Showing papers on "Somatosensory system published in 1999"
TL;DR: The bulk of the evidence now strongly supports a prominent and highly modulated role for S1 cortex in the sensory aspects of pain, including localization and discrimination of pain intensity.
Abstract: Anatomical, physiological, and lesion data implicate multiple cortical regions in the complex experience of pain. These regions include primary and secondary somatosensory cortices, anterior cingulate cortex, insular cortex, and regions of the frontal cortex. Nevertheless, the role of different cortical areas in pain processing is controversial, particularly that of primary somatosensory cortex (S1). Human brain-imaging studies do not consistently reveal pain-related activation of S1, and older studies of cortical lesions and cortical stimulation in humans did not uncover a clear role of S1 in the pain experience. Whereas studies from a number of laboratories show that S1 is activated during the presentation of noxious stimuli as well as in association with some pathological pain states, others do not report such activation. Several factors may contribute to the different results among studies. First, we have evidence demonstrating that S1 activation is highly modulated by cognitive factors that alter pain perception, including attention and previous experience. Second, the precise somatotopic organization of S1 may lead to small focal activations, which are degraded by sulcal anatomical variability when averaging data across subjects. Third, the probable mixed excitatory and inhibitory effects of nociceptive input to S1 could be disparately represented in different experimental paradigms. Finally, statistical considerations are important in interpreting negative findings in S1. We conclude that, when these factors are taken into account, the bulk of the evidence now strongly supports a prominent and highly modulated role for S1 cortex in the sensory aspects of pain, including localization and discrimination of pain intensity.
684 citations
TL;DR: Attentional processes could possibly explain part of the variability observed in previous PET reports and should therefore be considered in further studies on pain in both normal subjects and patients with chronic pain.
Abstract: Turning attention towards or away from a painful heat stimulus is known to modify both the subjective intensity of pain and the cortical evoked potentials to noxious stimuli. Using PET, we investigated in 12 volunteers whether pain-related regional cerebral blood flow (rCBF) changes were also modulated by attention. High (mean 46.6°C) or low (mean 39°C) intensity thermal stimuli were applied to the hand under three attentional conditions: (i) attention directed towards the stimuli, (ii) attention diverted from the stimuli, and (iii) no task. Only the insular/second somatosensory cortices were found to respond whatever the attentional context and might, therefore, subserve the sensory-discriminative dimension of pain ( intensity coding ). In parallel, other rCBF changes previously described as `pain-related' appeared to depend essentially on the attentional context. Attention to the thermal stimulus involved a large network which was primarily right-sided, including prefrontal, posterior parietal, anterior cingulate cortices and thalamus. Anterior cingulate activity was not found to pertain to the intensity coding network but rather to the attentional neural activity triggered by pain. The attentional network disclosed in this study could be further subdivided into a non-specific arousal component, involving thalamic and upper brainstem regions, and a selective attention and orientating component including prefrontal, posterior parietal and cingulate cortices. A further effect observed in response to high intensity stimuli was a rCBF decrease within the somatosensory cortex ipsilateral to stimulation, which was considered to reflect contrast enhancing and/or anticipation processes. Attentional processes could possibly explain part of the variability observed in previous PET reports and should therefore be considered in further studies on pain in both normal subjects and patients with chronic pain.
583 citations
TL;DR: The results suggest that descending axons, rather than apical dendrites, are primarily activated by MCS, and highlight the thalamus as the key structure mediating functional MCS effects, and propose a model of MCS action.
Abstract: Although electrical stimulation of the precentral gyrus (MCS) is emerging as a promising technique for pain control, its mechanisms of action remain obscure, and its application largely empirical. Using positron emission tomography (PET) we studied regional changes in cerebral flood flow (rCBF) in 10 patients undergoing motor cortex stimulation for pain control, seven of whom also underwent somatosensory evoked potentials and nociceptive spinal reflex recordings. The most significant MCS-related increase in rCBF concerned the ventral-lateral thalamus, probably reflecting cortico-thalamic connections from motor areas. CBF increases were also observed in medial thalamus, anterior cingulate/orbitofrontal cortex, anterior insula and upper brainstem; conversely, no significant CBF changes appeared in motor areas beneath the stimulating electrode. Somatosensory evoked potentials from SI remained stable during MCS, and no rCBF changes were observed in somatosensory cortex during the procedure. Our results suggest that descending axons, rather than apical dendrites, are primarily activated by MCS, and highlight the thalamus as the key structure mediating functional MCS effects. A model of MCS action is proposed, whereby activation of thalamic nuclei directly connected with motor and premotor cortices would entail a cascade of synaptic events in pain-related structures receiving afferents from these nuclei, including the medial thalamus, anterior cingulate and upper brainstem. MCS could influence the affective-emotional component of chronic pain by way of cingulate/orbitofrontal activation, and lead to descending inhibition of pain impulses by activation of the brainstem, also suggested by attenuation of spinal flexion reflexes. In contrast, the hypothesis of somatosensory cortex activation by MCS could not be confirmed by our results.
