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PubMed1998

State-dependent receptive-field restructuring in the visual cortex.

To extract important information from the environment on a useful timescale, the visual system must be able to adapt rapidly to constantly changing scenes. This requires dynamic control of visual resolution, possibly at the level of the responses of single neurons. Individual cells in the visual cortex respond to light stimuli on particular locations (receptive fields) on the retina, and the structure of these receptive fields can change in different contexts. Here we show experimentally that the shape of receptive fields in the primary visual cortex of anaesthetized cats undergoes significant modifications, which are correlated with the general state of the brain as assessed by electroencephalography: receptive fields are wider during synchronized states and smaller during non-synchronized states. We also show that cortical receptive fields shrink over time when stimulated with flashing light spots. Finally, by using a network model we account for the changing size of the cortical receptive fields by dynamically rescaling the levels of excitation and inhibition in the visual thalamus and cortex. The observed dynamic changes in the sizes of the cortical receptive field could be a reflection of a process that adapts the spatial resolution within the primary visual pathway to different states of excitability.

Animals↗

PubMed2004

Somatodendritic minicolumns of output neurons in the rat visual cortex.

The apical dendrites of the pyramidal neurons of the cerebral cortex form radial bundles in all species and areas. Using microtubule-associated protein (MAP)2 immunostaining and Voronoi tessellation analysis in the rat visual cortex, we obtained objective criteria to define dendritic bundles in tangential sections: in supragranular layers of the rat visual cortex we found bundles of 6-6.4 dendrites, at a density of 1929 bundles/mm(2) and a centre-to-centre distance of 27 micro m. Using lipophilic tracers to label different pyramidal cell populations, based on the same criteria as in MAP2-immunostained material, we found that in the rat visual cortex the bundles consist of neurons with specific targets. Neurons projecting to the ipsi- or contralateral cortex form bundles together and with neurons projecting to the striatum, but not with those projecting to the superior colliculus, dorsal division of the lateral geniculate nucleus or through the cerebral peduncle. The latter neurons form bundles with neurons projecting to the striatum. Thus, the cerebral cortex is organized in minicolumns of output neurons visible at the earliest ages studied (P3), which might have a higher probability of being interconnected than those outside.

Age Factors↗

PubMed2000

Sparse coding and decorrelation in primary visual cortex during natural vision.

Theoretical studies suggest that primary visual cortex (area V1) uses a sparse code to efficiently represent natural scenes. This issue was investigated by recording from V1 neurons in awake behaving macaques during both free viewing of natural scenes and conditions simulating natural vision. Stimulation of the nonclassical receptive field increases the selectivity and sparseness of individual V1 neurons, increases the sparseness of the population response distribution, and strongly decorrelates the responses of neuron pairs. These effects are due to both excitatory and suppressive modulation of the classical receptive field by the nonclassical receptive field and do not depend critically on the spatiotemporal structure of the stimuli. During natural vision, the classical and nonclassical receptive fields function together to form a sparse representation of the visual world. This sparse code may be computationally efficient for both early vision and higher visual processing.

Action Potentials↗

PubMed1983

[Double orientation tuning of visual cortex neurons in the cat].

Orientation tuning of 148 visual cortex neurons was investigated in immobilized unanaesthetized cats. The light slit of the optimal size flashing in the receptive field was used as a stimulus. It was found that 88 neurons (59%) had double orientation tuning: preferred and additional. Additional orientation was either orthogonal or at a sharp angle to the preferred orientation. In 64% neurons double orientation was found only after a change of the contrast between stimulus and background. This kind of tuning in many neurons appeared only at definite moments after the beginning of the stimulus. Model analysis showed that double orientation tuning might be a consequence of the specific structure of neuronal receptive fields. The functional meaning of double orientation tuning and its role in the detection of visual images signs are discussed.

Animals↗

PubMed1999

Influence of experience on orientation maps in cat visual cortex.

Experience is known to affect the development of ocular dominance maps in visual cortex, but it has remained controversial whether orientation preference maps are similarly affected by limiting visual experience to a single orientation early in life. Here we used optical imaging based on intrinsic signals to show that the visual cortex of kittens reared in a striped environment responded to all orientations, but devoted up to twice as much surface area to the experienced orientation as the orthogonal one. This effect is due to an instructive role of visual experience whereby some neurons shift their orientation preferences toward the experienced orientation. Thus, although cortical orientation maps are remarkably rigid in the sense that orientations that have never been seen by the animal occupy a large portion of the cortical territory, visual experience can nevertheless alter neuronal responses to oriented contours.

