by either steroid, and THDOC, but not alphaxalone, inhibited in some areas. Comparing regions, THDOC at high concentrations (10 microM) enhanced in human frontal lobe and primary sensory and motor cortex, with greater effect in deep layers than superficial. This steroid had no effect in other parts of parietal lobe and inhibited muscimol binding in temporal lobe, primary visual cortex, and other parts of occipital lobe. Concentration-dependence curves for THDOC showed regional variation, e.g., in the hippocampal formation and surrounding neocortex. These regional and species differences are consistent with the existence of multiple GABAA receptor subtypes that differ in pharmacology. This heterogeneity provides both the opportunity and the difficulty of targeting clinically useful medications such as antiepileptic drugs to the appropriate human brain regions, and the species differences in regional subtype expression suggest caution in use of animal models.

Autoradiography↗
PubMed1999

Visual stimulation elicits locked and induced gamma oscillations in monkey intracortical- and EEG-potentials, but not in human EEG.

Stimulus-related fast oscillations in the gamma-range (30-100 Hz) were clearly demonstrated with microelectrode recordings in visual cortex of awake monkeys, and they were also reported for recordings of human electroencephalograms (EEG). However, the presence of stimulus-related gamma-modulation in human EEG has repeatedly been disputed. To clarify this dispute, we recorded the scalp EEG of man and monkey as well as intracortical field potentials (LFP) from monkey primary visual cortex (V1) during identical visual stimulation (large-field sinusoidal gratings, which proved to induce the largest gamma-amplitudes in monkey V1 and V2). We found a strong stimulus-related increase of gamma-oscillations in monkey LFP and EEG, but no modulation of gamma-activity in human EEG. In contrast to previous results, gamma-oscillations in the monkey were strongly phase-locked to stimulus onsets in early response periods (80-160 ms) and became gradually independent in later periods. Our negative result on gamma-modulation in human subjects contradicts several published findings. We conclude from our results that visually evoked gamma-modulations in humans EEG are not as accessible as in the monkey.

Animals↗
PubMed1983

Selective disturbance of movement vision after bilateral brain damage.

A patient who suffered bilateral posterior brain damage exhibited disturbance of movement vision in a rather pure form. The patient had no impression of movement in depth, and could only discriminate between a stationary and a moving target in the periphery of her otherwise intact visual fields. She had some movement vision in the central part of her visual fields, provided that target velocity did not exceed 10 deg/s. Neither did she possess visual movement after effects nor apparent (phi) visual movement. In addition, visually guided eye and finger movements were impaired. In contrast to the disturbance of movement perception in the visual modality, movement perception elicited by acoustic and tactile stimuli was not impaired. On the basis of the localization of the cerebral damage (as judged by CT scanning and neuropsychological testing) it is concluded that the observed disorder in movement vision is due to bilateral cerebral lesions affecting the lateral temporo-occipital cortex and the underlying white matter. The selectivity of the visual disturbance supports the idea that movement vision is a separate visual function depending on neuronal mechanisms beyond the primary visual cortex.

Adult↗
PubMed1999

Motion opponency in visual cortex.

Perceptual studies suggest that visual motion perception is mediated by opponent mechanisms that correspond to mutually suppressive populations of neurons sensitive to motions in opposite directions. We tested for a neuronal correlate of motion opponency using functional magnetic resonance imaging (fMRI) to measure brain activity in human visual cortex. There was strong motion opponency in a secondary visual cortical area known as the human MT complex (MT+), but there was little evidence of motion opponency in primary visual cortex. To determine whether the level of opponency in human and monkey are comparable, a variant of these experiments was performed using multiunit electrophysiological recording in areas MT and MST of the macaque monkey brain. Although there was substantial variability in the degree of opponency between recording sites, the monkey and human data were qualitatively similar on average. These results provide further evidence that: (1) direction-selective signals underly human MT+ responses, (2) neuronal signals in human MT+ support visual motion perception, (3) human MT+ is homologous to macaque monkey MT and adjacent motion sensitive brain areas, and (4) that fMRI measurements are correlated with average spiking activity.

Animals↗
PubMed2005

A precritical period for plasticity in visual cortex.