523 citations
TL;DR: Evidence is presented that the multisensory parieto‐insular vestibular cortex is the human homologue of the Parieto-insular Vestibular Cortex (PIVC) in the monkey and is involved in the perception of verticality and self‐motion.
Abstract: The two major cortical functions of the vestibular system are spatial orientation and self-motion perception. These functions, however, are not exclusively vestibular; they also rely on visual and somatosensory input. All three systems (vestibular, visual and somatosensory) provide us with redundant information about the position and motion of our body relative to the external space. Although the vestibular cortex function is distributed among several multisensory areas in the parietal and temporal cortices, it is also integrated in a larger network for spatial attention and sensorimotor control of eye and body motion in space.
364 citations
TL;DR: It is proposed that sensory responses are dynamically modulated during active tactile exploration to optimize detection of different types of stimuli during quiet immobility, when the somatosensory system seems to be optimally tuned to detect the presence of single stimuli.
Abstract: We investigated the influence of four different behavioral states on tactile responses recorded simultaneously via arrays of microwires chronically implanted in the vibrissal representations of the rat ventral posterior medial nucleus (VPM) of the thalamus and the primary somatosensory cortex (SI). Brief (100 μsec) electrical stimuli delivered via a cuff electrode to the infraorbital nerve yielded robust sensory responses in VPM and SI during states of quiet immobility. However, significant reductions in tactile response magnitude and latency were observed in VPM and SI during large-amplitude, exploratory movements of the whiskers (at ∼4–6 Hz). During small-amplitude, 7–12 Hz whisker-twitching movements, a significant reduction in SI response magnitude and an increase in VPM and SI response latencies were observed as well. When pairs of stimuli with interstimulus intervals 25 msec. These response patterns were correlated with the amount and duration of postexcitatory firing suppression observed in VPM and SI during each of these behaviors. On the basis of these results, we propose that sensory responses are dynamically modulated during active tactile exploration to optimize detection of different types of stimuli. During quiet immobility, the somatosensory system seems to be optimally tuned to detect the presence of single stimuli. In contrast, during whisker movements and other exploratory behaviors, the system is primed to detect the occurrence of rapid sequences of tactile stimuli, which are likely to be generated by multiple whisker contacts with objects during exploratory activity.
350 citations
TL;DR: MEG patterns following stimulation of different peripheral nerves indicate activation of an extensive cortical network and the serial versus parallel processing in the cortical somatosensory network is still under debate.
Abstract: Magnetoencephalography (MEG) is a totally non-invasive research method which provides information about cortical dynamics on a millisecond time-scale. Whole-scalp magnetic field patterns following stimulation of different peripheral nerves indicate activation of an extensive cortical network. At the SI cortex, the responses reflect mainly the activity of area 3b, with clearly somatotopical representations of different body parts. The SII cortex is activated bilaterally and it also receives, besides tactile input, nociceptive afference. Somatically evoked MEG signals may also be detected from the posterior parietal cortex, central mesial cortex and the frontal lobe. The serial versus parallel processing in the cortical somatosensory network is still under debate.
337 citations
TL;DR: It is concluded that the PrV input mediates the near "one-to-one" correspondence between a neuronal response in V PM and a single mystacial whisker and alterations in VPM responses produced by changing the depth of anesthesia are due to its selective influence on the properties mediated by SpVi inputs at the level of the thalamus.
Abstract: Modulation of receptive field properties of thalamic somatosensory neurons by the depth of anesthesia. The dominant frequency of electrocorticographic (ECoG) recordings was used to determine the de...
277 citations
TL;DR: It is reported that sensory deprivation alters short-term synaptic dynamics in both vertical and horizontal excitatory pathways within the supragranular cortex, which helps to explain the functional cortical reorganization that follows persistent changes of sensory experience.