Animals↗

PubMed2002

Dorsal visual cortex activity elicited by posture change in a visuo-tactile matching task.

To investigate the process of crossmodal spatial recognition, we examined the effect of posture change on the recognition of a tactile stimulus position. The task was to judge whether a visual and a tactile stimulus, presented to the left or right, were on the same or different sides while subjects crossed or uncrossed their hands. Under a condition which removed the effect of response bias to the left and right, the dorsal visual cortex (area 18/19) and the precuneus were more activated in the crossed hands condition. The dorsal visual cortex activation suggests that the activity of brain areas classically considered to be visual cortex is affected by posture change, and reflects the reciprocal process across different modalities in spatial recognition.

Adult↗

PubMed1985

[Interrelation among properties of visual cortex neurons in the cat].

In acute experiments on immobilized cats 13 functional characteristics of 96 visual cortex neurons were investigated. Regressional and cluster analyses were used to divide these neurons into two subgroups with different density and degree of connections between characteristics. The receptive fields of cells of the first subgroup were localized relatively centrally in the visual field, those of the second subgroup were localized more often on the periphery. A valuable correlation was found in the half of the studied characteristics. In each subgroup the more centrally localized cells with small receptive fields had relatively shorter latencies, lower thresholds, shorter temporal summation, wider intensity range and greater differential sensitivity; their responses were phasic, with high-frequency discharges. The density of valuable correlation of the characteristics varied from 0.21 to 0.99. The amount of these correlations in the first subgroup was two times higher than in the second one. The possible mechanisms of the correlation between the properties of the visual cortex neurons are discussed, as well as their differences in two subgroups and in the cortex and LGB.

Animals↗

PubMed2002

Relationship of correlated spontaneous activity to functional ocular dominance columns in the developing visual cortex.

Utilizing a multielectrode array to record spontaneous and visually evoked activity of cortical neurons in area 17, we investigate the relationship between long-range correlated spontaneous activity and functional ocular dominance columns during early ferret postnatal development (P24-P29). In regions of visual cortex containing alternating ocular dominance patches, periodic fluctuations in correlated activity are observed in which spontaneous activity is most highly correlated between cortical patches exhibiting the same eye preference. However, these fluctuations are present even within large contralateral eye-dominated bands which lack any periodic alternations in ocular dominance. Thus, the organization of ocular dominance columns cannot fully account for the patterns of correlated activity we observe. Our results suggest that patterns of long-range correlated activity reflect an intrinsic periodicity of cortical connectivity that is constrained by segregated eye-specific LGN afferents.

Action Potentials↗

PubMed1983

Influence of the visual cortex on responses of retinal ganglion cells in the rat.

The objective of the present investigation was to answer the following question: Does the visual cortex affect the neuronal firing of retinal ganglion cells in the rat? To test this hypothesis, the visual cortex was inactivated by a reversible cryoblockade. Action potentials of a ganglion cell were recorded from its axon at the optic tract level prior to, during, and following cortical blockade. The results indicated that indeed the visual cortex influenced the retinal output since its inactivation led to a modification of the firing pattern evoked in response to a flash of light. In most cases the modification was an increase of the bursting pattern of the evoked discharges. By contrast cooling nonvisual areas failed to modify ganglion cells' discharge. A comparison between cortico-geniculate and cortico-retinal feedback loops seems to suggest that the first path is involved mostly with the spatial organization of center-surround receptive fields, whereas the second path is associated with temporal aspects of the retinal responses in the rat.

Action Potentials↗

PubMedDate not supplied

[Relationship between level of vigilance and changes in the receptive fields of the cat visual cortex].

The activity of 118 neurones of the primary visual cortex (17th field) was studied in unanesthetized cats, immobilized with d-tubocurarine, in a state of calm wakefulness and in strained attention, alarm. The strained attention was elicited by an air-puff directed to the corner of the closed eye, not used for the photic stimulation. Considerable rearrangments of the receptive field of neurones (94%) were observed in the state of strained attention. In the majority of the studied cortical cells (75%) during 10-30 minutes the excitatory centres of their receptive fields became narrower while their inhibitory periphery widened; in the cells with the inhibitory centres of the receptive fields they were larger in 80% of the cases. It is shown that with dark, mesopic and scotopic adaptation the major characteristics of the changes in the receptive fields remain unaltered. It is assumed that these effects have a behavioural value for the organism, because the created rise in the level of alertness is accompanied by a sharpening of the receptive fields, i.e. by a greater ability to a fine analysis and recognition of visual images.

