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The way in which information about complex objects and faces is represented in visual cortex is controversial. One model posits that information is processed in modules, highly specialized for different categories of objects; an opposing model appeals to a distributed representation across a large network of visual areas. A recent paper uses a novel imaging technique to address this controversy.
An experimental neurohistological study has been made of the intrinsic connections of the cortex of area 17 of the monkey, of the commissural connections of the visual cortex of the cat and monkey and of the association fibres passing into area 17 of the cat. In light microscopic studies the axonal degeneration method of Nauta has been used, and the site and mode of termination of the degenerating fibres has also been determined with the electron microscope...
We investigated the effect of 8-hydroxy-2-(N,N-dipropylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, on the induction of long-term potentiation in rat visual cortex slices. Perfusion of 8-OH-DPAT (0.1-10 microM) did not affect layer II/III field potentials evoked by test stimulation of layer IV, but significantly reduced long-term potentiation induced by tetanic stimulation. The inhibitory effect of 8-OH-DPAT was blocked by the 5-HT1A receptor antagonist, pindolol (10 microM), but not by the 5-HT2,7 receptor antagonist, ritanserin (100 microM), nor by the 5-HT3,4 receptor antagonist, MDL72222 (100 microM). These results suggest that the rat visual cortex long-term potentiation is inhibited by 5-HT1A receptor stimulation.
The colour-sensitive properties of the visual cortex neurons were studied in the squirrel. All the neurons responded to achromatic, green and blue visual stimuli; responses to red stimuli were slight or absent. According to responses to patterned visual stimuli the neurons were classified as non-selective, directionally-selective and orientation-selective (simple and complex). No colour-opponent properties were revealed in any of these neuronal groups: neuronal responses were qualitatively the same to achromatic, green and blue stimuli, and the receptive fields organization was independent of the stimulus colour. These data are discussed with respect to the fact of presence of colour-opponent cells in the retina and lateral geniculate nucleus of squirrel.
Neurotrophins have been suggested to act as liaison molecules between activity-dependent synaptic plasticity and the establishment of patterns of synaptic connectivity during postnatal developmental in different brain areas, including the visual cortex. In particular, recent studies have shown that Trk B ligands are involved in the formation of the ocular dominance columns during postnatal development. Here, we examined the contribution of endogenous Trk B activation to the regulation of different forms of synaptic plasticity including long-term potentiation (LTP), long-term depression (LTD) and LTP after LTD in the developing visual cortex. Rat cortical slices were incubated with a soluble form of Trk B receptor (TrkB IgG) preventing Trk B activation by endogenous ligands. LTP expression was also studied at P23 (postnatal), when the expression of brain-derived neurotrophic factor (BDNF) reaches a peak and the LTP expression is normally downregulated. The present results demonstrate that Trk B activation is required for the long-term maintenance, > 30 min, of both LTP and LTP after LTD at P17. At P23, a higher concentration of TrkB IgG was necessary to impair LTP. In contrast, neither amplitude nor duration of LTD were affected by Trk B ligands blockade. Taken together, these results indicate that endogenous Trk B ligands are necessary for the expression of LTP but not LTD at a critical time during postnatal cortical development.
The effects of attentional modulation on activity within the human visual cortex were investigated using magnetoencephalography. Chromatic sinusoidal stimuli were used to evoke activity from the occipital cortex, with attention directed either toward or away from the stimulus using a bar-orientation judgment task. For five observers, global magnetic field power was plotted as a function of time from stimulus onset. The major peak of each function occurred at about 120 ms latency and was well modeled by a current dipole near the calcarine sulcus. Independent component analysis (ICA) on the non-averaged data for each observer also revealed one component of calcarine origin, the location of which matched that of the dipolar source determined from the averaged data. For two observers, ICA revealed a second component near the parieto-occipital sulcus. Although no effects of attention were evident using standard averaging procedures, time-varying spectral analyses of single trials revealed that the main effect of attention was to alter the level of oscillatory activity. Most notably, a sustained increase in alpha-band (7-12 Hz) activity of both calcarine and parieto-occipital origin was evident. In addition, calcarine activity in the range of 13-21 Hz was enhanced, while calcarine activity in the range of 5-6 Hz was reduced. Our results are consistent with the hypothesis that attentional modulation affects neural processing within the calcarine and parieto-occipital cortex by altering the amplitude of alpha-band activity and other natural brain rhythms.
