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PubMed1995

Development and plasticity of cortical processing architectures.

One of the basic functions of the cerebral cortex is the analysis and representation of relations among the components of sensory and motor patterns. It is proposed that the cortex applies two complementary strategies to cope with the combinatorial problem posed by the astronomical number of possible relations: (i) the analysis and representation of frequently occurring, behaviorally relevant relations by groups of cells with fixed but broadly tuned response properties; and (ii) the dynamic association of these cells into functionally coherent assemblies. Feedforward connections and reciprocal associative connections, respectively, are thought to underlie these two operations. The architectures of both types of connections are susceptible to experience-dependent modifications during development, but they become fixed in the adult. As development proceeds, feedforward connections also appear to lose much of their functional plasticity, whereas the synapses of the associative connections retain a high susceptibility to use-dependent modifications. The reduced plasticity of feedforward connections is probably responsible for the invariance of cognitive categories acquired early in development. The persistent adaptivity of reciprocal connections is a likely substrate for the ability to generate representations for new perceptual objects and motor patterns throughout life.

Action Potentials↗
PubMed1992

Activity-dependent decrease in NMDA receptor responses during development of the visual cortex.

Plasticity of the developing visual system has been regarded as the best model for changes of neuronal connections under the influence of the environment. N-methyl-D-aspartate (NMDA) receptors are crucial for experience-dependent synaptic modifications that occur in the developing visual cortex. NMDA-mediated excitatory postsynaptic currents (EPSCs) in layer IV neurons of the visual cortex lasted longer in young rats than in adult rats, and the duration of the EPSCs became progressively shorter, in parallel with the developmental reduction in synaptic plasticity. This decrease in NMDA receptor-mediated EPSC duration is delayed when the animals are reared in the dark, a condition that prolongs developmental plasticity, and is prevented by treatment with tetrodotoxin, a procedure that inhibits neural activity. Application of L-glutamate to outside-out patches excised from layer IV neurons of young, but not of adult, rats activated prolonged bursts of NMDA channel openings. A modification of the NMDA receptor gating properties may therefore account for the age-dependent decline of visual cortical plasticity.

Action Potentials↗
PubMed1991

Where practice makes perfect in texture discrimination: evidence for primary visual cortex plasticity.

In terms of functional anatomy, where does learning occur when, for a basic visual discrimination task, performance improves with practice (perceptual learning)? We report remarkable long-term learning in a simple texture discrimination task where learning is specific for retinal input. This learning is (i) local (in a retinotopic sense), (ii) orientation specific but asymmetric (it is specific for background but not for target-element orientation), and (iii) strongly monocular (there is little interocular transfer of learning). Our results suggest that learning involves experience-dependent changes at a level of the visual system where monocularity and the retinotopic organization of the visual input are still retained and where different orientations are processed separately. These results can be interpreted in terms of local plasticity induced by retinal input in early visual processing in human adults, presumably at the level of orientation-gradient sensitive cells in primary visual cortex.

Discrimination, Psychological↗
PubMed1982

Central core control of developmental plasticity in the kitten visual cortex: II. Electrical activation of mesencephalic and diencephalic projections.

Fifteen dark-reared, 4- to 5-week-old kittens were stimulated monocularly with patterned light while they were anesthetized and paralyzed. Six of these kittens were exposed to the light stimuli only, in four kittens the light stimuli were paired with electric stimulation of the mesencephalic reticular formation and in five kittens with electric activation of the medial thalamic nuclei. Throughout the conditioning period, the ocular dominance of neurons in the visual cortex was determined from evoked potentials that were elicited either with electric stimulation of the optic nerves or with phase reversing gratings of variable spatial frequencies. In two kittens, ocular dominance changes were assessed after the end of the conditioning period by analyzing single unit receptive fields. Monocular stimulation with patterned light induced a marked shift of ocular dominance toward the stimulated eye, when the light stimulus was paired with electric activation of either the mesencephalic reticular formation or of the medial thalamus. Moreover, a substantial fraction of cells acquired mature receptive fields. No such changes occurred with light or electric stimulation alone. It is concluded that central core projections which modulate cortical excitability gate experience-dependent modifications of connections in the kitten visual cortex.

Animals↗
PubMed1992

Changes of drebrin expression in the visual cortex of the cat during development.

