Showing papers on "Somatosensory system published in 1998"
TL;DR: There is anatomical evidence which suggests that there are important differences in the extent of convergence of cortical projections in the primate versus the nonprimate amygdala.
1,201 citations
TL;DR: FMRI examination of neural responses when subjects experienced a tactile stimulus that was either self-produced or externally produced suggests that the cerebellum is involved in predicting the specific sensory consequences of movements, providing the signal used to cancel the sensory response to self-generated stimulation.
Abstract: A self-produced tactile stimulus is perceived as less ticklish than the same stimulus generated externally. We used fMRI to examine neural responses when subjects experienced a tactile stimulus that was either self-produced or externally produced. More activity was found in somatosensory cortex when the stimulus was externally produced. In the cerebellum, less activity was associated with a movement that generated a tactile stimulus than with a movement that did not. This difference suggests that the cerebellum is involved in predicting the specific sensory consequences of movements, providing the signal that is used to cancel the sensory response to self-generated stimulation.
1,169 citations
TL;DR: In this paper, the existence of a well-defined vestibular cortical system in humans has been verified from single-unit recordings and tracer studies in three different primate species.
431 citations
TL;DR: The growth of intracortical but not thalamocortical connections could account for much of the reorganization of the sensory maps in cortex in macaque monkeys with long-standing, accidental trauma to a forelimb.
Abstract: Distributions of thalamic and cortical connections were investigated in four macaque monkeys with long-standing, accidental trauma to a forelimb, to determine whether the growth of new connections plays a role in the reorganization of somatosensory cortex that occurs after major alterations in peripheral somatosensory inputs. In each monkey, microelectrode recordings of cortical areas 3b and 1 demonstrated massive reorganizations of the cortex related to the affected limb. Injections of tracers in area 1 of these monkeys revealed normal patterns of thalamocortical connections, but markedly expanded lateral connections in areas 3b and 1. Thus, the growth of intracortical but not thalamocortical connections could account for much of the reorganization of the sensory maps in cortex.
427 citations
TL;DR: The human cortical hand somatosensory area of 6 patients with focal dystonia of the hand is mapped and an abnormality of the normal homuncular organization of the finger representations in the primary somatoensory cortex is found.
Abstract: Abnormalities of the sensory system have been proposed as causative factors for dystonia. By mapping the human cortical hand somatosensory area of 6 patients with focal dystonia of the hand, we found an abnormality of the normal homuncular organization of the finger representations in the primary somatosensory cortex (S1). Although a remote antecedent event or even a developmental anomaly cannot entirely be ruled out, our findings may support the concept that abnormal plasticity is involved in the development of dystonia.
366 citations
TL;DR: A modest increase of tonic excitatory drive in conjunction with the activation of cholinergic muscarinic receptors can elicit persistent gamma frequency network oscillations in the rodent somatosensory cortex, andphasize the role of the Cholinergic ascending system in the cortical processing of sensory information.
Abstract: 1Concomitant application of the cholinergic agonist carbachol and nanomolar doses of kainate can elicit persistent gamma frequency oscillations in all layers of the mouse somatosensory cortex in vitro. Receptor pharmacology with bath-applied antagonists indicated that oscillatory network activity depended crucially on the participation of cholinergic muscarinic, (S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate and GABAA receptors.
2The timing of action potentials and the occurrence of excitatory as well as inhibitory postsynaptic events was highly correlated with the phasic change of extracellularly recorded population activity. Firing probability was lowest during the peak negativity of IPSPs and gradually increased during their ensuing decay. In conjunction with the effect of a barbiturate to decrease the frequency of gamma oscillations, this suggests a crucial role of IPSPs in phasing the suprathreshold activity of principal neurons.
3At nearby (< 1 mm) sites contained within any given cortical layer, oscillatory extra- and intracellular activity was highly synchronous with no apparent phase lag. However, interlaminar mapping experiments demonstrated a phase reversal of both extra- and intracellularly recorded activity near the lower border of thalamo-recipient layer 4, thus corroborating findings that have been obtained in vivo.
4In conclusion, a modest increase of tonic excitatory drive in conjunction with the activation of cholinergic muscarinic receptors can elicit persistent gamma frequency network oscillations in the rodent somatosensory cortex. These findings (re)emphasize the role of the cholinergic ascending system in the cortical processing of sensory information.
