[Squirrel visual cortex detector neurons discriminating the contours and rapid movements of visual stimuli].
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Mapping receptive fields in primary visual cortex.
Nearly 40 years ago, in the pages of this journal, Hubel and Wiesel provided the first description of receptive fields in the primary visual cortex of higher mammals. They defined two classes of cortical cells, "simple" and "complex", based on neural responses to simple visual stimuli. The notion of a hierarchy of receptive fields, where increasingly intricate receptive fields are constructed from more elementary ones, was introduced. Since those early days we have witnessed the birth of quantitative methods to map receptive fields and mathematical descriptions of simple and complex cell function. Insights gained from these models, along with new theoretical concepts, are refining our understanding of receptive field structure and the underlying cortical circuitry. Here, I provide a brief historical account of the evolution of receptive field mapping in visual cortex along with the associated conceptual advancements, and speculate on the shape novel theories of the cortex may take as a result these measurements.
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Functional magnetic resonance imaging evidence for binocular interactions in human visual cortex.
Using functional magnetic resonance imaging (fMRI), we explored the binocular interactions occurring when subjects viewed dichoptically presented checkerboard stimuli. A flickering radial checkerboard was presented to each eye of the subject, while T2*-weighted images were acquired over the visual cortex with gradient-echo, echoplanar sequences. We compared responses in striate and extrastriate visual cortex under four conditions: both eyes were stimulated at the same time (binocular condition), each eye was stimulated in alternation (monocular condition) or first the one eye then the other eye was stimulated (left eye first - right eye trailing, or vice versa). The results indicate that only the striate area, in and near the calcarine fissure, shows significant differences for these stimulation conditions. These differences are not evident in more remote extrastriate or associational visual areas, although the BOLD response in the stimulation-rest comparison was robust. These results suggest that the effect could be related to inhibitory interactions across ocular dominance columns in striate visual cortex.
Adult↗
Directional selectivity in the responses of units in cat primary visual cortex to passive eye movement.
The responses of single units in the primary visual cortex (Area 17) of anaesthetized, paralysed cats, to passive movement of the ipsilateral eye were studied. Responses to passive eye movement were found in about one-third of the cortical units isolated. Appropriate control experiments excluded visual, auditory and cutaneous inputs as the source of the effective signal during passive eye movement. The magnitudes of the responses to a number (usually four) of radial directions of passive eye movement were estimated from sets of peristimulus time histograms "interleaved" in time. Units were defined as "radially selective" if the responses to movement along one radius (e.g. vertically upwards) exceeded that along at least one other orthogonal radius (e.g. horizontal-temporal). Of 60 units tested, 53 (88%) were "radially selective" according to this definition. Some of the "radially selective" units showed an additional type of specificity to passive eye movement: (a) Some units responded preferentially to movement along one of the arcs of passive eye movement which were tested (e.g. vertical movement above the equator of the orbit). These units we have called "arc selective". (b) Other units were sensitive to the direction of movement and preferred movement in a particular direction over more than one arc (e.g. horizontal movement towards the temporal side in both nasal and temporal halves of the orbit). These we have called "direction selective". Twenty-one "radially selective" units showed one of these additional properties, nine were arc selective and twelve were direction selective. The implications of these results for the understanding of the function of orbital proprioceptive signals in the cortex are discussed briefly. Responses to passive eye movement were found in all of layers II-VI in Area 17 and the implications of this for the understanding of the pathway by which orbital proprioceptive signals reach the primary visual cortex are discussed. The experiments have shown that many units in cat visual cortex respond to passive eye movement and that most of these units have some specificity for particular radial directions of movement while some have additional specific properties. We believe that these properties of radial, directional and arc sensitivity are likely to be important in understanding the function of the orbital proprioceptive signal which arises during eye movement and they are particularly interesting in relation to the findings of others that this proprioceptive signal appears to be concerned in the normal development of visual properties in the cortex and in the control of visually guided movement in adult cats.
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The organization of subcortical projections of the hamster's visual cortex.
