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PubMed2004

Topographic and laminar maturation of striate cortex in early postnatal marmoset monkeys, as revealed by neurofilament immunohistochemistry.

The maturation of pyramidal neurons in the primary visual cortex (V1) of marmoset monkeys was investigated using an antibody (SMI-32) to non-phosphorylated neurofilament protein (NNF). Analysis of animals aged between birth and postnatal day 91 (PD 91, which corresponds approximately to the peak of synaptogenesis in this species) revealed discrete changes in both the laminar and the areal distribution of NNF. At PD 0, the upper part of layer 6 contained darkly labelled neurons and associated neuropil, including axons. In this layer a centroperipheral gradient, with more labelled cells in the foveal representation, was apparent at PD 0. This topographic gradient gradually disappeared, and by PD 91 a similar density of labelled layer 6 cells was observed throughout V1. Labelled cells were not apparent in layer 3C until PD 7, and were not distributed according to a topographic gradient. Labelled cells were first observed in layer 3B(alpha) at PD 28, when they formed a centroperipheral gradient similar to that seen in layer 6. This gradient was still evident in an adult animal. These results demonstrate an inside-out profile of postnatal cortical development, with the topographic pattern of maturation of V1 mimicking the centroperipheral gradient of maturation in the retina.

Animals↗
PubMed2005

Orientation selectivity without orientation maps in visual cortex of a highly visual mammal.

In mammalian neocortex, the orderly arrangement of columns of neurons is thought to be a fundamental organizing principle. In primary visual cortex (V1), neurons respond preferentially to bars of a particular orientation, and, in many mammals, these orientation-selective cells are arranged in a semiregular, smoothly varying map across the cortical surface. Curiously, orientation maps have not been found in rodents or lagomorphs. To explore whether this lack of organization in previously studied rodents could be attributable to low visual acuity, poorly differentiated visual brain areas, or small absolute V1 size, we examined V1 organization of a larger, highly visual rodent, the gray squirrel. Using intrinsic signal optical imaging and single-cell recordings, we found no evidence of an orientation map, suggesting that formation of orientation maps depends on mechanisms not found in rodents. We did find robust orientation tuning of single cells, and this tuning was invariant to stimulus contrast. Therefore, it seems unlikely that orientation maps are important for orientation tuning or its contrast invariance in V1. In vertical electrode penetrations, we found little evidence for columnar organization of orientation-selective neurons and little evidence for local anisotropy of orientation preferences. We conclude that an orderly and columnar arrangement of functional response properties is not a universal characteristic of cortical architecture.

Action Potentials↗
PubMedDate not supplied

Functional and cortical adaptations to central vision loss.

Age-related macular degeneration (AMD), affecting the retina, afflicts one out of ten people aged 80 years or older in the United States. AMD often results in vision loss to the central 15-20 deg of the visual field (i.e. central scotoma), and frequently afflicts both eyes. In most cases, when the central scotoma includes the fovea, patients will adopt an eccentric preferred retinal locus (PRL) for fixation. The onset of a central scotoma results in the absence of retinal inputs to corresponding regions of retinotopically mapped visual cortex. Animal studies have shown evidence for reorganization in adult mammals for such cortical areas following experimentally induced central scotomata. However, it is still unknown whether reorganization occurs in primary visual cortex (V1) of AMD patients. Nor is it known whether the adoption of a PRL corresponds to changes to the retinotopic mapping of V1. Two recent advances hold out the promise for addressing these issues and for contributing to the rehabilitation of AMD patients: improved methods for assessing visual function across the fields of AMD patients using the scanning laser ophthalmoscope, and the advent of brain-imaging methods for studying retinotopic mapping in humans. For the most part, specialists in these two areas come from different disciplines and communities, with few opportunities to interact. The purpose of this review is to summarize key findings on both the clinical and neuroscience issues related to questions about visual adaptation in AMD patients.