One of the seminal discoveries in developmental neuroscience is that altering visual experience through monocular deprivation can alter both the physiological and the anatomical representation of the two eyes, called ocular dominance columns, in primary visual cortex. This rearrangement is restricted to a critical period that starts a few days or weeks after vision is established and ends before adulthood. In contrast to the original hypothesis proposed by Hubel and Wiesel, ocular dominance columns are already substantially formed before the onset of the critical period. Indeed, before the critical period there is a period of ocular dominance column formation during which there is robust spontaneous activity and visual experience. Recent findings raise important questions about whether activity guides ocular dominance column formation in this 'precritical period'. One developmental event that marks the passage from the precritical period to the critical period is the activation of a GABAergic circuit. How these events trigger the transition from the precritical to critical period is not known.

Animals↗
PubMed1988

Functional anatomy of macaque striate cortex. I. Ocular dominance, binocular interactions, and baseline conditions.

A series of experiments was carried out using 14C-2-deoxy-d-glucose (DG) in order to examine the functional architecture of macaque striate (primary visual) cortex. This paper describes the results of experiments on uptake during various baseline (or reference) conditions of visual stimulation (described below), and on differences in the functional architecture following monocular versus binocular viewing conditions. In binocular "baseline" experiments, monkeys were stimulated either (1) in the dark, (2) with a diffuse gray screen, or (3) with a very general visual stimulus composed of gratings of varied orientation and spatial frequency. In all of these conditions, DG uptake was found to be topographically uniform within all layers of parafoveal striate cortex. In monocular experiments that were otherwise similar, uptake was topographically uniform within the full extent of the eye dominance strip, in all layers. Certain other visual stimuli produce high uptake in the blobs, and still another set of visual stimuli (including high-spatial-frequency gratings) produce highest uptake between the blobs at parafoveal eccentricities, even in an unanesthetized, unparalyzed monkey. Eye movements per se had no obvious effect on striate DG uptake. Endogenous uptake in the blobs (relative to that in the interblobs) appears higher in the squirrel monkey than in the macaque. The pattern of DG uptake produced by binocular viewing was found to deviate in a number of ways from that expected by linearly summing the component monocular DG patterns. One of the most interesting deviations was an enhancement of the representation of visual field borders between stimuli differing from each other in texture, orientation, direction, etc. This "border enhancement" was confined to striate layers 1-3 (not appearing in any of the striate input layers), and it only appeared following binocular, but not monocular, viewing conditions. The border enhancement may be related to a suppression of DG uptake that occurs during binocular viewing conditions in layers 2 + 3 (and perhaps layers 1 and 4B), but not in layers 4Ca, 4Cb, 5 or 6. Another major class of binocular interaction was a spread of neural activity into the "unstimulated" ocular dominance strips following monocular stimulation. Such an effect was prominent in striate layer 4Ca, but it did not occur in layer 4Cb. This "binocular" spread of DG uptake into the inappropriate eye dominance strip in 4Ca may be related to the appearance of orientation tuning and orientation columns in that layer. No DG effects were seen that depended on the absolute disparity of visual stimuli in macaque striate cortex.

Animals↗
PubMed1986

Tangential organization of the infragranular fiber plexus in rat cerebral cortex.

Cylindrical lesions (diameter 300-500 microns) were formed by poking needles into various parts of the cerebral cortex of adult albino rats. Degenerating axons were visualized in horizontal sections through the 'flattened' cortex using the silver impregnation method of Gallyas et al. [Stain Technol. 55: 291-297 (1980)] which stains degenerating axoplasm. The density and distribution of tangentially oriented axons were evaluated in the infragranular layers by TV image analysis. The sampling fields were concentrically arranged around the lesion at distances of 200, 400, 700 and 1,100 microns. The results indicate that the distribution patterns of degenerating (associational) axons covary with the cytoarchitectonic regions into which the lesions were placed. In the motor cortex, the majority of axons run in the antero-posterior direction. The density is generally lower around lesions in frontal regions than in parietal regions. The most extended degeneration was found around lesions near the border of or within the retrosplenial cortex, indicating an exceptionally strong internal connectivity in this area. Since only few degenerating axons were seen around lesions in the center of area 17, the high density of myelinated axons in the primary visual cortex seems to be due to fibers that originate in peristrate areas. It is concluded that the number and extension of fibers that degenerate tends to covary with some aspects of cortical architecture, but it is not area-specific.

Animals↗
PubMedDate not supplied

Cell type specificity of local cortical connections.

