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PubMed1994

Association and intrinsic connections of human extrastriate visual cortex.

Human extrastriate visual cortex contains several functionally defined visual areas of which four (V2, VP, V4 and V5) can be identified using cyto- and myeloarchitectonic criteria. A case with a small lesion in the superolateral part of cytoarchitectonic area 19, outside the previously defined visual areas, has been studied with the Nauta method for anterogradely degenerating axons. The infarcted part of cortex formed dense intrahemispheric connections in some visual areas (V1, V2, V3 and probably a part of V4) and weaker connections in others (VP, V5). Connections to V1 and to V2 were mainly directed to the representation of the lower paramedian part of the visual field. Intrinsic connections, visualized by the degenerating axon segments in the cortex around the lesion, had a patchy distribution. In conclusion, intrahemispheric corticocortical connections link preferentially some of the visual areas and within these areas certain parts.

Afferent Pathways↗
PubMed2000

Arrangement of orientation pinwheel centers around area 17/18 transition zone in cat visual cortex.

In the primary visual cortex of higher mammals, orientation preferences are represented continuously except for singular points, so-called pinwheel centers. In spite of the uniqueness of orientation pinwheel centers, very little is known about the pattern of their arrangement. In this study we examined the arrangement of orientation pinwheel centers in the cat visual cortex by optical imaging of intrinsic signals. Our results demonstrate that orientation pinwheel centers are arranged in a unique geometric pattern around the area 17/18 transition zone: pinwheel centers of the same type are arranged in rows parallel to the transition zone, and rows of clockwise and counterclockwise pinwheel centers are arranged alternately. We suggest that the areal border imposes a strong restriction on the pattern formation of orientation preference maps in the visual cortex.

Animals↗
PubMed2004

Influence of contrast on orientation and temporal frequency tuning in ferret primary visual cortex.

Neurons in primary visual cortex are highly sensitive to the contrast, orientation, and temporal frequency of a visual stimulus. These three stimulus properties can be varied independently of one another, raising the question of how they interact to influence neuronal responses. We recorded from individual neurons in ferret primary visual cortex to determine the influence of stimulus contrast on orientation tuning, temporal-frequency tuning, and latency to visual response. Results show that orientation-tuning bandwidth is not affected by contrast level. Thus neurons in ferret visual cortex display contrast-invariant orientation tuning. Stimulus contrast does, however, influence the structure of orientation-tuning curves as measures of circular variance vary inversely with contrast for both simple and complex cells. This change in circular variance depends, in part, on a contrast-dependent change in the ratio of null to preferred orientation responses. Stimulus contrast also has an influence on the temporal-frequency tuning of cortical neurons. Both simple and complex cells display a contrast-dependent rightward shift in their temporal frequency-tuning curves that results in an increase in the highest temporal frequency needed to produce a half-maximum response (TF(50)). Results show that the degree of the contrast-dependent increase in TF(50) is similar for cortical neurons and neurons in the lateral geniculate nucleus (LGN) and indicate that subcortical mechanisms likely play a major role in establishing the degree of effect displayed by downstream neurons. Finally, results show that LGN and cortical neurons experience a contrast-dependent phase advance in their visual response. This phase advance is most pronounced for cortical neurons indicating a role for both subcortical and cortical mechanisms.

Action Potentials↗
PubMed1989

Patterns of connections in rat visual cortex.