Adaptation, Biological↗
PubMed1997

A network model of motion processing in area MT of primates.

A simple and biologically plausible model is proposed to simulate the visual motion processing taking place in the middle temporal (MT) area of the visual cortex in the primate brain. The model is a hierarchical neural network composed of multiple competitive learning layers. The input layer of the network simulates the neurons in the primary visual cortex (V1), which are sensitive to the orientation and motion velocity of the visual stimuli, and the middle and output layers of the network simulate the component MT and pattern MT neurons, which are selectively responsive to local and global motions, respectively. The network model was tested with various simulated motion patterns (random dots of different direction correlations, transparent motion, grating and plaid patterns, and so on). The response properties of the model closely resemble many of the known features of the MT neurons found neurophysiologically. These results show that the sophisticated response behaviors of the MT neurons can emerge naturally from some very simple models, such as a competitive learning network.

Animals↗
PubMed2002

Neural correlates of perceptual learning: a functional MRI study of visual texture discrimination.

Visual texture discrimination has been shown to induce long-lasting behavioral improvement restricted to the trained eye and trained location in visual field [Karni, A. & Sagi, D. (1991) Proc. Natl. Acad. Sci. USA 88, 4966-4970]. We tested the hypothesis that such learning involves durable neural modifications at the earliest cortical stages of the visual system, where eye specificity, orientation, and location information are mapped with highest resolution. Using functional magnetic resonance imaging in humans, we measured neural activity 24 h after a single session of intensive monocular training on visual texture discrimination, performed in one visual quadrant. Within-subject comparisons between trained and untrained eye for targets presented within the same quadrant revealed higher activity in a corresponding retinotopic area of visual cortex. Functional connectivity analysis showed that these learning-dependent changes were not associated with an increased engagement of other brain areas remote from early visual cortex. We suggest that these new data are consistent with recent proposals that the cellular mechanisms underlying this type of perceptual learning may involve changes in local connections within primary visual cortex. Our findings provide a direct demonstration of learning-dependent reorganization at early processing stages in the visual cortex of adult humans.

Adult↗
PubMedDate not supplied

A neural model of the cortical representation of egocentric distance.

Neurons in the visual cortex of monkeys respond selectively to the disparity between the images in the two eyes. Recent recordings have shown that some of the disparity-selective neurons in the primary visual cortex and the posterior parietal cortex are modulated by the distance of fixation. A population of such gain-modulated, disparity-selective neurons forms a set of basis functions of horizontal disparity and distance of fixation that can be used as an intermediate representation for computing egocentric distance. This distributed representation is consistent with psychophysical studies of human depth perception; in contrast, neurons explicitly tuned to distance are not consistent with how we perceive distance. In a population model that includes noise in the firing rates of neurons, the perceived distance is shown to be the estimate of geometrical distance that minimizes the variance of the estimation.

Animals↗
PubMed1988

Functional anatomy of macaque striate cortex. IV. Contrast and magno-parvo streams.

Macaque monkeys were shown achromatic gratings of various contrasts during 14C-2-deoxy-d-glucose (DG) infusion in order to measure the contrast sensitivity of different subdivisions of primary visual cortex. DG uptake is essentially saturated at stimulus contrasts of 50% and above, although the saturation contrast varies with layer and with different criteria. Following visual stimulation with gratings of 8% contrast, stimulus-driven uptake was relatively high in striate layer 4Ca (which receives primary input from the magnocellular LGN layers), but was absent in layer 4Cb (which receives primary input from the parvocellular layers). In this same (magnocellular-specific) stimulation condition, striate layers 4B, 4Ca, and 6 showed strong stimulus-induced DG uptake, and layers 2, 3, 4A, and 5 showed only light or negligible uptake. By comparison to other cases that were shown stimuli of systematically higher contrast, and to a wide variety of DG cases shown very different stimuli, it is evident that information derived from the magnocellular and parvocellular layers in the LGN remains partially, or largely, segregated in its passage through striate cortex, and projects in a still somewhat segregated fashion to different extrastriate areas. The sum of all available evidence suggests that the magnocellular information projects strongly through striate layers 4Ca, 4B, and 6, with moderate input into the blobs in layers 2 + 3, and to blob-aligned portions of layer 4A. Parvocellular-dominated regions of striate cortex include both the blob and interblob portions of layers 2 + 3, 4A, 4Cb, and 5. Because the major striate input to V2 arrives from striate layers 2 + 3, and because the major striate input to MT originates in layer 4B and 6, it appears that area V2 receives information derived largely from the parvocellular LGN layers, and that area MT receives information derived mainly from the magnocellular layers.