The function of the cerebral cortex is dependent on the precise organization of the circuits formed by its component neurons. The connections between neurons are not random, but are specific at multiple levels of organization. For example, each cortical area connects to only a selected subset of other areas and within any given area the axonal and dendritic arbors of individual neurons arborize in precise, layer-specific patterns (for review see Felleman & Van Essen, 1991; Callaway, 1998). In each layer there are dendrites from multiple cell types including cells with somata both within and outside that layer. Anatomical studies have shown that axons arborizing in a particular cortical layer can connect selectively onto dendrites of some cell types in the layer, while avoiding the dendrites of other cell types (e.g. Freund & Gulyas, 1991; Hornung & Celio, 1992; Staiger et al., 1996). These cell type specific connections are, however, difficult to elucidate with anatomical methods, so the frequency of such specificity has remained elusive. Recent experimental methods combining intracellular recording of single neurons with focal neuronal stimulation by uncaging glutamate with light ("photostimulation") have made the analysis of cell type specific cortical connections more tractable. These studies show that cell type specificity of connections is prevalent in cortex. Here I review photostimulation-based studies investigating the laminar sources of cortical input to distinct cell types in the visual and somatosensory cortices of rats and the primary visual cortex of monkeys.

Animals↗
PubMed2002

Color opponent neurons in V1: a review and model reconciling results from imaging and single-unit recording.

The signals in visual cortex that ultimately give rise to color perception remain poorly understood. Controversy has particularly surrounded one aspect of color's encoding in the visual system-opponent processing in primary visual cortex. Early single-unit studies suggested that V1 contains relatively few color-opponent neurons. Neuroimaging measurements, however, have suggested that such neurons might be relatively numerous. Here, we reconcile these apparently discrepant results and conclude that V1 contains relatively large numbers of color-opponent neurons. We first review results from each method and find that most neuroimaging studies provide evidence of substantial color opponency in V1, and that despite apparent controversy, most single-unit studies agree that relatively large numbers of V1 neurons show some sort of color opponency. To reconcile the results from different techniques more formally, we used electrophysiological data to predict the outcomes of neuroimaging experiments. We simulated the expected fMRI response in V1 to spatial patterns of different color, based on the neurons' properties, as reported in Johnson, Hawken, and Shapley, (2001). The simulated responses to stimuli used in Engel, Zhang, and Wandell, (1997) agree well with the actually observed fMRI results. The model identifies several factors that led to the apparent discrepancy between techniques, and makes testable predictions about how these factors influence the magnitude of color-opponent signals. fMRI and single-unit data converge to show that large numbers of color-opponent neurons exist in V1.

Color Perception↗
PubMed1999

Severe neuronal losses with age in the parietal cortex and ventrobasal thalamus of mice transgenic for the human NF-L neurofilament protein.

Transgenic mice expressing human light neurofilament protein (NF-L) display early perikaryal accumulations of disarrayed neurofilaments in layers II/III of the parietal cortex and in the ventrobasal complex of thalamus. This cytoskeletal abnormality, reflected by strong NF-L immunoreactivity, is transient in the developing cortex but persists until old age in the thalamus. To investigate whether it leads to neuronal death, the unbiased cell counting method of the dissector was applied to the parietal cortex and the thalamus of normal and transgenic mice at various postnatal (P10, P20, P90) and advanced ages (14-18 months). Similar data were also obtained from the primary visual cortex free of NF-L accumulation. Compared with normal, the total number of neurons in the parietal (but not occipital) cortex of transgenic mice showed little change during the postnatal period, but decreased markedly with old age, particularly in layers II/III. Severe neuronal loss was also documented in the thalamic ventrobasal complex of aged transgenic mice. The delayed neuronal death in the parietal cortex, occurring long after recovery from the NF-L accumulations, was suggestive of a combination of deleterious factors, including the early overproduction of neurofilament protein and subsequent loss of afferent input from the affected somatosensory thalamic nuclei. Furthermore, strong accumulation of lipofuscin in the neurons of aged transgenic mice suggested that oxidative stress partakes in the mechanisms through which NF-L overproduction compromises neuronal viability.

Aging↗
PubMed1987

Local circuit neurons of macaque monkey striate cortex: I. Neurons of laminae 4C and 5A.