The subcortical projections of the hamster's visual cortex were determined by use of injections of tritiated proline and heat lesions placed in different cortical loci. The brains were processed for autoradiography and silver impregnation of degenerating axons. Striate cortex was shown to project ipsilaterally to the dorsocaudal region of the caudate nucleus, a dorsolateral area within the thalamic reticular nucleus (RT), a laterodorsal region of the nucleus lateralis anterior (LA), the rostral half of nucleus lateralis posterior (LP), the whole territory of the dorsal (dLGN) and ventral (vLGN) geniculate nuclei, the anterior (PA) and posterior (PP) pretectal nuclei, the superior colliculus (SC), and the precerebellar pontine nuclei. In addition, the medial visual area (18b) was shown to project to a medial band of LA and part of the caudal half of LP, while the adjoining parietal cortex was seen to terminate in a lateral part of the caudate, a ventral band of LA, and the ventral half of rostral LP. Segregation of different cortical inputs was clear in LA, LP, caudate, and pons. The projections to dLGN, vLGN, SC, LP, and PA were retinotopically organized. Clear evidence of some topography was found within RT, PP, and the pons, although a consistent map could not be derived from the data.
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[Expansion and thickness of the visual cortex of microphthalmic mice and healthy Wistar animals].
In connection with our studies about the development and the structure of the visual system of the brain of normal and genetically microphthalmic mice in this paper we have investigated the extend of the visual cortex and the thickness of the primary visual cortex (area Oc1). The most important results are: The size of the surface of the neocortex and of the areas Oc1 and Oc2m shows no significant differences between the normal and the microphthalmic mice. The whole thickness of area Oc1 is at day 20 in the microphthalmic mice significantly greater than in normal animals. In the microphthalmic mice lamina I, III and V are thicker than in normal mice. Lamina IV is in microphthalmic mice significantly thinner than in normal animals. The interpretation of these results is difficult because at PD 20 the cortex and his laminae are not full developed. But there are in the microphthalmic mice not such clear changes (reduction) on the cortical level as in the subcortical structures of the visual system. The reduction of the thickness of lamina IV maybe caused by a reduced projection from the lateral geniculate nucleus.
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The quantitative effects of dark-rearing and light exposure on the laminar composition and depth distribution of neurons and glia in the visual cortex (area 17) of the rat.
The effects of dark-rearing and light-exposure on the distribution of neurons and glial cells types in the rat visual cortex (area 17) have been investigated. Three groups of animals were studied: rats reared in the dark until weaning at 21 days post natum (21 DPN) and subsequently light-exposed for 31 days (Group 21/31); rats dark-reared until 52 DPN and then exposed to light for 3 days (Group 3 dL); and rats totally dark-reared until 52 DPN (Group 52 dD). Semithin sections tangential to the pial surface were obtained at sampling intervals 50 micron apart throughout the depth of the left visual cortex. The volume numerical densities of neurons, astroglia, oligodendroglia, and microglia, at each sampling strata in the cortex were calculated using stereological techniques. The laminer density and distribution of neurons was not significantly different between the three groups. In comparison with group 21/31 there was a marked reduction in the densities of astroglia, oligodendroglia, and microglia in lower layer 5 of groups 3 dL and 52 dD. Additionally, the density of microglia in thalamorecipeint layer 4 was greatly increased in group 3 dL compared with groups 21/31 and 52 dD. These results indicate specific alterations in the glial cell composition of the rat visual cortex following periods of dark-rearing and light-exposure. Furthermore, changes in the density of glial cells in layer 5 may reflect functional modifications in neurons projecting to the superior colliculus.
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Functional specialisation in the visual cortex of the rhesus monkey.
Anatomical and functional studies of the visual cortex of the rhesus monkey have shown that it is made up of a multiplicity of distinct areas. These seem to be functionally specialised to analyse different features of the visual environment.
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Difference in the representation of cardinal and oblique contours in cat visual cortex.