Abstract: Many representations of sensory stimuli in the neocortex are arranged as topographic maps. These cortical maps are not fixed, but show experience-dependent plasticity1,2. For instance, sensory deprivation causes the cortical area representing the deprived sensory input to shrink, and neighbouring spared representations to enlarge, in somatosensory3, auditory4 or visual cortex5. In adolescent and adult animals, changes in cortical maps are most noticeable in the supragranular layers at the junction of deprived and spared cortex6,7,8,9. However, the cellular mechanisms of this experience-dependent plasticity are unclear. Long-term potentiation and depression have been implicated10,11,12, but have not been proven to be necessary or sufficient for cortical map reorganization. Short-term synaptic dynamics have not been considered. We developed a brain slice preparation involving rat whisker barrel cortex in vitro. Here we report that sensory deprivation alters short-term synaptic dynamics in both vertical and horizontal excitatory pathways within the supragranular cortex. Moreover, modifications of horizontal pathways amplify changes in the vertical inputs. Our findings help to explain the functional cortical reorganization that follows persistent changes of sensory experience.
270 citations
TL;DR: Tracing of auditory cortical connections suggests that the primate auditory system, like the visual and somatosensory systems, may be organized into 'what' and 'where' pathways.
Abstract: Tracing of auditory cortical connections suggests that the primate auditory system, like the visual and somatosensory systems, may be organized into 'what' and 'where' pathways.
266 citations
TL;DR: It is likely that the contribution of proprioceptive input to the activation of the premotor cortex, SMA, cerebellum and basal ganglia, if any, is small, and the present results do not rule out the possibility that the cutaneous afferent input or the combination of cutaneous and proprioception input participates in theactivation of those areas during the active movement.
Abstract: A PET study was performed in six normal volunteers to elucidate the functional localization of the sensory afferent component during finger movement. Brain activation during the passive movement driven by a servo-motor was compared with that during an auditory-cued active movement which was controlled kinematically in the same way as the passive one. A newly developed device was used for selectively activating proprioception with a minimal contribution from tactile senses. Active movement was associated with activation of multiple areas, including the contralateral primary sensorimotor cortex, premotor cortex, supplementary motor area (SMA), bilateral secondary somatosensory areas and basal ganglia and ipsilateral cerebellum. In contrast, only the contralateral primary and secondary somatosensory areas were activated by the passive movement. It is likely that the contribution of proprioceptive input to the activation of the premotor cortex, SMA, cerebellum and basal ganglia, if any, is small. However, the present results do not rule out the possibility that the cutaneous afferent input or the combination of cutaneous and proprioceptive input participates in the activation of those areas during the active movement.
261 citations
TL;DR: The result suggests that the thalamic plasticity that is seen immediately after a peripheral deafferentation is dependent upon both descending corticofugal projections and ascending trigeminothalamic projections.
Abstract: Multiple neuron ensemble recordings were obtained simultaneously from both the primary somatosensory (SI) cortex and the ventroposterior medial thalamus (VPM) before and during the combined administration of reversible inactivation of the SI cortex and a reversible subcutaneous block of peripheral trigeminal nerve fibers. This procedure was performed to quantify the contribution of descending corticofugal projections on (i) the normal organization of thalamic somatosensory receptive fields and (ii) the thalamic somatosensory plastic reorganization that immediately follows a peripheral deafferentation. Reversible inactivation of SI cortex resulted in immediate changes in receptive field properties throughout the VPM. Cortical inactivation also significantly reduced but did not completely eliminate the occurrence of VPM receptive field reorganization resulting from the reversible peripheral deafferentation. This result suggests that the thalamic plasticity that is seen immediately after a peripheral deafferentation is dependent upon both descending corticofugal projections and ascending trigeminothalamic projections.
TL;DR: The study indicates that fMRI enables examination of cortical networks subserving pain perception at an anatomical detail not available with other brain imaging techniques and shows that this cortical network underlying pain perception shares components with the networks underlying touch perception and motor execution.
TL;DR: Modulation of S1 and S2 supports concepts of early selection in tactile attention, and the posterior parietal region may be subdivided into modality-specific subregions, each of which processes information needed to attend to a specific modality.