Animals↗
PubMed1997

Synchronization of oscillatory responses in visual cortex correlates with perception in interocular rivalry.

In subjects suffering from early onset strabismus, signals conveyed by the two eyes are not perceived simultaneously but in alternation. We exploited this phenomenon of interocular suppression to investigate the neuronal correlate of binocular rivalry in primary visual cortex of awake strabismic cats. Monocularly presented stimuli that were readily perceived by the animal evoked synchronized discharges with an oscillatory patterning in the gamma-frequency range. Upon dichoptic stimulation, neurons responding to the stimulus that continued to be perceived increased the synchronicity and the regularity of their oscillatory patterning while the reverse was true for neurons responding to the stimulus that was no longer perceived. These differential changes were not associated with modifications of discharge rate, suggesting that at early stages of visual processing the degree of synchronicity rather than the amplitude of responses determines which signals are perceived and control behavioral responses.

Animals↗
PubMed1988

Segregation of ON and OFF afferents to ferret visual cortex.

1. ON-center and OFF-center cells are found in separate sublaminae of the ferret's lateral geniculate nucleus (LGN). The purpose of these experiments was to determine whether this segregation is maintained in the projection from the LGN to primary visual cortex (area 17). 2. The distribution of the geniculocortical afferents within area 17 was studied by recording in layer IV after cortical neurons were silenced with kainic acid. 3. In 28 radial penetrations made into layer IV of five kainate-treated ferrets, the center types of 289 single units with response characteristics identical to those of geniculate cells were noted. A Monte Carlo analysis of these data demonstrated that the geniculocortical afferents cluster according to center type. 4. There was no tendency for ON and OFF afferents to occupy separate sublayers within layer IV. 5. The organization of the afferents in the plane of layer IV was studied by making closely spaced electrode penetrations across the dorsal exposed surface of the cortex in three kainate-treated ferrets. A Monte Carlo analysis of these results demonstrated that afferents segregate on the basis of center type, as well as on the basis of ocular dominance, into patches in the plane of layer IV. 6. The surface-mapping results and the results of experiments in which electrode penetrations were made tangential to layer IV indicated that center-type patches can extend over several hundred micrometers. A Monte Carlo analysis of the sizes of the ocular dominance patches and center-type patches provided further support for this conclusion.

Animals↗
PubMed1995

Optical imaging of intrinsic signals as a tool to visualize the functional architecture of adult and developing visual cortex.

One of the most common principles of cortical organization is that neurons with similar response properties are clustered together in space. Thereby the environment is represented in an orderly fashion on the cortical surface in a so-called "cortical map". In primary visual cortex, for instance, neurons with similar orientation preferences are grouped together, forming the orientation preference map. Optical imaging of intrinsic signals allows to investigate the organization of such maps in vivo. Neuronal activity was measured utilizing the fact that the transition from oxy-hemoglobin to hemoglobin in active brain areas can be detected optically by recording changes in light reflectance with a high resolution CCD-camera. When using this technique to look at the exact patterning of orientation preference maps in cat visual cortex a novel principle for the organization of cortical maps was observed: orientation was not organized in parallel bands as had previously been thought but iso-orientation domains were organized radially; orientations from 0 to 180 degrees were laid out in a pinwheel-like fashion around singularities which we termed "orientation-centers". After observing pinwheel patterns in orientation preference maps in adult cat visual cortex it was also investigated how these meticulously arranged maps develop in the cortex of young kittens. Performing chronical recordings in kittens from the age of postnatal day 17 on we were able to observe how orientation maps form already during the third week of life and--under normal conditions--remain largely unchanged thereafter.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1991

A novel cytoarchitectonic area induced experimentally within the primate visual cortex.