342 citations
TL;DR: A large part of the somatosensory homunculus was reproduced quantitatively on an individual brain MRI using magnetoencephalography, which is compatible with the bizarre proportion of the Homunculus with a large tongue, lips, and fingers.
314 citations
TL;DR: Reorganization of brainstem and thalamic nuclei associated with slow transneuronal atrophy is likely to be a progressive process and when coupled with divergence of ascending connections, it islikely to make a substantial contribution to representational changes in cortex.
Abstract: After long-term denervation of an upper limb in macaque monkeys, the representation of the face in somatosensory cortex expands over many millimeters into the silenced representation of the hand. Various brainstem and cortical mechanisms have been proposed to explain this phenomenon. Reorganization in the thalamus has been largely ignored. In monkeys with deafferented upper limbs for 12 to 20 years, it was found that the brainstem cuneate and the thalamic ventral posterior nuclei had undergone severe transneuronal atrophy, and physiological mapping in the thalamus revealed that the face and trunk representations were adjoined while the normally small representation of the lower face had expanded comparable to the expansion in cortex. Reorganization of brainstem and thalamic nuclei associated with slow transneuronal atrophy is likely to be a progressive process. When coupled with divergence of ascending connections, it is likely to make a substantial contribution to representational changes in cortex.
246 citations
TL;DR: The results suggest that reorganization of sensory pathways occurs very soon after amputation in humans, potentially due to the unmasking of ordinarily silent inputs rather than sprouting of new axon terminals.
Abstract: We studied a patient after amputation of an arm and found that in less than 24 h stimuli applied on the ipsilateral face were referred in a precise, topographically organized, modality-specific manner to distinct points on the phantom. Functional magnetic resonance imaging (fMRI) performed one month later showed that brush-evoked activity in the brain demonstrates objective signal changes which correlate with perceptual changes in the phantom hand. This finding in humans corresponds to the observations of immediate plasticity in cortical pathways described in animals, including primates. The results suggest that reorganization of sensory pathways occurs very soon after amputation in humans, potentially due to the unmasking of ordinarily silent inputs rather than sprouting of new axon terminals.
224 citations
TL;DR: One role of the mammalian corticofugal system may be to modify subcortical sensory maps in response to sensory experience, as investigated in the big brown bat.
Abstract: The auditory system, like the visual and somatosensory systems, contains topographic maps in its central neural pathways. These maps can be modified by sensory deprivation, injury and experience in both young and adult animals. Such plasticity has been explained by changes in the divergent and convergent projections of the ascending sensory system. Another possibility, however, is that plasticity may be mediated by descending corticofugal connections. We have investigated the role of descending connections from the cortex to the inferior colliculus of the big brown bat. Electrical stimulation of the auditory cortex causes a downward shift in the preferred frequencies of collicular neurons toward that of the stimulated cortical neurons. This results in a change in the frequency map within the colliculus. Moreover, similar changes can be induced by repeated bursts of sound at moderate intensities. Thus, one role of the mammalian corticofugal system may be to modify subcortical sensory maps in response to sensory experience.
213 citations
TL;DR: These findings demonstrate a separation in functional contributions of lateral parietal opercular cortex and IPA, and indicate different cortical processing streams for the somatosensory submodalities microgeometry and macrogeometry.
Abstract: Somatosensory stimuli are known to activate the postcentral gyrus, and neurons there fire when the skin is in contact with objects. Also neurons in the lateral fissure, the parietal operculum, fire when rough surfaces are felt. However the localization of somatosensory association areas in humans is largely unknown and differences in functional contributions between somatosensory association areas has not been previously demonstrated. For these reasons the regional cerebral blood flow was measured with 15O-butanol and positron-emission tomography in two groups of young volunteers discriminating the lengths, shapes, and roughness of objects with their right hand. Roughness discrimination activated the lateral parietal opercular cortex significantly more than did length or shape discrimination. A Boolean intersection of the cluster images showing the statistical significant increases of length and shape discrimination demonstrated that shape and length discrimination activated the same cortical field lining the anterior part of the intraparietal sulcus (IPA). Shape and length discrimination activated IPA significantly more than did roughness discriminaton. These findings demonstrate a separation in functional contributions of lateral parietal opercular cortex and IPA. The results indicate different cortical processing streams for the somatosensory submodalities microgeometry and macrogeometry.
TL;DR: This finding demonstrates a dramatic 'top-down' influence of cortex on receptive field size in the somatosensory thalamus and indicates that changes in 'higher-order' areas of the brain can trigger extensive changes in the receptive field characteristics of neurons located earlier in the processing pathway.