The definition of visual areas is one of the central problems in visual cortex research. Rodent extrastriate cortex offers a striking example of the complexity of this issue, in that different parcelation schemes identify within it from 2 to as many as 13 separate visual areas. In the experiments reported here, patterns of connections within rat visual cortex were studied in an effort to better define its organizational layout. The experimental paradigm used consisted of the following steps: first, the pattern of callosal connections was revealed in vivo with the fluorescent tracer bisbenzimide. Then, using the callosal pattern as a landmark, single injections of WGA-HRP were placed at various sites in striate and extrastriate cortex. Subsequently, the relation between the tangential distribution of ipsilateral corticocortical connections, the callosal connections, and the borders of striate cortex were examined in the flattened cortex preparation. The experiments revealed widespread, patchy connections within rat visual cortex. These connections appeared to reflect 3 organizational trends. First, neighboring sites were more extensively connected than distant ones. Second, extrastriate sites receiving common striate cortex inputs tended to be interconnected. Finally, projections from opposite poles in striate cortex tended to form interdigitating patterns of connections in regions of overlap. Altogether these trends suggest that the extrastriate band adjoining striate cortex has a single, global map organization. However, within the global map, a clear modular organization was evident, which appeared to correspond to the multiple visuotopic representations reported for this region. Based on its location, and some organizational similarities. it is suggested that the global map may constitute the rat homolog of area V2 in cat and monkey.

Animals↗
PubMed2003

Patterned vision causes CRE-mediated gene expression in the visual cortex through PKA and ERK.

Normal visual experience during postnatal development is necessary for the maturation of visual cortical circuits and acts through molecular mechanisms that are still poorly understood. Recently, it has been shown that ERK (extracellular signal-regulated kinase) 1/2, protein kinase A (PKA), and CREB (cAMP response element-binding protein) are crucial factors for experience-dependent development of the visual cortex, but very little is known about the role of visual experience in their activation. Here, we show that visual stimulation after a brief period of dark rearing caused a transient ERK activation in the visual cortex. Visually induced ERK activation occurred primarily in excitatory neurons of layers II-III and VI and was prevented by binocular lid suture. ERK phosphorylation was strongly reduced by cortical infusion with the cAMP-PKA inhibitor Rp-8-Cl-cAMPS, thus establishing a link between PKA and ERK activation. To analyze the downstream consequences of ERK and PKA signaling, we studied the action of visual stimulation on transcription of genes controlled by CREB in transgenic mice carrying the LacZ reporter gene under the control of the CRE (cAMP response element) promoter. Visual stimulation triggered a prolonged episode of CRE-mediated gene expression in the visual cortex that was suppressed by infusion with the ERK inhibitor U0126. Cortical administration of Rp-8-Cl-cAMPS attenuated the experience-dependent activation of CRE-mediated gene transcription. These results show that ERK phosphorylation in visual cortical neurons represents a molecular readout of patterned visual stimuli and that visual activation of ERK involves the cAMP-PKA system. Finally, because CRE-mediated gene expression was totally dependent on ERK activation, we suggest that PKA action on CRE-mediated gene expression is mediated by ERK.

Anesthetics, Intravenous↗
PubMed2003

Spatial patterns of spontaneous local field activity in the monkey visual cortex.

Spontaneous activity among visually responsive neurons is often considered to consist of random neural events, or perhaps to reflect an irrelevant by-product of brain homeostasis. However, recent studies have emphasized that such ongoing activity is strongly synchronized over large cortical distances, and can have a marked impact on the responsiveness of neurons to visual stimuli, suggesting that such activity may indeed be highly relevant to the brain's interpretation of its sensory input. In the current study, we examined the spatiotemporal nature of local field potential (LFP) fluctuations in the visual cortex of two macaque monkeys that were awake, but in a state of relaxation with minimal visual stimulation. Using an array of 16 electrodes spaced by several millimeters, we simultaneously monitored the LFP at many sites over a large region of the visual cortex. In agreement with the literature, we found that the coherence in the raw LFP signal fell off quickly with both frequency and distance. However, when we examined slower fluctuations in the LFP power, we found that power signals, including those derived from the high y-range frequencies, had high coherence that fell off only very slowly with cortical distance. Finally, we performed an additional experiment, with several electrodes placed on either side of a sulcus, to demonstrate that the decline in local field synchrony with cortical distance was so reliable that the interruption in the cortical sheet corresponding to the opening of the sulcus could be easily identified by monitoring just a few minutes of spontaneous LFP activity. These experiments reveal that a significant portion of spontaneous LFP fluctuations in the visual cortex is contributed by global mechanisms, imposing synchrony that is, first and foremost, a function of cortical separation between any two points.