Adaptation, Physiological↗

PubMed1991

Ontogenesis of the depressant activity of carbachol on synaptic activity in rat visual cortex.

We studied the ontogeny of muscarinic depression in the developing rat visual cortex using carbachol (a nonhydrolyzable cholinergic agonist) application to neocortical slices obtained from four postnatal age groups: 9-10 days, 15 days, 30-40 days and 18 months. Carbachol suppressed the evoked synaptic response of layers II-III to stimulation of layer II-III afferents. Atropine eliminated the carbachol effect, suggesting that it is mediated by muscarinic receptors. The results indicate a significant increase in muscarinic efficacy in the developing rat visual cortex.

Animals↗

PubMed2000

Task-related modulation of visual cortex.

We performed a series of experiments to quantify the effects of task performance on cortical activity in early visual areas. Functional magnetic resonance imaging (fMRI) was used to measure cortical activity in several cortical visual areas including primary visual cortex (V1) and the MT complex (MT+) as subjects performed a variety of threshold-level visual psychophysical tasks. Performing speed, direction, and contrast discrimination tasks produced strong modulations of cortical activity. For example, one experiment tested for selective modulations of MT+ activity as subjects alternated between performing contrast and speed discrimination tasks. MT+ responses modulated in phase with the periods of time during which subjects performed the speed discrimination task; that is, MT+ activity was higher during speed discrimination than during contrast discrimination. Task-related modulations were consistent across repeated measurements in each subject; however, significant individual differences were observed between subjects. Together, the results suggest 1) that specific changes in the cognitive/behavioral state of a subject can exert selective and reliable modulations of cortical activity in early visual cortex, even in V1; 2) that there are significant individual differences in these modulations; and 3) that visual areas and pathways that are highly sensitive to small changes in a given stimulus feature (such as contrast or speed) are selectively modulated during discrimination judgments on that feature. Increasing the gain of the relevant neuronal signals in this way may improve their signal-to-noise to help optimize task performance.

Attention↗

PubMed1975

Timing of major ontogenetic events in the visual cortex of the rhesus monkey.

The place and time of origin, the migration and eventual disposition of neurons of the monkey visual cortex were studied by autoradiography in animals killed at various intervals after 3H-thymidine pulse labeling at embryonic (E) and early postnatal (P) stages. All neurons destined for the visual cortex are generated during about a 2-month period between E45 and E102. Neuron position in the cortical laminae correlates systematically with time of cell origin; neurons destined for deeper cortical positions are generated earlier, and more superficial ones progressively later. Thus, most neurons in layer VI are born between E45 and E60, in layer V between E60 and E70, in layer IV between E70 and E80, and in layers III and II between E80 and E102. No neurons, but numerous glia, are generated within the cortical plate itself. Initially young neurons are produced almost exclusively in the ventricular zone. Later they are probably generated in both ventricular and subventricular zones, and by the end of the proliferative period the subventricular zone becomes the predominant source of new cells. At the time when all neurons destined for the monkey visual cortex have already been produced, primary fissures are barely indicated on the cerebral surface and no secondary fissure appears as yet. Autoradiographic analyses indicate that at early stages young neurons move to the cortical plate relatively synchronously and at a fast rate, whereas at later stages there are considerable differences in the rates of cell migration. At early stages when the migration pathway is relatively short, the external process of the ventricular cell may stretch across almost the entire migratory distance. It is possible that nuclei move without interruption within their own cylinders of cytoplasm, a mechanism which might account for the rapid, synchronous movement of cell bodies as seen in the autoradiographic material. During later stages when young neurons, mostly of subventricular origin, move across a distance which is more than 10 times the length of their leading process, migrating cells follow radial glial guides across the widened intermediate zone and through densely packed cortical plate. This type of cell displacement seems to proceed less synchronously and require more time. It is proposed that two different mechanisms of cell displacement might exist: one in operation at early stages of cortical formation when mostly ventricular cells migrate, and another utilized by subventricular cells at later stages.

Animals↗

PubMed2002

An amplitude equation approach to contextual effects in visual cortex.