The retinotopic organization of the middle temporal visual area (MT) was determined in six adult owl monkeys and one adult marmoset 69 d to 10 months after lesions of the dorsolateral primary visual cortex (V1). The lesions removed were limited to extensive parts of the representation of the lower visual quadrant in V1. Microelectrodes were used to record from neurons at numerous sites in MT to determine whether parts of MT normally devoted to the lower visual quadrant (1) were unresponsive to visual stimuli, (2) acquired responsiveness to inputs from intact portions of V1, or (3) became responsive to some other visually driven input such as a relay from the superior colliculus via the pulvinar to MT. All monkeys (n = 6) with moderate to moderately large lesions had unresponsive portions of MT even after 10 months of recovery. These unresponsive regions were retinotopically equivalent to the removed parts of V1 in normal animals. Thus, there was no evidence for an alternative source of activation. In addition, these results indicate that any retinotopic reorganization of MT based on inputs from intact portions of V1 was not extensive, yet neurons near the margins of responsive cortex may have acquired new receptive fields, and the smallest 5 degrees lesion of V1 failed to produce an unresponsive zone. Deprived portions of MT were not remarkably changed in histological appearance in cytochrome oxidase, Nissl, and Wisteria floribunda agglutinin preparations. Nevertheless, some reduction in myelin staining and other histological changes were suggested. We conclude that MT is highly dependent on V1 for activation in these monkeys, and alternative sources do not become effective over months when normal activation is absent. Additionally, remaining V1 inputs have only a limited capacity to expand their activation territory into deprived portions of MT.
Evidence for structural modifications in the brain following environmental changes have been provided during the last decades. The most pronounced alterations following environmental manipulations have been found in the visual cortex. These plastic changes are supposed to reflect reorganization of neuronal connections involved in postnatal development and adult adjustments of connections involved in sensori-perceptual processing and learning. Potential candidates to mediate these changes are neurotrophins. Nerve growth factor (NGF) has been associated with cognitive functions and shown to improve the performance of aged rats in spatial learning and memory task. In the central nervous system, NGF is of importance for development and maintenance of cholinergic neurons and atrophy of cholinergic neurons is strongly correlated with learning and memory impairments. Exposure to enriched environmental conditions improves learning and problem-solving ability and results in plastic changes in the brain. This study examined the effect of environmental enrichment on expression of NGF mRNA in the rat visual cortex and hippocampus. Rats housed in groups in a stimulus-rich environment for 30 days had significantly higher levels of NGF mRNA than rats housed individually in single cages without stimulus-enrichment. We have recently presented results showing higher levels of neurotrophin-3 (NT-3) mRNA and improved spatial learning following environmental enrichment, and suggest that an interplay involving the neurotrophins NGF and NT-3 may be mediating experience-induced structural changes.
Multiple neuronal activity, recorded through chronically implanted electrodes, was analyzed. During acquisition and consolidation of alimentary conditioned reflexes to electrostimulation of the lateral geniculate body or optical tract, the patterns of neuronal activity in the visual cortex and sensory motor cortex became organized in a way different from that observed during pseudoconditioning. The majority of the neurons showed change in activity during both the isolated action of the stimuli and their simultaneous presentation. In stabilized conditioned reflexes, the activity of neurons in the sensory motor and visual cortex was interdependent. Neuronal indices of backward conditioned connections during activation of the reinforcing structures were analogous to the reactions of visual cortical neurons during the conditioned stimulus action and were manifested by an increase of discharge activity.
Lactate and the other monocarboxylates are a major energy source for the developing brain. We investigated the immunocytochemical expression of two monocarboxylate transporters, MCT1 and MCT2, in the human visual cortex between 13 and 26 post-ovulatory weeks. We used immunoperoxidase and immunofluorescence techniques to determine whether these transporters co-localized with markers for blood vessels (CD34), neurons (microtubule-associated protein 2 [MAP2], SMI 311), radial glia (vimentin), or astrocytes (glial fibrillary acidic protein [GFAP], S100beta protein). MCT1 immunoreactivity was visible in blood vessel walls as early as the 13th week of gestation mainly in the cortical plate and subplate. At this stage, less than 10% of vessels in the ventricular layer expressed MCT1, whereas all blood vessels walls showed this immunoreactivity at the 26th gestational week. Starting at the 19th week of gestation, sparse MCT1 positive cell bodies were detected, some of them co-localized with MAP2 immunoreactivity. MCT2 immunoreactivity was noted in astrocytic cell bodies from week 19 and spread subsequently to the astrocyte end-feet in contact with blood vessels. MCTs immunoreactivities were most marked in the subplate and deep cortical plate, where the most differentiated neurons were located. Our findings suggest that monocarboxylate trafficking between vessels (MCT1), astrocytes (MCT2) and some postmitotic neurons (MCT1) could develop gradually toward 20 gestational weeks (g.w.). These data suggest that lactate or other monocarboxylates could represent a significant energy source for the human visual cortex at this early stage.