The expression of and developmental changes in drebrin were studied in cat visual cortex using immunohistochemistry and immunoblot analysis. Drebrin is a developmentally regulated brain protein which in the chicken has characteristic changes in expression related to developmental stage. A monoclonal antibody (MAb M2F6) raised against drebrin, was found to label the neuropil of the kitten visual cortex in the early postnatal period. At 1-3 weeks of age, the staining was prominent in layer IV of the visual cortex. The immunoreactivity, however, was found to be dramatically decreased around the end of the sensitive period for ocular dominance plasticity (approximately 3 months of age). In the adult visual cortex, almost no immunostaining was observed. These developmental changes revealed by an immunohistochemical method were confirmed using immunoblot analysis. Upon immunoblot analysis after SDS-PAGE of protein from the kitten visual cortex, MAb M2F6 was found to recognize two protein bands with molecular weights of 130 kDa (drebrin E) and 140 kDa (drebrin A). The developmental profile of the intensity of the two bands of the drebin closely parallels in time the postnatal changes in cortical susceptibility to visual deprivation. These results indicate that the expression of drebrin in kitten visual cortex is restricted to the early postnatal period and suggest that it may play an important role in the experience-dependent modification of cortical circuitry during the sensitive period.

Aging↗
PubMed1990

Excitatory amino acid receptors and synaptic plasticity.

Excitatory amino acid receptors are the mediators of synaptic transmission at many synapses that can undergo use-dependent modifications of synaptic efficiency. They also play an essential role in the induction of these plastic changes. Graham Collingridge and Wolf Singer describe how NMDA receptors can endow synapses with hebbian-like properties and discuss how these may be used by vertebrates for associative learning and experience-dependent modifications of synaptic connections during development. The role of AMPA receptors in the maintenance of long-term potentiation is also discussed.

Humans↗
PubMed1982

Central core control of developmental plasticity in the kitten visual cortex: I. Diencephalic lesions.

In five, dark-reared, 4-week-old kittens the posterior two thirds of the corpus callosum were split, and a lesion comprising the intralaminar nuclei was made of the left medial thalamic complex. In addition, the right eye was closed by suture. Post-operatively, the kittens showed abnormal orienting responses, neglecting visual stimuli presented in the hemifield contralateral to the side of the lesion. Sudden changes in light, sound, or somatosensory stimulation elicited orienting responses that all tended toward the side of the lesion. These massive symptoms faded within a few weeks but the kittens continued to neglect visual stimuli in the hemifield contralateral to the lesion when a second stimulus was presented simultaneously in the other hemifield. Electrophysiologic analysis of the visual cortex, performed after the end of the critical period, revealed marked interhemispheric differences. In the visual cortex of the normal hemisphere most neurons were monocular and responded exclusively to stimulation of the open eye, but otherwise had normal receptive field properties. In the visual cortex of the hemisphere containing the thalamic lesion, the majority of the neurons remained binocular. In addition, the selectivity for stimulus orientation and the vigor of responses to optimally aligned stimuli were subnormal on this side. Thus, the same retinal signals, which in the control hemisphere suppressed the pathways from the deprived eye and supported the development of normal receptive fields, failed to do either in the hemisphere containing the thalamic lesion. Apparently, experience-dependent changes in the visual cortex require both retinal stimulation and the functioning of diencephalic structures which modulate cortical excitability and control selective attention.

Animals↗
PubMed1990

Long-term potentiation and depression in the cerebral neocortex.

Long-term potentiation of synaptic efficacy following tetanic synaptic inputs was described originally in the hippocampus, and it has been studied extensively based on the hypothesis that it represents a synaptic model of learning and memory in the brain. In the cerebral neocortex, studies on LTP have burgeoned later, and have progressed less rapidly than those in the hippocampus. Recently, however, experimental data describing the phenomenology and the mechanisms underlying LTP have accumulated in the neocortex, particularly in the visual, somatosensory, and motor cortices. In the developing visual cortex, LTP has been induced by afferent tetanic stimulation at relatively low frequencies, for long duration. Thus, particular attention has been given to parameters of the tetanus optimal for the induction of cortical LTP, and the differences between these and those effective in inducing hippocampal LTP have been reviewed. In the motor cortex, the associative LTP following combined activation of separate sites as well as homosynaptic LTP following activation of single pathways have been reported and these types of synaptic plasticity have been suggested as being a basis for a certain type of motor learning. Long-lasting depression (LTD) of synaptic efficacy also has been reported in the developing visual cortex and suggested as a neural basis for experience-dependent modifications of visual cortical neurons. LTD has been found in other areas of the neocortex as well, although the probability of its induction is relatively low and its functional significance is not yet clear. Among the possible mechanisms for the induction of LTP and LTD, those including the involvement of NMDA receptors, protein kinase C, Ca2+/calmodulin-dependent kinase II, and membrane-associated cytoskeletal proteins have been reviewed, although the results obtained so far are only fragmentary and are premature for definitive conclusions to be drawn.