A mathematical theory of interacting hypercolumns in primary visual cortex (V1) is presented that incorporates details concerning the anisotropic nature of long-range lateral connections. Each hypercolumn is modeled as a ring of interacting excitatory and inhibitory neural populations with orientation preferences over the range 0 to 180 degrees. Analytical methods from bifurcation theory are used to derive nonlinear equations for the amplitude and phase of the population tuning curves in which the effective lateral interactions are linear in the amplitudes. These amplitude equations describe how mutual interactions between hypercolumns via lateral connections modify the response of each hypercolumn to modulated inputs from the lateral geniculate nucleus; such interactions form the basis of contextual effects. The coupled ring model is shown to reproduce a number of orientation-dependent and contrast-dependent features observed in center-surround experiments. A major prediction of the model is that the anisotropy in lateral connections results in a nonuniform modulatory effect of the surround that is correlated with the orientation of the center.

Animals↗

PubMed1978

Visual discrimination in cats: physical parameters of stimuli and effects of visual cortex ablation.

Correct reaction probability and minimum exposition time in visual form discrimination task were studied under the conditions of varying pattern parameters. The minimal exposition time depended on pattern contrast and pattern background ratio and on the number of screen position variations. However, it remained unchanged when the background luminance and pattern size varied within the suprathreshold range. The deficits of discrimination in the minimal exposition time conditions in animals with damages to the striate and parastriate cortical regions suggest that the visual cortex participates in the analysis of shortterm stimuli and is responsible for prolongation of the visual trace within the time necessary for object discrimination.

Animals↗

PubMed1977

Disinhibition of perigeniculate reticular neurons following chronic ablation of the visual cortex in rats.

Effects of chronic ablation of the visual cortex (VC) were studied in the perigeniculate reticular neurons (PGR neurones) which were located in the thalamic reticular nucleus immediately adjacent to the dorsal nucleus of the lateral geniculate body and identified as the I-cells of Burke and Sefton. In rats with the intact VC the PGR neurons responded to single shock stimulation of the optic tract (OT) with bursts of spike spaced regularly. During the inter-burst period the neurons were inhibited, indicating that except for the primary spike burst, others were postinhibitory rebound excitation. In the VC-ablated PGR neurons there were no changes in the primary spike burst, but the remaining ones were very weak or sometimes missing, suggesting that the inhibition was poorly developed. With double shock stimulation of OT it was established that after showing the primary spike burst, the VC-ablated PGR neurons suffered a less intense inhibition than control. To a diffuse, sustained illumination, the normal PGR neurons showed on- and off- responses, whereas the VC-ablated ones were tonically activated during the presence of illumination. These findings were taken as indicating that the inhibitory mechanism for the PGR neurons were made less active after the VC had been ablated chronically.

Animals↗

PubMed2001

The effects of aging on layer 1 of primary visual cortex in the rhesus monkey.

The effect of age on layer 1 in primary visual cortex was determined in 19 rhesus monkeys of various ages. Twelve of the monkeys had been behaviorally tested. With age layer 1 becomes thinner and the glial limiting membrane becomes thicker. In the neuropil of layer 1 many of the dendrites in old monkeys appear to be degenerating and, as a consequence, electron micrographs from old monkeys display fewer dendritic and spine profiles per unit area than in young monkeys. As determined using both the disector and size-frequency methods, there is also a concomitant decrease in the numerical density of synapses with age. Although there is a significant correlation between the thinning of layer 1 in area 17 and age, there is no significant correlation between either the thinning of layer 1 or its loss of synapses and any of the behavioral measures of memory function obtained from the 12 behaviorally tested monkeys. Similar morphological changes with age occur in layer 1 of prefrontal cortex of these same monkeys, but in area 46 both the thinning of layer 1 and the loss of synapses show a significant correlation with behavioral measures of memory function. These differences between layer 1 in these two cortical areas presumably relate to the fact that prefrontal cortex has a greater role in subserving cognition than does primary visual cortex.

Aging↗

PubMed1989

The effect of bilateral visual cortex lesions on the development of eye movements and perception.

We studied the eye movements and visual perception in a man with extensive bilateral occipital lesions acquired at birth. He had 3/500 acuity, limited visual fields, and extrafoveal fixation attributable to an intact left superior visual cortex. Analysis of digitized electro-oculography and magnetic search coil data showed normal saccade dynamics. He could make voluntary saccades. Smooth pursuit of suprathreshold targets was mostly saccadic and did not improve with added nonvisual cues. Our results support the hypothesis that the visual cortex is crucial to the development of eye movements. The foveal representation in the occipital lobes, missing in this subject, is needed for development of normal smooth pursuit.

Adolescent↗