Abstract: The influence of cortical feedback on receptive field organization in the thalamus was assessed in the primate somatosensory system. Chronic and acute suppression of neuronal activity in primary somatosensory cortex resulted in a striking enlargement of receptive fields in the ventroposterior thalamus. This finding demonstrates a dramatic 'top-down' influence of cortex on receptive field size in the somatosensory thalamus. In addition, this result has important implications for studies of adult neuronal plasticity because it indicates that changes in 'higher-order' areas of the brain can trigger extensive changes in the receptive field characteristics of neurons located earlier in the processing pathway.
TL;DR: A grid translation of columnar somatosensory cortical inputs into striatum and a detailed map for the rat sensorimotor zone are shown, showing the lattice‐like grid is a novel functional/neuroanatomic organization that is ideal for distributing, combining, and integrating information for sensorsimotor and cognitive processing.
Abstract: The neuroanatomic organizing principles underlying integrative functions in the striatum are only partially understood. Within the three major subdivisions of the striatum-sensorimotor, associative, and limbic--longitudinal zones of axonal plexuses from the cerebral cortex end in bands and clusters that innervate cell groups. To identify organizing principles of the corticostriate bands and clusters, we localized somatosensory cortical cells receptive to light touch on the hindlimb, forelimb, or vibrissae by extracellular recording, and we labeled their projections by iontophoretic application of dextran anterograde tracers. The results show that cortical cells in columnar groups project to the striatum in the form of successive strips, or laminae, that parallel the curve of the external capsule. The vibrissae somatosensory cortex projects to the most lateral lamina. Just medial to the vibrissae projection, the major axonal arborizations arising from hindlimb and forelimb somatosensory cortex are organized within a common lamina, where they interdigitate and overlap as well as remain separate. In addition, the hindlimb and forelimb cortex send small projections to the vibrissae lamina, and vice versa, forming broken, radially oriented lines of points across the laminar strips. The major somatosensory projections are in the dorsolateral, calbindin-poor sensorimotor striatum, whereas the radially oriented projection points extend into the medial, calbindin-rich associative striatum. Extending previous studies of corticostriate projections, this report shows a grid translation of columnar somatosensory cortical inputs into striatum and a detailed map for the rat sensorimotor zone. The lattice-like grid is a novel functional/neuroanatomic organization that is ideal for distributing, combining, and integrating information for sensorimotor and cognitive processing.
TL;DR: Data suggest that following phasic activation of the LC efferent system, the efficacy of signal transmission through sensory networks in mammalian brain is enhanced.
Book•
01 Jan 1998
TL;DR: This chapter discusses Neuroscience at the Cellular Level, which focuses on the development of the Nervous System, and Cerebrum, which is concerned with the role of the cerebrum in clinical applications.
Abstract: 1: Introduction to Neuroscience Part 1: Neuroscience at the Cellular Level 2. Physical and Electrical Properties of Cells in the Nervous System 3. Synapses and Synaptic Transmissions 4. Neuroplasticity Part 2: Development of the Nervous System 5. Development of the Nervous System Part 3: Neuroscience at the System Level 6. Somatosensory System 7. Somatosensation: Clinical Applications 8. Neuropathic Pain, Pain Matrix Dysfunction, and Pain Syndromes 9. Autonomic Nervous System 10. Normal Motor System: Motor Neurons 11. Basal Ganglia, Cerebellum, and Movement Part 4: Neuroscience at the Regional Level 12. Peripheral Nervous System 13. Spinal Region 14. Cranial Nerves 15. Brain Stem Region 16. Vestibular and Visual Systems 17. Cerebrum 18. Cerebrum: Clinical Applications Part 5: Support Systems 19. Support Systems: Blood Supply and Cerebrospinal Fluid System Appendices Appendix A: Gross Anatomy Atlas Glossary
TL;DR: It is suggested that isometric contraction facilitates activation of SII cortices to tactile stimuli, possibly by decreasing inhibition from the SI cortex, and may be related to tuning of S II neurons towards relevant tactile input arising from the region of the body where the muscle activation occurs.
TL;DR: It is proposed that activation of median nerve muscle afferents can suppress the excitability of cortical areas controlling the antagonist forearm extensor muscles acting on the hand.
Abstract: To find out whether muscle afferents influence the excitability of corticospinal projections to antagonist muscles, we studied sixteen healthy subjects and one patient with a focal brain lesion.