Animals↗
PubMed2002

Unmasking motion-processing activity in human brain area V5/MT+ mediated by pathways that bypass primary visual cortex.

Most models of the human visual system argue that higher-order motion-processing cortical regions receive their inputs only via the primary visual cortex (striate cortex), rather than also via direct projections from the thalamus that bypass primary visual cortex. However, recent evidence in non-human primates, along with some evidence in humans with damaged primary visual cortex (e.g., "blindsight" for motion in the blind visual hemifield), have argued for the existence of a direct thalamic-to-extrastriate projection for motion processing. This evidence remains controversial. Here we tested the idea that direct thalamic input to extrastriate motion processing areas exists in humans but might be masked in scalp recordings by activity from early visual areas. To do this, we employed stimuli that induced strong refractory effects in primary visual cortex--thereby creating a brief "reversable lesion" in primary visual cortex--immediately before the presentation of a motion stimulus. Under these conditions, we then assessed whether motion areas of cortex were still able to process the motion stimuli by recording event-related potentials (ERPs) and event-related magnetic fields (ERFs/MEG). We found robust motion-related activity in extrastriate motion processing areas in the ERP and MEG signals even when primary visual cortex was heavily suppressed by our manipulation. This finding provides evidence for a direct thalamic functional pathway to extrastriate visual cortical motion processing areas in the human that bypasses primary visual cortex.

Adult↗
PubMed1994

Development of laminar expression of the m2 muscarinic cholinergic receptor gene in rat visual cortex and the effect of monocular visual deprivation.

The postnatal development of laminar pattern of the m2 muscarinic acetylcholine receptor subtype mRNA in the visual cortex of both normally raised and monocularly deprived rats (one eyelid sutured at the age of 11 days) was studied using in situ hybridization histochemistry and computer-assisted image analysis. In normally raised rats, on birth, the m2 transcript was found to be more concentrated in the superficial zones of the cortex. This laminar pattern alters to a more homogeneous distribution of the label throughout the cortex already detectable on day 7. From day 10 onwards a bimodal laminar pattern gradually develops with increased mRNA levels in layer IV and upper layer VI. From postnatal day 21 onwards the hybridization peak in layer VI decreases as compared to the peak level in layer IV resulting in an adult distribution with highest labeling in layer IV, low labeling in layer I to III and moderate labeling in layers V and VI. Monocular deprivation results in decreased m2 mRNA levels in visual cortical layers IV-VI in both deprived and non-deprived cortices already detectable at the age of 18 days and persisting up to the age of 21 days; but this effect disappears following further deprivation until adulthood. The data suggest that the changes in m2 receptor level from a more homogeneous distribution to a bimodal pattern during postnatal development seem to be related to synaptogenesis and final tuning of connectional pattern within the rat visual cortex.

Animals↗
PubMed1990

Change in the pattern of behavioural specialization of neurons in the motor cortex of the rabbit following lesion of the visual cortex.

In order to find out whether damage of the visual cortex (area 17) of the brain results in a functional reorganization of the motor cortex, experiments were carried out with freely moving rabbits performing a food acquisition task in an experimental cage. Two rabbits served as controls, while in three rabbits the visual cortex was bilaterally damaged. Analysis of the activity of 575 neurons in the control and operated rabbits after the recovery of the original instrumental food acquisition behaviour revealed a marked difference in the behavioural specialization of the neurons in the motor cortex of two operated rabbits compared with the control animals. Although the same types of units as in the control rabbits could be found in the operated rabbits (M neurons activated in relation to body and limb movements, S neurons activated in relation to food seizure and L neurons activated in relation to learned food acquisition task), the number of S units was about half of that in the controls and the number of L units about double. The relative number of activations of the neurons in the operated rabbits was significantly less frequent during the food seizure and more frequent during the learned behaviour. This difference indicates a change in the pattern of behavioural specialization of the neurons in the motor cortex due to the damage of the visual cortex. In this reorganization, the motor cortex became more like (but not identical to) visual and limbic cortices that normally contain noticeably more L neurons than the motor cortex. The number of neurons activated in relation to the behaviour in the operated rabbits, as compared with the control animals, was smaller in the upper and larger in the lower layers of the motor cortex. This may indicate recruitment of new neurons from the lower cortical layers.