It has been shown that prenatal malnutrition results at birth in increased concentration of noradrenaline (NA) in the brain. Besides, it is known that NA is an important regulator of normal regressive processes occurring during synaptogenesis such as cell death, axonal pruning and synaptic elimination. The present study was designed to investigate (i) whether prenatal malnutrition enhances the NA release in the visual cortex and (ii) whether or not chronic administration of clonidine during gestation may prevent long-term deleterious effects of fetal malnutrition on functional properties of interhemispheric connections of the visual cortex and on the interhemispheric asymmetry of visual evoked responses. Prenatal malnutrition was induced by restricting food consumption to pregnant rats from Day 8 postconception to parturition. Results show that at birth, prenatally malnourished rats had higher NA release than normals. At 45-50 days of age, the malnourished group exhibited (a) reduced peak-to-peak amplitude and diminished extent of the projecting field of transcallosal evoked responses, and (b) abolished interhemispheric asymmetry of visual evoked responses. Clonidine administration to malnourished mothers from Day 14 postconception to parturition (10 g/kg/day s.c.), prevented in the offspring disorders induced by prenatal malnutrition on cortical NA release, on interhemispheric connectivity of visual areas and on interhemispheric bioelectrical asymmetry, probably by restoring the normal trophic role of NA during synaptogenesis. Results are discussed in relationship to normal regressive events occurring during early brain development.
Past observations revealed a relationship between mammalian retinal ganglion cell distribution and cortical and subcortical projection of the rabbit's visual field. High ganglion cell density sectors of the retina claimed larger areas in their cortical and subcortical projections (with a high magnification factor) than retinal areas with low ganglion cell density. In this study, it was observed that distribution of nonpyramidal neurons in laminae IV and VI of the pigmented rabbit's visual cortex was not uniform. The nonuniformity in layer IV roughly followed the variation in the magnification factor in the cortical projection of the animal's visual field. The highest density of nonpyramidal neurons in layer IV was observed in the cortical area with the highest magnification factor.
The adult cerebral cortex can adapt to environmental change. Using monocular deprivation as a paradigm, we find that rapid experience-dependent plasticity exists even in the mature primary visual cortex. However, adult cortical plasticity differs from developmental plasticity in two important ways. First, the effect of adult, but not juvenile monocular deprivation is strongly suppressed by administration of barbiturate just prior to recording visual evoked potentials, suggesting that the effect of adult experience can be inactivated acutely. Second, the effect of deprivation is less persistent over time in adults than in juveniles. This correlates with the known decline in CREB function during maturation of the visual cortex. To compensate for this decline in CREB function, we expressed persistently active VP16-CREB and find that it causes adult plasticity to become persistent. These results suggest that in development and adulthood, the regulation of a trans-synaptic signaling pathway controls the adaptive potential of cortical circuits.
The visual cortex in primates consists of an array of anatomically and chemically identifiable cellular modules (hypercolumns) with distinct physiological properties. For example, layers II/III in the macaque monkey contain a regular array of cytochrome oxidase-rich blobs. Furthermore, the surrounding cytochrome oxidase-poor interblob regions have a higher density of neuropeptide Y-positive aspiny stellate cells. Neurons in the blobs are thought to mediate predominantly low spatial frequencies and color vision, while those in the interblobs appear to be engaged in pattern vision and high spatial frequency analysis. In this study we examined the role of the retina in the development of hypercolumns. A bilateral retinal ablation was performed in embryos at midgestation, before any photoreceptors had established contacts with other retinal neurons and before layers II/III of the cortex--or their synaptic connection--had been generated. We found that the cortex in operated animals had cytochrome oxidase blobs and that their size and spacing were normal. In addition, neuropeptide Y-containing neurons were preferentially distributed in the interblob region as in control animals. Our findings indicate that some basic aspects of the cyto- and chemoarchitectonic organization of the cerebral cortex, which presumably evolved for the analysis of form and color, can emerge in the absence of cues from the retinal photoreceptors that mediate these attributes of vision.