Abstract: We used positron emission tomography to study cortical regions mediating tactile attention. Cues selectively directed subjects to attend to the roughness or duration of contact with embossed gratings that rubbed against a single fingertip with controlled speed and force. The task required discriminating between paired gratings that differed in tactile features of roughness and/or length. For different blocks of trials, cues directed attention to one tactile feature or indicated a divided attention strategy to a change in either feature. All attention conditions unambiguously activated several somatosensory foci in the parietal cortex, including somatotopically appropriate portions of the primary somatosensory cortex in the postcentral gyrus (S1) and the secondary somatosensory region (S2) within parietal opercular regions. There was no evidence for separate processing foci for selective and divided attention strategies, or for selectively attending to roughness versus stimulus duration. We observed a greater magnitude blood flow change in S2 versus S1 during attention tasks, which suggests that S2 might actually influence S1 activity. Despite these differences, modulation of S1 and S2 supports concepts of early selection in tactile attention. There were also examples of non-sensory foci in frontal cortex, anterior cingulate gyrus and bilateral superior parietal regions at the fundus of the postcentral sulcus. Posterior parietal regions observed in this study did not overlap foci seen in studies of visual attention. Thus, the posterior parietal region may be subdivided into modality-specific subregions, each of which processes information needed to attend to a specific modality. These non-sensory areas may constitute a network that provides a source of modulating influences on the earlier stage, sensory areas.
TL;DR: The analysis of the literature in each sensory modality indicates that relationships between learning-induced sensory plasticity and behavioural performance can, or cannot, be found depending on the tasks that were used.
TL;DR: This prediction is accurate when tactile stimuli are self-produced relative to when they are externally produced, and is therefore used to attenuate the somatosensory response to the former type of tactile stimulation but not the former, and supports the proposal that the cerebellum is involved in predicting the sensory consequences of movements.
TL;DR: A bilateral activation of the lateral pain system as well as involvement of the medial pain system during dynamic mechanical allodynia in patients with mononeuropathy is revealed.
Abstract: The objective of this study was to investigate the central processing of dynamic mechanical allodynia in patients with mononeuropathy. Regional cerebral blood flow, as an indicator of neuronal activity, was measured with positron emission tomography. Paired comparisons were made between three different states; rest, allodynia during brushing the painful skin area, and brushing of the homologous contralateral area. Bilateral activations were observed in the primary somatosensory cortex (S1) and the secondary somatosensory cortex (S2) during allodynia compared to rest. The S1 activation contralateral to the site of the stimulus was more expressed during allodynia than during innocuous touch. Significant activations of the contralateral posterior parietal cortex, the periaqueductal gray (PAG), the thalamus bilaterally and motor areas were also observed in the allodynic state compared to both non-allodynic states. In the anterior cingulate cortex (ACC) there was only a suggested activation when the allodynic state was compared with the non-allodynic states. In order to account for the individual variability in the intensity of allodynia and ongoing spontaneous pain, rCBF was regressed on the individually reported pain intensity, and significant covariations were observed in the ACC and the right anterior insula. Significantly decreased regional blood flow was observed bilaterally in the medial and lateral temporal lobe as well as in the occipital and posterior cingulate cortices when the allodynic state was compared to the non-painful conditions. This finding is consistent with previous studies suggesting attentional modulation and a central coping strategy for known and expected painful stimuli. Involvement of the medial pain system has previously been reported in patients with mononeuropathy during ongoing spontaneous pain. This study reveals a bilateral activation of the lateral pain system as well as involvement of the medial pain system during dynamic mechanical allodynia in patients with mononeuropathy.
TL;DR: The studies discussed in this article suggest that, despite its original conception as a uniquely segregated cortex, rat SI has a wide array of dynamic interactions, and that the study of this region will provide insight into the general mechanisms of cortical dynamics engaged by sensory systems.
TL;DR: It is reported that NMDA receptor-independent LTD is the major form of long-term plasticity in the anterior cingulate cortex (ACC), and it is suggested that synaptic LTD in the ACC may contribute to enhanced neuronal responses to subsequent somatosensory stimuli after amputation.
Abstract: Two forms of activity-dependent long-term depression (LTD) in the CNS, as defined by their sensitivity to the blockade of NMDA receptors, are thought to be important in learning, memory, and development. Here, we report that NMDA receptor-independent LTD is the major form of long-term plasticity in the anterior cingulate cortex (ACC). Both L-type voltage-gated calcium channels and metabotropic glutamate receptors are required for inducing LTD. Amputation of a third hindpaw digit in an adult rat induced rapid expression of immediate early genes in the ACC bilaterally and caused a loss of LTD that persisted for at least 2 weeks. Our results suggest that synaptic LTD in the ACC may contribute to enhanced neuronal responses to subsequent somatosensory stimuli after amputation.
TL;DR: Results indicate that SII and/or associated cortices in parietal operculum, often viewed as higher-order processing areas for somatosensory perception, are coactivated with SI during the early processing of intermittent somatoensory input.