Animals↗
PubMed2002

Determination of individual stimulus--response curves in the visual cortex.

Activation in the visual cortex is typically studied using group average changes in an on-off paradigm for a single flicker frequency. We used functional magnetic resonance imaging (fMRI) to characterize the stimulus-response curve in the visual cortex as a function of flicker frequency in individual subjects, using LED goggles with 17 frequency steps between 0 and 30 Hz. Ten healthy young individuals were studied on two different occasions (mean interval; 22 days). In all but one subject, a third-order polynomial curve could be fitted to the data. From the response curve we calculated the peak response (the frequency where the response amplitude was maximal), the percentage change (relative difference) of the response amplitudes between 8 Hz and the peak frequency, and the average slope of response (towards the peak). On both occasions we could determine a peak response for each subject with small within-subject variability. The average absolute difference in peak response between both sessions was 1.37 Hz (range, 0.2-4.3 Hz), indicating that the peak frequency is rather stable for a given individual. In conclusion, our study illustrates the ability of fMRI to examine the stimulus-response curve in individual subjects in the visual cortex. Based on our findings, the peak response and the slope of response seem highly reproducible within subjects. A similar analysis of the stimulus-response curve may be applicable to other types of stimuli.

Adult↗
PubMed1993

Postnatal ontogeny of GABAA and benzodiazepine receptors in individual layers of rat visual cortex and the effect of visual deprivation.

The postnatal development of GABAA and benzodiazepine receptors was studied in the rat visual cortex with [3H]muscimol and [3H]flunitrazepam, respectively. The studies were performed by quantitative receptor autoradiography in the different cortical layers from normally raised rats and from monocularly deprived animals. In all cortical layers of normal rats there was a sharp (approx 3-fold) increase in [3H]muscimol binding from days 1 to 10. Between postnatal days 10 and 20 there was a considerable decrease in GABAA receptors; thereafter this decline continued until adulthood but at a more moderate rate. In all cortical layers, [3H]flunitrazepam binding increased sharply, also from day 1 to 10; at this age the adult values were practically attained. Monocular deprivation, by unilateral eyelid suture at the age of 11 days onwards lead to an increase in [3H]muscimol binding in cortical layers IV-VI. This increase in GABAA receptors was detectable at the age of 25 days onwards. In contrast, monocular deprivation did not alter the density of benzodiazepine receptors in any of the cortical layers, and this lack of effects was regardless of the duration of visual deprivation. The present results suggest that the development of GABAA receptors in the deeper cortical layers, but not that of benzodiazepine receptors, is seemingly regulated by the visual experience at a critical period of brain maturation.

Animals↗
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↗
PubMed1999

BDNF regulates the maturation of inhibition and the critical period of plasticity in mouse visual cortex.

Maturation of the visual cortex is influenced by visual experience during an early postnatal period. The factors that regulate such a critical period remain unclear. We examined the maturation and plasticity of the visual cortex in transgenic mice in which the postnatal rise of brain-derived neurotrophic factor (BDNF) was accelerated. In these mice, the maturation of GABAergic innervation and inhibition was accelerated. Furthermore, the age-dependent decline of cortical long-term potentiation induced by white matter stimulation, a form of synaptic plasticity sensitive to cortical inhibition, occurred earlier. Finally, transgenic mice showed a precocious development of visual acuity and an earlier termination of the critical period for ocular dominance plasticity. We propose that BDNF promotes the maturation of cortical inhibition during early postnatal life, thereby regulating the critical period for visual cortical plasticity.