Animals↗
PubMed1990

Synaptic plasticity in fear conditioning circuits: induction of LTP in the lateral nucleus of the amygdala by stimulation of the medial geniculate body.

Electrical stimulation of the medial geniculate body in the anesthetized rat produces an evoked potential in the lateral nucleus of the amygdala. The potential varies in amplitude with stimulus intensity and reaches peak amplitude in 8.5 msec on the average. High-frequency stimulation of the pathway produces long-lasting increases in the amplitude and slope of the potential. These robust and enduring experience-dependent modifications in neural transmission occur in a pathway known to be involved in the formation of emotional memories and may offer a means for examining the cellular mechanisms of emotional learning, as well as a new approach to questions concerning the relevance of long-term potentiation to normal mnemonic processes.

Amygdala↗
PubMed1995

Functional MRI evidence for adult motor cortex plasticity during motor skill learning.

Performance of complex motor tasks, such as rapid sequences of finger movements, can be improved in terms of speed and accuracy over several weeks by daily practice sessions. This improvement does not generalize to a matched sequence of identical component movements, nor to the contralateral hand. Here we report a study of the neural changes underlying this learning using functional magnetic resonance imaging (MRI) of local blood oxygenation level-dependent (BOLD) signals evoked in primary motor cortex (M1). Before training, a comparable extent of M1 was activated by both sequences. However, two ordering effects were observed: repeating a sequence within a brief time window initially resulted in a smaller area of activation (habituation), but later in larger area of activation (enhancement), suggesting a switch in M1 processing mode within the first session (fast learning). By week 4 of training, concurrent with asymptotic performance, the extent of cortex activated by the practised sequence enlarged compared with the unpractised sequence, irrespective of order (slow learning). These changes persisted for several months. The results suggest a slowly evolving, long-term, experience-dependent reorganization of the adult M1, which may underlie the acquisition and retention of the motor skill.

Adult↗
PubMed1988

Experience-dependent modifications of kitten striate cortex are not prevented by thalamic lesions that include the intralaminar nuclei.

It has been shown previously that surgical lesions of the antero-medial thalamus interfere with ocular dominance modifications that normally result from monocular deprivation in young kittens (Singer 1982). The aim of the present study was to determine whether this effect was due specifically to the destruction of the visual cortical projections of the anterior intralaminar nuclei. We report here that large excitotoxin lesions of the anterior dorsal thalamus have no effect on the cortical response to monocular deprivation. These data indicate that the intralaminar projection is not essential for ocular dominance plasticity.

Animals↗
PubMed1991

Stimulation of phosphoinositide turnover by excitatory amino acids. Pharmacology, development, and role in visual cortical plasticity.

Theoretical analysis suggests that in the visual cortex during early postnatal development, afferent activity can yield either an increase or a decrease in synaptic strength depending on the pattern of EAA receptor activation in cortical neurons. This motivated us to study the mechanism of EAA-stimulated phosphoinositide turnover in visual cortex. Available evidence suggests that PI hydrolysis is stimulated by EAAs primarily at a single receptor site (Q2 receptor), and that this site is distinct from both the traditional quisqualate (Q1) receptor and the NMDA receptor. NMDA does, however, inhibit EAA-stimulated PI turnover in visual cortex, confirming that the Q2 receptor is on visual cortical neurons (as opposed to glia). We find that Q2 receptors in the neocortex are expressed transiently during postnatal development. The developmental time-course of EAA-stimulated PI turnover correlates precisely with the critical period when synaptic modifications are most readily elicited in visual cortex by changes in sensory experience. The compound AP3 can inhibit EAA-stimulated PI turnover, probably by acting as a partial Q2 agonist, and under some circumstances AP3 evidently can interfere with experience-dependent synaptic modifications. Increases in synaptic strength in visual cortex, as elsewhere, have been linked specifically to activation of NMDA receptors. We propose that decreases in synaptic strength may be specifically related to activation of the Q2 receptor. Further tests of this hypothesis will require the development of selective and potent antagonists.

Amino Acids↗
PubMed1992

Plasticity of binocular visual connections in the frog, Xenopus laevis: reversibility of effects of early visual deprivation.