Abstract: Simultaneous early processing of sensory input in human primary (SI) and secondary (SII) somatosensory cortices. The anatomic connectivity of the somatosensory system supports the simultaneous part...
TL;DR: The results suggest that the human SI and SII cortices may be sequentially activated within one hemisphere, whereas SII ipsilateral to the stimulation may receive direct input from the periphery, at least when normal input from SI is interrupted.
Abstract: To study the effects of parietal lesions on activation of the human somatosensory cortical network, we measured somatosensory evoked fields to electric median nerve stimuli, using a whole-scalp 122-channel neuromagnetometer, from six patients with cortical right-hemisphere stroke and from seven healthy control subjects. In the control subjects, unilateral stimuli elicited responses which were satisfactorily accounted for by modelled sources in the contralateral primary (SI) and bilateral secondary (SII) somatosensory cortices. In all patients, stimulation of the right median nerve also activated the SI and SII cortices of the healthy left hemisphere. However, the activation pattern was altered, suggesting diminished interhemispheric inhibition via callosal connections after right-sided stroke. Responses to left median nerve stimuli showed large interindividual variability due to the different extents of the lesions. The strength of the 20-ms response, originating in the SI cortex, roughly reflected the severity of the tactile impairment. Right SII responses were absent in patients with abnormal right SI responses, whereas the left SII was active in all patients, regardless of the responsiveness of the right SI and/or SII. Our results suggest that the human SI and SII cortices may be sequentially activated within one hemisphere, whereas SII ipsilateral to the stimulation may receive direct input from the periphery, at least when normal input from SI is interrupted.
TL;DR: Responses of Golgi cells to mechanical stimulation of the face, in Crus I‐II of ketamine‐xylazine anaesthetized rats, revealed a slow firing rhythm at rest and excitatory responses to tactile input.
Abstract: Golgi cells are the only elements within the cerebellar cortex that inhibit granule cells. Despite their unique position there is little information on how Golgi cells respond to afferent input. We studied responses of Golgi cells to mechanical stimulation of the face, in Crus I-II of ketamine-xylazine anaesthetized rats. In 41 rats, 87 putative Golgi cells were identified, based on spike characteristics and on location of electrolytic lesions in the granular layer. They displayed a slow firing rhythm at rest (8.4 spikes/s). Most Golgi cells (84%) showed excitatory responses to tactile input. Their receptive fields (RFs) included, in 78%, the entire ipsilateral infraorbital nerve territory, and extended, in 14%, to other trigeminal nerve branches and, in 48%, to the contralateral face. Excitatory responses consisted of multiple, precisely timed (+/- 1 ms) spikes. Most peristimulus time histograms (PSTHs) (69%) showed an early (5-10 ms) and a late (13-26 ms) excitatory component, with each component consisting of a single PSTH peak. In some PSTHs the early component was a double peak (< 4 ms interval). In others, only one, early or late, PSTH peak was observed. The excitatory components were followed by a silent period (28-69 ms latency), the duration of which (13-200 ms) varied with response amplitude. In single cells, response profiles changed with stimulus location. In simultaneously recorded cells, evoked profiles differed for identical stimuli. Differences in RF size between early 'double' and 'single' peaks suggested that they resulted from direct mossy fibre and parallel fibre input, respectively. Late PSTH peaks were assumed to reflect corticopontine activation.
TL;DR: During somatosensory stimulation at various frequencies, cerebral blood flow is coupled to integrated neuronal activity but not to averaged evoked potentials, suggesting that frequency is an important determinant of neuronal activity.
TL;DR: Morphological analysis of vestibular neurones demonstrated that vestibulo‐trigeminal neurones are relatively small and belong to a different population than those receiving projections from the trigeminal nuclei.