The cerebral cortex is divisible into a number of cytoarchitectonic areas, but developmental mechanisms that regulate their number and size remain unknown. Here we provide evidence that reducing the population of selected thalamic fibers projecting into the primary visual cortex (area 17) of monkeys during midgestation induces the formation of a novel cytoarchitectonic area situated along the border of and embedded within area 17. This region, termed area X, differs cytoarchitectonically from both area 17 and the adjacent secondary visual cortex (area 18). We propose that an aberrant combination of thalamic and cortical connections acting on a portion of prospective area 17 deprived of its normal thalamic input may result in formation of a hybrid cortex. Our results support the protomap hypothesis of cortical parcellation and suggest how during evolution new cytoarchitectonic regions may arise by cell-cell interactions that depend on a unique combination of intrinsic properties of cortical neurons and afferent fibers.

Animals↗
PubMed2000

Contralateral monocular dominance in anterior visual cortex confirmed by functional magnetic resonance imaging.

PURPOSE: Although it is known that the damage to anterior striate cortex results in temporal peripheral visual field loss of the contralateral eye in patients with cerebral visual disturbance, the monocularity of anterior striate cortex has not been demonstrated in normal living humans. The aim of this study was to investigate whether this could be shown noninvasively using functional magnetic resonance imaging of the human visual cortex. METHODS: Eleven normal volunteers were studied with functional magnetic resonance imaging during alternating monocular visual stimulation using a 1.5 Tesla scanner. The data were motion corrected and spatially normalized to the standard brain. The monocular activation of the visual cortex was compared with the activation by the other eye. RESULTS: In the individual data analysis, contralateral eye dominance was always observed in the anterior striate cortex. In the group analysis from 11 subjects, the area with contralateral eye dominance was found in the most anterior part of primary visual cortex where the calcarine fissure merged with the parieto-occipital sulcus. CONCLUSIONS: This study shows that the contralateral eye dominance of anterior striate cortex can be detected noninvasively with functional magnetic resonance imaging during monocular visual stimulation. The finding confirms that the anterior striate cortex, where the monocular temporal crescent is represented, is primarily monocular, but the fact that greatest density of retinal ganglion cells and photoreceptors is in the nasal hemiretina must also be taken into account when interpreting these results.

Dominance, Cerebral↗
PubMedDate not supplied

Neural and perceptual adjustments to dim light.

At reduced luminance levels, the visual system integrates light over extended periods of time. Although the general effects of this process are known, specific changes in the visual cortex have not been identified. We have studied the physiological changes that occur during a transition from high to low luminance by measurements of single neurons in the cat's primary visual cortex. Under low-luminance conditions, we find increased latencies, expanded temporal responses, and a loss of temporal structure. This results in temporal-frequency tuning curves that are peaked at relatively low frequencies. To examine parallel perceptual changes, we compared perceived temporal frequency in human subjects under high- and low-luminance conditions. Low-luminance flickering patterns are perceived to modulate at relatively high rates. This occurs even though peak sensitivity is shifted to relatively low temporal frequencies. To explore further the perceptual component, we measured perceived temporal frequency in human subjects with unilateral optic neuritis for whom optic nerve transmission is known to be relatively slow and generally similar to the normal physiological state under low luminance. These subjects also perceive relatively high modulation rates through their affected eye. Considered together, these results demonstrate an inverse relationship between the physiological and the perceptual consequences of reduced stimulus luminance. This relationship may be accounted for by shifts of neuronal population responses between high- and low-luminance levels.