A study has been made, using Golgi preparations, of the organization of neurons with smooth or sparsely spined dendrites, here called local circuit neurons, of the macaque monkey primary visual cortex. Since these neurons include those responsible for inhibitory circuitry of the cortex, a better understanding of their anatomical organization is essential to concepts of functional organization of the region. This account describes those neurons found with cell body and major dendritic spread within the thalamic recipient zone of lamina 4C and its border zone with lamina 5A. The neurons are grouped firstly in terms of in which laminar division the soma occurred--4C beta, 4C alpha or the border zone of 5A-4C beta--and secondly, into varieties on the basis of the interlaminar projection patterns of their axons. Most, if not all, of the local circuit neurons of these divisions have interlaminar axon projections as well as an arbor local to their cell body and dendritic field. These interlaminar projections are highly specific, targeting from one to five laminar divisions depending on the variety of neuron; on this basis 17 varieties of local circuit neuron are described. While the number of varieties appears dauntingly large in terms of understanding the functional circuitry of the region, the clear-cut organization of the interlaminar links may provide clues as to the information processing that concerns each neuron. The local circuit neuron axon projections can be related to a wealth of information already available concerning the laminar organization of afferent axons and efferent cell groups, the organization of spiny neuron intrinsic relays (presumed to be excitatory), and physiological properties of different laminar divisions. It is hoped that the information derived from this study can serve as a guide for correlated physiological-anatomical studies on single cells of the region.

Animals↗
PubMedDate not supplied

Organization of texture segregation processing in primate visual cortex.

We investigated the intracortical organization of neuronal mass activity that is related to texture segregation on the basis of orientation contrast. Evoked potentials were recorded to a stimulus, signalling a contribution from texture segregation-sensitive mechanisms by means of specific response components. The specific components could only be recorded when textons had a spatial organization that leads to the percept of image segmentation. Equivalent dipole estimations of the specific response components suggested the presence of texture segregation-related activity in the primary visual cortex. These results were corroborated by current-source-density analysis of intracortical recordings in the awake monkey. A specific involvement of layers 2/3 and 5 of area 17 in the global process of image segmentation could be demonstrated.

Animals↗
PubMed1993

Functional mapping of the human visual cortex at 4 and 1.5 tesla using deoxygenation contrast EPI.

The effects of photic stimulation on the visual cortex of human brain were studied by means of gradient-echo echo-planar imaging (EPI). Whole-body 4 and 1.5 T MRI systems, equipped with a small z axis head gradient coil, were used. Variations of image intensity of up to 28% at 4 T, and up to 7% at 1.5 T, were observed in primary visual cortex, corresponding to an increase of blood oxygenation in regions of increased neural activity. The larger effects at 4 T are due to the increased importance of the susceptibility difference between deoxygenated and oxygenated blood at high fields.

Adult↗
PubMed2002

Orientation and color columns in monkey visual cortex.

The literature on orientation and color columns in monkey visual cortex is reviewed. The orientation column model most consistent with existing data is one containing 'stripes' of alternating positive and negative orientation 'singularities' (cytochrome oxidase blobs) which run along the centers of ocular dominance (OD) columns, with horizontal and vertical orientations alternating at interblob centers. Evidence is summarized suggesting that color is mapped continuously across the monkey's primary visual cortex, with the ends of the spectrum located at 'red' and 'blue' cytochrome oxidase blobs and extra-spectral purple located between adjacent red and blue blobs in the same OD column. In the orientation column model, the 'linear zones' of Obermayer and Blasdel have the appearance of the lines on a pumpkin. A pinwheel model of color columns, consistent with existing data, includes spectral and extra-spectral colors as spokes. Spectral iso-color lines run across iso-orientation lines in linear zones, while extra-spectral iso-color lines occupy the 'saddle points' of Obermayer and Blasdel. The color column model accounts for closure of the perceptual color circle, as proposed by Isaac Newton in 1704, but does not account for color opponency.

Animals↗
PubMed1998

The role of visual experience in the development of columns in cat visual cortex.

The role of experience in the development of the cerebral cortex has long been controversial. Patterned visual experience in the cat begins when the eyes open about a week after birth. Cortical maps for orientation and ocular dominance in the primary visual cortex of cats were found to be present by 2 weeks. Early pattern vision appeared unimportant because these cortical maps developed identically until nearly 3 weeks of age, whether or not the eyes were open. The naïve maps were powerfully dominated by the contralateral eye, and experience was needed for responses to the other eye to become strong, a process unlikely to be strictly Hebbian. With continued visual deprivation, responses to both eyes deteriorated, with a time course parallel to the well-known critical period of cortical plasticity. The basic structure of cortical maps is therefore innate, but experience is essential for specific features of these maps, as well as for maintaining the responsiveness and selectivity of cortical neurons.

Animals↗
PubMed1989

Allylglycine and brain excitability. Electrophysiological investigation in cats.