Abstract: In order to study the connection patterns between the sensory trigeminal and the vestibular nuclei (VN), injections of anterogradely and/or retrogradely transported neuronal tracers were made in the rat. Trigeminal injections resulted in anterogradely labelled fibres, with an ipsilateral preponderance, within the VN: in the ventrolateral part of the inferior nucleus (IVN), in the lateral part of the medial nucleus (MVN), in the lateral nucleus (LVN) with a higher density in its ventral half, and in the superior nucleus (SVN), more in the periphery than in the central part. Moderate trigeminal projections were observed in the small vestibular groups f, x and y/l and in the nucleus prepositus hypoglossi. Additional retrogradely labelled neurones were seen in the IVN, MVN, and LVN, in the same regions as those receiving trigeminal afferents. Morphological analysis of vestibular neurones demonstrated that vestibulo-trigeminal neurones are relatively small and belong to a different population than those receiving projections from the trigeminal nuclei. The trigeminovestibular and vestibulo-trigeminal relationships were confirmed by tracer injections in the VN. The results show that, in the VN, there is sensory information from facial receptors in addition to those reported from the neck and body. These facial afferents complement those from the neck and lower spinal levels in supplying important somatosensory information from the face and eye muscles. The oculomotor connections of the respective zones of the VN receiving trigeminal afferents suggest that sensory inputs from the face, including extraocular proprioception, may, through this pathway, influence the vestibular control of eye and head movements.
TL;DR: Functional magnetic resonance imaging data show that the electrical stimuli used for SEP recording lead to a functional activation of S1 as well as M1.
TL;DR: The rat primary somatosensory cortex does not appear to operate as a static decoder of tactile information, and data suggest that tactile processing in rats is likely to involve the on-going interactions between populations of broadly tuned neurons in the thalamocortical pathway.
Abstract: Animals in their natural environments actively process spatiotemporally complex sensory signals in order to guide adaptive behavior. It therefore seems likely that the properties of both single neurons and neural ensembles should reflect the dynamic nature of such interactions. During exploratory behaviors, rats move their whiskers to actively discriminate between different tactile features. We investigated whether this dynamic sensory processing was reflected in the spatial and temporal properties of neurons in layer V of the ‘whisker area’ in the rat primary somatosensory cortex. We found that the majority of layer V neurons had large (8.5 ± 4.9 whiskers) spatiotemporal receptive fields (i.e. individual cells responded best to different whiskers as a function of post-stimulus time), and that the excitatory responses of surround whiskers formed a spatial gradient of excitation that seemed to reflect the greater use of the ventral and caudal whiskers during natural behaviors. Analyses of ensembles of layer V neurons revealed that single-whisker stimuli activated a portion of layer V that extends well beyond a single cortical column (average of 5.6 barrel cortical columns). Based on these results, we conclude that the rat primary somatosensory cortex does not appear to operate as a static decoder of tactile information. On the contrary, our data suggest that tactile processing in rats is likely to involve the on-going interactions between populations of broadly tuned neurons in the thalamocortical pathway.
TL;DR: It is suggested that the primate insular cortex may be involved in the integration of cardiovascular function with somatosensory (principally nociceptive) input and supports the emerging role of theinsular cortex as an important forebrain site of viscerosomatoensory regulation with clinical implications for cardiovascular regulation under conditions of stress and arousal.
TL;DR: The results suggest that trigeminal afferents participate in modulation of the activity of primary motor cortex output neurons via primary somatosensory cortex-to-primary motor cortex associational connections, even under anaesthesia.
TL;DR: It is found that the developmental expression of the NR2A subunit is delayed compared with the somatosensory cortex, in agreement with the prolonged critical period in the visual cortex, and sensory deprivation further delays the developmental change in the NMDA receptor subunit composition.
Abstract: The susceptibility of cortical networks to use-dependent modifications declines with age (critical period) and this decline of neuronal plasticity during development is paralleled by the shortening of NMDA receptor EPSCs. We showed previously in the somatosensory cortex that the shortening of NMDA receptor kinetics correlates with a developmentally-regulated increase in the NR2A subunit expression. Here we examine whether this developmental regulation of NR2A expression is related to the duration of critical periods and whether it is influenced by experience. Functional NMDA receptors and their molecular characteristics are studied in identified layer IV neurons of rat visual cortex. In this structure the time course of the critical period differs from that in the somatosensory cortex and can be changed by sensory deprivation, thus permitting examination of correlations between the time course of receptor expression and the duration of the critical period. We find that the developmental expression of the NR2A subunit is delayed compared with the somatosensory cortex, in agreement with the prolonged critical period in the visual cortex. Moreover, sensory deprivation further delays the developmental change in the NMDA receptor subunit composition, demonstrating the activity dependence of this process and strengthening the correlation between changes in subunit composition and the time course of the critical period.
TL;DR: It is suggested that the mechanisms underlying the 'gating' effect during explorative finger movements in the 30 ms time range predominantly arise in the motor cortex.
TL;DR: The studies reviewed here reveal important roles for ionotropic glutamate receptors in the mediation of sensory inputs to the SC and in transmission between the superficial and deep layers.