In frogs, each lobe of the midbrain optic tectum receives two spatially aligned maps of binocular visual space, one through each eye. The map from the contralateral eye is the product of direct retinal input to the tectum, while that from the ipsilateral eye is mediated by a commissural system of connections that relays binocular visual information from one tectal lobe to the other. In Xenopus laevis, dark-reared from early life, the visuotectal map through the ipsilateral eye exhibits significant signs of disorder indicating that the normal development of the underlying "intertectal" system involves experience-dependent processes. Here we investigate the potential of this system for recovery from such deprivation-induced disruption. Three groups of animals were first dark-reared from embryonic stages until 3 months, 12 months or 2 years after metamorphosis. These short-, medium- and long-term periods of visual deprivation are associated with a progressive disturbance of the intertectal system (Grant and Keating 1989b). The animals then experienced 12-18 months in a normal visual environment, before an electrophysiological mapping experiment was carried out to reveal the status of their intertectal system. Quantitative analyses of the visuotopic order in the ipsilateral visuotectal map and of its spatial alignment with the map derived from the contralateral eye revealed that these features were completely normal in all 3 experimental groups. It is concluded that defects present in the intertectal system of dark-reared Xenopus can be repaired by subsequent visual experience, and that the reparative capacity is not limited either by the animal's age or by the existing degree of prior disruption.

Animals↗
PubMed1995

Differential rearing alters spine density on medium-sized spiny neurons in the rat corpus striatum: evidence for association of morphological plasticity with early response gene expression.

Morphological plasticity of medium-sized spiny neurons of the striatum was examined in Long-Evans hooded rats reared in complex or individual cage environments. Rat pups, aged 28-32 days, were housed either individually in standard laboratory cages or as a group in a large toy- and object-filled environment for 30 days. The spine density on dendrites of medium-sized spiny neurons in the dorsolateral striatum was then examined using the Golgi-Cox method. Rats reared in the complex environment displayed an increase of approximately 30% in spine density relative to those reared individually. These results demonstrate experience-dependent changes in neural structure in the striatum and suggest that the mechanisms for information storage in response to experience may be more widespread in the forebrain than previously believed.

Animals↗
PubMed1984

Somatosensory cortical map changes following digit amputation in adult monkeys.

The cortical representations of the hand in area 3b in adult owl monkeys were defined with use of microelectrode mapping techniques 2-8 months after surgical amputation of digit 3, or of both digits 2 and 3. Digital nerves were tied to prevent their regeneration within the amputation stump. Successive maps were derived in several monkeys to determine the nature of changes in map organization in the same individuals over time. In all monkeys studied, the representations of adjacent digits and palmar surfaces expanded topographically to occupy most or all of the cortical territories formerly representing the amputated digit(s). With the expansion of the representations of these surrounding skin surfaces (1) there were severalfold increases in their magnification and (2) roughly corresponding decreases in receptive field areas. Thus, with increases in magnification, surrounding skin surfaces were represented in correspondingly finer grain, implying that the rule relating receptive field overlap to separation in distance across the cortex (see Sur et al., '80) was dynamically maintained as receptive fields progressively decreased in size. These studies also revealed that: the discontinuities between the representations of the digits underwent significant translocations (usually by hundreds of microns) after amputation, and sharp new discontinuous boundaries formed where usually separated, expanded digital representations (e.g., of digits 1 and 4) approached each other in the reorganizing map, implying that these map discontinuities are normally dynamically maintained. Changes in receptive field sizes with expansion of representations of surrounding skin surfaces into the deprived cortical zone had a spatial distribution and time course similar to changes in sensory acuity on the stumps of human amputees. This suggests that experience-dependent map changes result in changes in sensory capabilities. The major topographic changes were limited to a cortical zone 500-700 micron on either side of the initial boundaries of the representation of the amputated digits. More distant regions did not appear to reorganize (i.e., were not occupied by inputs from surrounding skin surfaces) even many months after amputation. The representations of some skin surfaces moved in entirety to locations within the former territories of representation of amputated digits in every monkey studied. In man, no mislocation errors or perceptual distortions result from stimulation of surfaces surrounding a digital amputation.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1990

Ocular dominance plasticity and developmental changes of 5'-nucleotidase distributions in the kitten visual cortex.