The effect of DL-allylglycine (DL-AG) in two doses (40 and 60 mg/kg i.v.) was studied on cats under acute conditions. DL-AG provoked the appearance of epileptiform EEG patterns either spontaneous or in response to intermittent light stimulation with a frequency of 5-25 Hz. The effect was dose-dependent. The paroxysmal EEG was restricted to or began in the cortex and later it appeared in the thalamus and mesencephalic reticular formation. This finding together with the observation that after DL-AG the visual evoked potentials increased in the primary visual cortex and decreased in the centre median nucleus and in the reticular formation support the view about the role of the cortex in the increased brain excitability and in the increased photosensitivity after allylglycine. The photosensitive of spontaneous epileptiform EEG activity in cats pretreated with DL-allylglycine could be an useful experimental model of epilepsy for assessing the efficacy of anticonvulsant drugs.

Allylglycine↗
PubMed1978

Physiological consequences for the cat's visual cortex of effectively restricting early visual experience with oriented contours.

1. The early visual experience of nine cats was restricted to viewing horizontal or vertical lines inside opaque goggles. 2. When the kittens were 3-4 mo old, extracellular recordings were made in the primary visual cortex. To obtain a representative sample of cortical cells, units were studied at regularly spaced intervals along the course of electrode penetrations traveling oblique to the cortical surface. An automated assessment of preferred orientation using a computer-driven optical display was employed, and during the recording session the experimenters did not know which orientation(s) each animal had viewed in early life. 3. In the cats that viewed horizontal lines with one eye and vertical lines with the other during rearing, two major findings of previous workers (14) were confirmed. First, a majority of units were not selective for orientation. Second, units with preferred orientations near vertical tended to be activated exclusively by the eye that had viewed vertical, and likewise for horizontal. 4. In cats that viewed lines of the same orientation with both eyes during rearing, a substantially smaller proportion of units were selective for orientation; the preferred orientations of these units also tended to match the orientation to which the cats had been exposed. 5. Portions of some electrode penetrations showed an orderly arrangement of cells according to preferred orientation similar to that seen in normal cats, but with regions over which only nonselective cells were found. Many penetrations appeared less orderly. 6. The results are consistent with a role for early visual experience in maintaining the responsiveness and innate selectivity of cortical neurons, although they cannot entirely rule out the possibility that experience may alter or determine the preferred orientation of some cells.

Animals↗
PubMed2002

Visual cortex neurons of monkeys and cats: temporal dynamics of the contrast response function.

Cortical neurons display two fundamental nonlinear response characteristics: contrast-set gain control (also termed contrast normalization) and response expansion (also termed half-squaring). These nonlinearities could play an important role in forming and maintaining stimulus selectivity during natural viewing, but only if they operate well within the time frame of a single fixation. To analyze the temporal dynamics of these nonlinearities, we measured the responses of individual neurons, recorded from the primary visual cortex of monkeys and cats, as a function of the contrast of transient stationary gratings that were presented for a brief interval (200 ms). We then examined 1) the temporal response profile (i.e., the post stimulus time histogram) as a function of contrast and 2) the contrast response function throughout the course of the temporal response. We found that the shape and complexity of the temporal response profile varies considerably from cell to cell. However, within a given cell, the shape remains relatively invariant as a function of contrast and appears to be simply scaled and shifted. Stated quantitatively, approximately 95% of the variation in the temporal responses as a function of contrast could be accounted for by scaling and shifting the average poststimulus time histogram. Equivalently, we found that the overall shape of the contrast response function (measured every 2 ms) remains relatively invariant from the onset through the entire temporal response. Further, the contrast-set gain control and the response expansion are fully expressed within the first 10 ms after the onset of the response. Stated quantitatively, the same, scaled Naka-Rushton equation (with the same half-saturation contrast and expansive response exponent) provides a good fit to the contrast response function from the first 10 ms through the last 10 ms of the temporal response. Based upon these measurements, it appears as though the two nonlinear properties, contrast-set gain control and response expansion, are present in full strength, virtually instantaneously, at the onset of the response. This observation suggests that response expansion and contrast-set gain control can influence the performance of visual cortex neurons very early in a single fixation, based on the contrast within that fixation. In the DISCUSSION, we consider the implications of the results within the context of 1) slower types of contrast gain control, 2) discrimination performance, 3) drifting steady-state measurements, 4) functional models that incorporate response expansion and contrast normalization, and 5) structural models of the biochemical and biophysical neural mechanisms.

Animals↗