Animals↗
PubMed1994

GABAA receptor subunit immunoreactivity in primate visual cortex: distribution in macaques and humans and regulation by visual input in adulthood.

Subunit proteins that make up functional GABAA receptors were localized immunocytochemistry in the primary visual cortex (area 17) of adult monkeys and humans. Immunoreactivity for the alpha 1, beta 2/3, and gamma 2 subunits is greatest in layers (II-III, IVA and IVC) of monkey area 17 that contain the highest density of GABA neurons and terminals. Immunostaining for each subunit is unevenly distributed in layers II and III, where patches of immunoreactivity correspond to regions of intense cytochrome oxidase (CO) staining, and in layer IVA, where intense immunoreactivity forms a honeycomb pattern identical to the CO staining pattern. Immunoreactivity for the subunits is localized principally within the neuropil, which, by simultaneous comparison with the distribution of microtubule-associated protein immunostaining, was found to include bundles of thin dendrites and zones of numerous dendritic segments. In addition, gamma 2 immunostaining surrounds the somata of a subpopulation of GABAergic neurons, immunoreactive for the calcium-binding protein parvalbumin. All three subunits are present in the somata and processes of neurons that occupy the white matter subjacent to monkey area 17. In human visual cortex, the alpha 1, beta 2/3, and gamma 2 subunits are distributed in a manner similar to that found in monkeys, with relatively intense immunostaining in layers IVC and IVA. In layer IVC, vertical stripes of intense receptor immunostaining (20-30 microns wide) alternate with wider stripes of pale immunostaining (30-60 microns wide). In the upper and lower halves of IVC beta, these stripes form lattices similar to those in layers IVC and IVA of monkeys. Following monocular deprivation by intravitreal injections of TTX in adult monkeys, immunoreactivity for each subunit in layer IVC consists of alternating intensely and lightly stained stripes. Comparison with the pattern of CO staining indicates that intense immunostaining for alpha 1, beta 2/3, and gamma 2 occurs in normal-eye stripes while abnormally light immunostaining is present in deprived-eye stripes. For all three subunits, immunoreactivity in deprived-eye stripes is reduced within 5 d of monocular deprivation and remains abnormally low for deprivations that extend to at least 30 d. These findings indicate that each of several GABAA receptor subunits adopt similar laminar and compartmental distributions in monkey and human area 17 and are likely to be expressed by the same neurons. The deprivation-dependent reduction in immunoreactivity for alpha 1, beta 2/3, and gamma 2 subunits suggests that all are regulated by visually driven activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗
PubMed1981

[Retrograde transneural degeneration in the visual system: neuromorphological study and problems (author's transl)].

The unilateral resection have been done entirely or in part of the primary visual cortex in 10 cats with various postoperational delays. In all cases, some large and medium axons in the primary optic tract show signs of degeneration, leading to their disappearance along with an important glial reaction. As there in similar signs of degeneration, but apparently less important in all normal cats used for control, it is worth-while to reconsider the notion of retrograde transneuronal degeneration after cortical lesion.

Animals↗
PubMed1989

Arborisation pattern and postsynaptic targets of physiologically identified thalamocortical afferents in striate cortex of the macaque monkey.