The distribution of the adenosine-producing ecto-enzyme 5'-nucleotidase was investigated histochemically in the visual cortex of normally reared and monocularly deprived kittens and cats. In normally reared kittens aged between 11 to 44 days, 5'-nucleotidase activity formed a band of intense neuropil staining throughout cortical layer IV of areas 17 and 18. The other layers were almost devoid of reaction product. Between the 4th and 6th week, this band had a patchy appearance in area 17, the center-to-center spacing of 5'-nucleotidase patches being approximately 1 mm. Monocular enucleation accentuated these patches of enhanced 5'-nucleotidase activity or made them reappear at developmental stages at which they had normally faded. Simultaneous visualization of ocular dominance columns by transneuronal transport of intraocularly injected 3H-proline showed that the patches of enhanced 5'-nucleotidase activity coincided with the territories of afferents from the intact eye. With increasing age and normal visual development, the patches disappeared and 5'-nucleotidase activity spread to the supra- and infragranular layers. The adult pattern was characterized by dense staining of all cortical laminae in both areas 17 and 18 and was established at about 8 weeks of age. At approximately 7 weeks of age, when the patches in layer IV had disappeared in the course of normal development, monocular enucleation caused a reappearance of the discontinuous pattern of 5'-nucleotidase activity in layer IV. These results reveal a close relation between the distribution of 5'-nucleotidase and the time course of the developmental phase during which the visual cortex is susceptible to experience-dependent alterations. As suggested by the correlation between sites of enzyme activity and eye dominance columns, the expression of 5'-nucleotidase patches in layer IV appears to be associated with the remodelling of ocular dominance territories that occurs both in normal development and after manipulation of afferent retinal input. Thus, 5'-nucleotidase is likely to serve a function in activity-dependent modifications of cortical circuitry. Moreover, 5'-nucleotidase activity is the only endogenous marker known to date that exhibits a columnar pattern in cat visual cortex.

5'-Nucleotidase↗
PubMed1992

Agonists of cholinergic and noradrenergic receptors facilitate synergistically the induction of long-term potentiation in slices of rat visual cortex.

Acetylcholine (ACh) and noradrenaline (NA) have been shown to facilitate experience-dependent modifications of synaptic connectivity during postnatal development of the kitten visual cortex. To further investigate the mechanisms of this facilitation we studied the effects of these neuromodulators in an in vitro model of use-dependent synaptic plasticity. We have chosen long-term potentiation (LTP) in rat visual cortex slices because it shares several features with the in vivo model. In both cases induction of synaptic modifications requires that postsynaptic activation reaches a critical threshold and in both cases changes are induced more easily in young animals and when N-methyl-D-aspartate (NMDA) receptor-gated conductances are activated. Intracellular recordings were obtained from regular spiking cells in supragranular layers of rat visual cortex and LTP was induced by tetanic stimulation of the underlying white matter. Both cholinergic and noradrenergic agonists raised the probability that tetanic stimuli induced LTP and as in vivo they acted synergistically. These effects were mediated by agonists of muscarinic and beta-receptors, respectively. The agonists of both receptor systems enhanced the depolarizing response to the tetanus and increased NMDA receptor-gated conductances during this response. We suggest that this mode of action also accounts for the facilitatory effects which ACh and NA have on use-dependent synaptic plasticity in the developing visual cortex.

Acetylcholine↗
PubMed1991

Synaptic plasticity in visual cortex: comparison of theory with experiment.

1. The aim of this work was to assess whether a form of synaptic modification based on the theory of Bienenstock, Cooper, and Munro (BCM) can, with a fixed set of parameters, reproduce both the kinetics and equilibrium states of experience-dependent modifications cortex. 2. According to the BCM theory, the connection strength of excitatory geniculocortical synapses varies as the product of a measure of input activity (d) and a function (phi) of the summed postsynaptic response. For all postsynaptic responses greater than spontaneous but less than a critical value called the "modification threshold" (theta), phi has a negative value. For all postsynaptic responses greater than theta, phi has a positive value. A novel feature of the BCM theory is that the value of theta is not fixed, but rather "slides" as a nonlinear function of the average postsynaptic response. 3. This theory permits precise specification of theoretical equivalents of experimental situations, allowing detailed, quantitative comparisons of theory with experiment. Such comparisons were carried out here in a series of computer simulations. 4. Simulations are performed by presenting input to a model cortical neuron, calculating the summed postsynaptic response, and then changing the synaptic weights according to the BCM theory. This process is repeated until the synaptic weights reach an equilibrium state. 5. Two types of geniculocortical input are simulated: "pattern" and "noise." Pattern input is assumed to correspond to the type of input that arises when a visual contour of a particular orientation is presented to the retina. This type of input is said to be "correlated" when the two sets of geniculocortical fibers relaying information from the two eyes convey the same patterns at the same time. Noise input is assumed to correspond to the type of input that arises in the absence of visual contours and, by definition, is uncorrelated. 6. By varying the types of input available to the two sets of geniculocortical synapses, we simulate the following types of visual experience: 1) normal binocular contour vision, 2) monocular deprivation, 3) reverse suture, 4) strabismus, 5) binocular deprivation, and 6) normal contour vision after a period of monocular deprivation. 7. The constraints placed on the set of parameters by each type of simulated visual environment, and the effects that such constraints have on the evolution of the synaptic weights, are investigated in detail.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