We evaluated the binocular interaction and horizontal disparity sensitivity in neurons recorded from macaque visual cortex. Neurons from V1 of three awake Macaca mulatta monkeys were isolated by means of extracellular recording and tested for disparity sensitivity with dynamic random dot stereograms. Neurons sensitive to horizontal disparities were stimulated both monocularly and binocularly with flashing bars and their responses quantified. ANOVA and regression tests were used for data analysis. Sixty-six cells out of 185 (66/185, 36%) showed sensitivity to horizontal disparity. Disparity sensitive cells were grouped into near (25/66, 38%), tuned inhibitory (16/66, 24%), far (13/66, 20%) and tuned excitatory (12/66, 18%). Receptive fields of tuned cells were located more centrally in the visual field than those of near and far cells. The binocular interaction in tuned inhibitory cells increased linearly along with ocular unbalance. Most of tuned excitatory cells (10/12, 83%) showed facilitatory binocular interaction, characterized by a stronger response to binocular stimulation than to the stimulation of the dominant eye. On the contrary, most of tuned inhibitory cells (14/16, 88%) showed suppressory binocular interaction, characterized by a weaker response to binocular stimulation than to the stimulation of the dominant eye. Near and far cells showed both types of interaction in similar percentages. The binocular response showed a linear relationship with the sum of both monocular responses in tuned excitatory, tuned inhibitory and near cells, but not in far cells. Sensitivity to horizontal disparity may be a result of facilitatory and suppressive interactions between left and right inputs.
Immunocytochemistry with polyclonal antibodies directed against protein kinase C (PKC) was utilized to investigate the development of the kinase in kitten visual cortex neurons. The immunoreaction product was found at postsynaptic sites at all ages studied. However, PKC was localized in presynaptic terminals only during the first few weeks of postnatal life, during the period when the cortex is most susceptible to visual experience. The overall level of PKC immunoreactivity was high at early postnatal ages (up to 6 weeks) and declined afterwards till adulthood. This decline in reactivity was not equal across the cortex and was particularly marked in the middle cortical layers, especially layer IV. The reduction of PKC immunoreactivity in all cortical layers but layer IV was abolished by isolating a portion of cortex from its neuronal inputs early in life. Indirect evidence points to the lateral geniculate nucleus as the source of input that is required for input-dependent maturational changes in the kinase level. The results reported here suggest that the expression of PKC in kitten visual cortex in not only developmentally regulated but is also use-dependent.
Zif268 transcription factor is expressed throughout Cebus apella visual cortex at high basal levels. Monocular eyelid suture alters the levels of Zif268 on neurons connected to the deprived eye, revealing ocular dominance columns in the striate cortex of Cebus as previously demonstrated in Old World monkeys (Chaudhuri and Cynader, Brain Res., 605 (1993) 349-353). Zif268 ocular dominance columns are revealed in adult Cebus monkey after 24-48 h of monocular deprivation, but not in infant monkeys up to 3 months of age. In 6-month-old Cebus monkeys, Zif268 ocular dominance columns are still poorly defined. These results suggest that Zif268 ocular dominance columns establish late during normal primate visual system development, and that some degree of visual plasticity is still present at this age in the Cebus monkey.
Selective attention is an important aspect of brain function that we need in coping with the immense and constant barrage of sensory information. One model of attention (Feature Integration Theory) that suggests an early selection of spatial locations of objects via an attentional spotlight would also solve the 'binding problem' (that is how do different attributes of each object get correctly bound together?). Our experiments have demonstrated modulation of specific locations of interest at the level of the primary visual cortex both in visual discrimination and memory tasks, where the actual locations of the targets was also important in being able to perform the task. It is suggested that the feedback mediating the modulation arises from the posterior parietal cortex, which would also be consistent with its known role in attentional control. In primates, the magnocellular (M) and parvocellular (P) pathways are the two major streams of inputs from the retina, carrying distinctly different types of information and they remain fairly segregated in their projections to the primary visual cortex and further into the extra-striate regions. The P inputs go mainly into the ventral (temporal) stream, while the dorsal (parietal) stream is dominated by M inputs. A theory of attentional gating is proposed here where the M dominated dorsal stream gates the P inputs into the ventral stream. This framework is used to provide a neural explanation of the processes involved in reading and in learning to read. This scheme also explains how a magnocellular deficit could cause the common reading impairment, dyslexia.