Optical imaging based on intrinsic signals was used to analyze the structure of orientation column in area 18 of cat visual cortex. Recordings were obtained in 12 adult cats during presentation of contours in various orientations. In order to quantitatively evaluate the size of the response area, a method combining both information on the amplitude of the changes and the statistical significance of the changes was proposed. The results showed that the proportion of the cortical area activated reached a peak during the presentation of horizontal or vertical contours, whereas the areas activated by oblique contours were relatively smaller. Cardinal contours (0 degrees and 90 degrees ) activated 50.63 +/- 1.88% (mean +/- SE) of the cortical area in the recording site, whereas oblique contours (45 degrees and 135 degrees ) activated only 45.25 +/- 2.02% of the area. This difference in area was statistically significant (t = 2.38, d.f. = 11, and P < 0.05). The results demonstrated a difference in the representations of the cardinal and oblique contours in area 18 of the cat visual cortex.
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Emotional facilitation of sensory processing in the visual cortex.
A key function of emotion is the preparation for action. However, organization of successful behavioral strategies depends on efficient stimulus encoding. The present study tested the hypothesis that perceptual encoding in the visual cortex is modulated by the emotional significance of visual stimuli. Event-related brain potentials were measured while subjects viewed pleasant, neutral, and unpleasant pictures. Early selective encoding of pleasant and unpleasant images was associated with a posterior negativity, indicating primary sources of activation in the visual cortex. The study also replicated previous findings in that affective cues also elicited enlarged late positive potentials, indexing increased stimulus relevance at higher-order stages of stimulus processing. These results support the hypothesis that sensory encoding of affective stimuli is facilitated implicitly by natural selective attention. Thus, the affect system not only modulates motor output (i.e., favoring approach or avoidance dispositions), but already operates at an early level of sensory encoding.
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Primary visual cortex volume and total neuron number are reduced in schizophrenia.
A number of studies that assessed the visual system in subjects with schizophrenia found impairments in early visual processing. Furthermore, functional imaging studies suggested changes in primary visual cortex activity in subjects with schizophrenia. Interestingly, postmortem studies of subjects with schizophrenia reported an increased density of neurons in the primary visual cortex (Brodmann's area 17, BA17). The observed changes in visual processing may thus be reflected in structural changes in the circuitry of BA17. To characterize the structural changes further we used stereological methods based on unbiased principles of sampling (Cavalieri's principle and the optical fractionator) to estimate the total volume and neuron number of BA17 in postmortem brains from 10 subjects with schizophrenia and 10 matched normal comparison subjects. In addition, we assessed cortical thickness. We found a marked and significant reduction in total neuron number (25%) and volume (22%) of BA17 in the schizophrenia group relative to the normal comparison subjects. In contrast, we found no changes in neuronal density or cortical thickness between the two groups. Subjects with schizophrenia therefore have a smaller cortical area allocated to primary visual perception. This finding suggests the existence of a schizophrenia-related change in cortical parcellation.
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Local circuits in primary visual cortex of the macaque monkey.
The basic laminar organization of excitatory local circuitry in the primary visual cortex of the macaque monkey is similar to that described previously in the cat's visual cortex (Gilbert 1983). This circuitry is described here in the context of a two-level model that distinguishes between feedforward and feedback connections. Embedded within this basic framework is a more complex organization. Within the strictly feedforward pathway, these circuits distribute unique combinations of magno-, parvo-, and koniocellular input from the lateral geniculate nucleus (LGN) to neurons in layers 2-4B. Their input is dependent on the extrastriate cortical areas they target. The local feedback connections from deep layers (5 and 6) arise from a diverse population of pyramidal neurons. Each type forms local connections with a unique relationship to more superficial layers. In the case of layer 6 neurons, these connections are closely related to layer 4 subdivisions receiving input from different functional streams.
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Modulation of neural activities during visual processing in the human extrastriate visual cortex.
Magnetoencephalographic responses to single letters (Japanese monosyllabic characters) presented in the left visual field were measured during visual cognition tasks, in which subjects judged matching of characters in two different processes based on phonological and graphical cues. Equivalent current dipoles, which represent focal neural activities, were localized in the extrastriate visual cortex of the occipital to occipitotemporal regions. The main activities were observed in the right lateral area when the subjects detected shape, and also in the medial and inferior areas of both hemispheres when they detected rhyme. These results suggest that the neural activities in the extrastriate cortex, which are related to early processing of familiar visual forms, can be modified by the top-down control.