The monosynaptic targets of different functional types of geniculocortical axons were compared in the primary visual cortex of monkeys. Single thalamocortical axons were recorded extracellularly in the white matter by using horseradish-peroxidase-filled pipettes. Their receptive fields were mapped and classified as corresponding to those of parvi- or magnocellular neurons in the lateral geniculate nucleus. The axons were then impaled and injected intraaxonally with horseradish peroxidase. Two magnocellular (MA) and two parvicellular (PA) axons were successfully recovered and reconstructed in three dimensions. The two MA axons arborised mainly in layer 4C alpha, as did the two PA axons in layer 4C beta. Few collaterals formed varicosities in layer 6. Both MA axons had two large, elongated clumps of bouton (approx. 300-500 x 600-1,200 microns each) and a small clump. One PA axon had two clumps (each with a core appr. 200 microns in diameter); the other had only one (appr. 150-200 microns in axon had 1,380; one MA axon had 3,200 boutons; and those of the more extensive MA axon were not counted. The distribution of postsynaptic targets as well as the number of synapses per bouton has been established for a sample of 150 PA boutons and 173 MA boutons from serial ultrathin sections. The MA axons made on average 2.1 synapses per bouton compared to 1.79 for one PA axon and 2.6 for the other. The sample of boutons taken from the two physiological types of axons contacted similar proportions of dendritic spines (52-68%), shafts (33-47%), and somata (0-3%). The postsynaptic elements were further characterized by immunostaining for GABA. All postsynaptic perikarya and some of the dendrites (4.5-9.5% of all targets) were positive for the amino acid. Near the thalamic synapse GABA-negative dendritic shafts frequently contained lamellar bodies, an organelle identical in structure to spine apparatus. Dendritic shafts and spines postsynaptic to the thalamocortical boutons frequently received an adjacent synapse from GABA-immunoreactive boutons. The similarity between the magno-and parvicellular axons in their targeting of postsynaptic elements, including the GABAergic neurons, suggests that the structural basis of the physiological differences between 4C alpha and 4C beta neurons should be sought in other aspects of the circuitry of layer 4C, such as local cortical circuits, or in the far greater horizontal extent of the thalamocortical and GABAergic axons in layer 4C alpha compared to those in the beta subdivision.

Animals↗
PubMed1993

Development of individual geniculocortical arbors in cat striate cortex and effects of binocular impulse blockade.

This study analyzes the morphological changes in geniculocortical axons terminating in the primary visual cortex of the cat, during the period in which, in normal development, the terminals in layer IV undergo an eye-specific segregation. Geniculocortical afferent fibers were filled anterogradely by the Phaseolus lectin (PHA-L) injected into the main laminae of the LGN. After standard immunohistochemical procedures, single axons were serially reconstructed in two or three dimensions. Experiments were performed in normal kittens and in kittens in which retinal activity was continuously blocked by repeated intraocular injections of TTX. In normal kittens, arbors were reconstructed at four different ages (19, 23, 30-31, and 39 days postnatally) spanning the period during which the geniculocortical projection segregates into eye-specific columns in layer IV (LeVay et al., 1978). Results reveal that sparse but widely extending branches characteristic of young arbors are eliminated during normal development at the same time as selected portions of the arbor grow considerably in length and complexity. The terminal arborizations also subdivide into distinct patches of terminals, consistent with the segregation of left and right eye afferents. In TTX-treated animals, axonal arbors reconstructed at postnatal days 23, 29, and 39 show a complexity and extent of terminal arborization similar to that of normal animals, though more variable in size and degree of elaboration. No progressive changes are evident with age. Further, the majority of arbors reconstructed from TTX-treated animals lack the patchy organization typical of normal animals.

Afferent Pathways↗
PubMed1978

[Evoked responses of the lateral geniculate body to photic stimulation in intact and visually deprived rabbits].

Flash-evoked responses were recorded from the contralateral lateral geniculate nucleus and from the primary visual cortex in 2--3-month-old dark-reared rabbits. Experiments were performed under nembutal anaesthesia in dark-adapted animals. It was found that, in contrast to a significant decrease in the cortical primary response amplitude of visually deprived rabbits, there were no apparent differences either in amplitude or latency and duration of the geniculate nucleus response between dark-reared and control animals. These results suggest that the abnormalities evoked in visually deprived rabbits were localized mainly at the cortical level. Hypothetical mechanisms of functional abnormalities in dark-reared rabbits are discussed.

Animals↗