Age Factors↗
PubMed2006

The generation of receptive-field structure in cat primary visual cortex.

Cells in primary visual cortex show a remarkable variety of receptive-field structures. In spite of the extensive experimental and theoretical effort over the past 50 years, it has been difficult to establish how this diversity of functional-response properties emerges in the cortex. One of the reasons is that while functional studies in the early visual pathway have been usually carried out in vivo with extracellular recording techniques, investigations about the precise structure of the cortical network have mainly been conducted in vitro. Thus, the link between structure and function has rarely been explicitly established, remaining a well-known controversial issue. In this chapter, I review recent data that simultaneously combines anatomy with physiology at the intracellular level; trying to understand how the primary visual cortex transforms the information it receives from the thalamus to generate receptive-field structure, contrast-invariant orientation tuning and other functional-response properties.

Animals↗
PubMed1988

Temporal sequence of neurotransmitter expression by developing neurons of fetal monkey visual cortex.

The developing fetal monkey visual cortex was studied immunocytochemically from 110-155 days post-conception in order to localize cell populations immunoreactive (ir) for gamma-aminobutyric acid, Substance P, cholecystokinin-octapeptide, somatostatin, neuropeptide Y, and proenkephalin A peptide (BAM-18). The area 17/18 border and all cortical laminae identified in the adult visual cortex were discernible from the youngest age examined. All ir-cell populations studied were present at each fetal age. However, despite a relatively adult-like cytoarchitecture, all ir-cell populations studied displayed patterns of immunostaining which were unlike those described in adult visual cortex, and showed significant changes in laminar distribution, morphology, and numbers over the time course of gestation examined. Despite the differences in the patterns of immunostaining between the fetal and adult visual cortex, ir-cell populations intrinsic to the developing visual cortex exhibited adult-like combinations of co-localized transmitters and peptides. The developing monkey cortex also contains ir-cell populations, particularly BAM-18-ir cells, which have not been detected immunocytochemically in the adult monkey cortex. Differences between the fetal and the adult ir-cell populations might be accounted for by cell death, morphological transformation, secondary migration or changes in gene expression for neurotransmitters and neuropeptides.

Animals↗
PubMed1997

Transplant of Schwann cells allows normal development of the visual cortex of dark-reared rats.

Visual experience is necessary for the correct development of the visual cortex. Dark-rearing from birth affects normal maturation of the functional properties of mammalian visual cortex: cortical cells show rapid habituation to repeated stimulation, decreased orientation selectivity, and enlarged receptive fields. Spatial resolution and response latency are also impaired. Recent experiments have demonstrated that visual deprivation reduces the expression of neurotrophins in the visual cortex. We formulated the hypothesis that visual experience drives the maturation of functional properties of the visual cortex by regulating cortical levels of neurotrophins. If this hypothesis is correct, exogenous supply of neurotrophins during dark-rearing from birth should prevent, or at least ameliorate, the effects of a lack of visual experience. Since Schwann cells are efficient biological minipumps of neurotrophic factors, we transplanted 1.0 or 1.5 x 10(6) Schwann cells or infused vehicle solution as a control into the lateral ventricles of 13 day old rats reared in total darkness from birth until the end of the critical period (postnatal day 45). Single-cell responses and visual-evoked potentials were recorded from the binocular zone of the primary visual cortex of each group. We found that in Schwann cell-transplanted animals all parameters tested were significantly improved upon those of dark-reared control rats and were in the range of normal adult values. Thus, Schwann cell transplant contributed to the normal development of visual response properties in the visual cortex, compensating for a complete absence of visual experience.

Animals↗