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Colour coding in the cerebral cortex: the responses of wavelength-selective and colour-coded cells in monkey visual cortex to changes in wavelength composition.
The reaction of wavelength-selective (WL), wavelength-opponent (WLO) and colour-coded (CO) cells in monkey visual cortex to changes in the wavelength composition of the light reflected from the area in their receptive fields was studied, using multicoloured displays. Wavelength-selective and wavelength-opponent cells were found to be very sensitive to changes in the wavelength composition of the light reflected from the areas in their receptive fields, irrespective of their perceived natural and void colours. Changes in the wavelength composition of the light reflected from surrounding areas did not affect their responses. They were also sensitive to the order in which lights of various wavelengths illuminated the areas in their receptive fields. Colour-coded cells were not affected by changes in the wavelength composition nor were they sensitive to the sequence with which the area in their receptive fields was illuminated by lights of different wavelengths. However, they required that the display, with the area of their preferred colour in their receptive fields, be trichromatically illuminated. This and other evidence suggested that such cells were sensitive not only to the illumination of the area in their receptive fields, but of surrounding areas as well. This evidence reinforces further the distinction between wavelength-selective and colour-coded cells and leads to the conclusion that one function of the wavelength-selective cells must be to register the changes in wavelength composition which occur throughout the day.
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GAP-43 in the cat visual cortex during postnatal development.
GAP-43 levels have been determined by immunoassay in cat visual cortex during postnatal development to test the idea that GAP-43 expression could be related to the duration of the critical period for plasticity. For comparison, GAP-43 levels have also been assayed in primary motor cortex, primary somatosensory cortex, and cerebellum at each age. GAP-43 levels were high in all regions at 5 d (with concentrations ranging from 7-10 ng/microgram protein) and then declined 60-80% by 60 d of age. After 60 d of age, GAP-43 concentrations in each region continued a slow decline to adult values, which ranged from 0.5-2 ng/microgram protein. To test for the involvement of GAP-43 in ocular dominance plasticity during the critical period, the effect of visual deprivation on GAP-43 levels was investigated. Monocular deprivation for 2-7 d, ending at either 27 or 35 d of age, had no effect on total membrane levels of GAP-43. The concentrations of membrane-associated GAP-43 prior to 40 d of age correlate with events that occur during postnatal development of the cat visual cortex. However, the slow decline in membrane-associated GAP-43 levels after 40 d of age may be an index of relative plasticity remaining after the peak of the critical period.
Aging↗
[Modulation of the activity of the visual cortex neurons by various non-visual stimuli in awake rabbits].
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[Disinhibition as a mechanism of tuning of cross-like figures in the visual cortex neurons].
A discrete neural net was used for simulation of cross-sensitivity in 40% of neurones of the cat visual cortex' area 17th. It is based on disinhibition of the end-stopping inhibition in receptive field from the side-disinhibitory zone. Highly selective or invariant sensitivity of the simulated neurone in respect to shape and orientation of a cross-like figure was observed under changes of location, size and weight of the receptive field zones. The disinhibitory mechanism seems to be critically involved in the selection of the second-order features of the images in the primary visual cortex.
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Blockade of postsynaptic activity in sleep inhibits developmental plasticity in visual cortex.
Ocular dominance plasticity is enhanced by sleep and reduced by sleep deprivation or when all neural activity in the sleeping visual cortex is reversibly inhibited. These latter findings demonstrate that the mechanisms responsible for the effects of sleep on cortical plasticity are activity dependent. To better isolate this activity dependent mechanism, we investigated the role of postsynaptic activity by inactivating the sleeping visual cortex after a period of monocular deprivation with the gamma-amino-n-butyric acid agonist muscimol. Microelectrode recordings showed that ocular dominance plasticity was significantly reduced in cortices reversibly silenced during sleep compared with cortices infused with vehicle only. These findings demonstrate that postsynaptic activity during sleep is required for the consolidation of experience-dependent cortical plasticity.
Action Potentials↗