Using transcranial magnetic and electrical brain stimulation we tested the excitability of corticomotoneuronal connections to right forearm muscles at rest after conditioning stimulation of the median nerve at the elbow. Somatosensory potentials evoked by median nerve stimulation were also recorded in each subject.
Test stimuli delivered at 13–19 ms after median nerve stimulation significantly inhibited EMG responses elicited in forearm extensor muscles by transcranial magnetic stimulation, but did not inhibit responses to electrical stimulation. In contrast, magnetically and electrically elicited responses in forearm flexor muscles were suppressed to the same extent.
The higher the intensity of the test shocks, the smaller was the amount of median nerve-elicited inhibition. Inhibition in extensor muscles was also smaller during tonic wrist extension, or if the induced electrical stimulating current in the brain flowed from posterior to anterior over the motor strip rather than vice versa. Test responses evoked by magnetic transcranial stimulation in the first dorsal interosseous and in brachioradialis muscles were not inhibited after median nerve stimulation at the elbow. Stimulation of digital nerves failed to inhibit motor potentials in extensor muscles.
Test stimuli delivered at 15 and 17 ms after radial nerve stimulation significantly inhibited EMG responses elicited in forearm flexor muscles by magnetic transcranial stimulation.
In the patient with a focal thalamic lesion, who had dystonic postures and an absent N20 component of the somatosensory-evoked potentials but normal strength, median nerve stimulation failed to inhibit magnetically evoked responses in forearm extensor muscles.
We propose that activation of median nerve muscle afferents can suppress the excitability of cortical areas controlling the antagonist forearm extensor muscles acting on the hand. The inhibitory effect occurs at short latency and might assist spinal pathways mediating reciprocal inhibition by contrasting the co-activation of antagonistic pools of corticospinal cells.
Motor commands from many areas of the brain as well as afferent information from the body can influence the activity of corticospinal cells. Stimulation of cutaneous and proprioceptive afferents activates precentral cells at short latencies (see Porter & Lemon, 1993; Rothwell, 1994). This short-latency facilitation of corticospinal cells is the functional correlate of the long-latency component of the stretch reflex in distal muscles. In humans, studies using transcranial brain stimulation have shown that afferent input to the motor cortex increases the excitability of corticomotoneuronal connections to the homonymous muscle (Troni et al. 1988; Deuschl et al. 1991; Mariorenzi et al. 1991; Kasai et al. 1992; Wolfe & Hayes, 1995; Baldissera & Leocani, 1995; Terao et al. 1995; Nielsen et al. 1997). However, the effect that muscle input has on corticospinal outputs to antagonist muscles remains obscure. In this paper we investigated the effect of median nerve muscle afferent stimulation on cortimotoneuronal connections to antagonist muscles in humans. We found a pattern of cortical reciprocal inhibition similar to that in the spinal cord.
TL;DR: Somatotopic maps in the mature brain reorganize in response to deafferentation by peripheral nerve cut, amputations, or spinal lesions, and an understanding of these mechanisms could guide interventions that potentiate recovery from such injuries.
Abstract: Somatotopic maps in the mature brain reorganize in response to deafferentation by peripheral nerve cut, amputations, or spinal lesions. Mechanisms underlying these changes may range from altered tonic inhibition and synaptic efficacy to neuronal sprouting. An understanding of these mechanisms could guide interventions that potentiate recovery from such injuries.
TL;DR: The present results suggest that a chronic pathological modification of peripheral sensorimotor inputs is associated with changes in neural activity at multiple sites of the somatosensory system.
Abstract: The human somatosensory cortex (S1) is capable of modification after partial peripheral deafferentation, but it is not known whether spinal and brainstem changes contribute to this process. We recorded spinal, brainstem and cortical somatosensory evoked potentials following ulnar nerve stimulation in patients affected by unilateral carpal tunnel syndrome with EMG evidence of chronic alterations in median nerve sensorimotor conduction at the wrist lasting at least 4 weeks, and compared them with those from the unaffected hand and with those obtained in a control group. Amplitudes of spinal N13 and brainstem P14 potentials following stimulation of the ulnar nerve ipsilateral to the deafferented median nerve were greater than those following stimulation of the contralateral ulnar nerve. Side-to-side amplitude differences in N13 and P14 were greater in patients than in the control group. Parietal N20 and P27 potentials, supposedly generated in S1, were also significantly increased. The present results suggest that a chronic pathological modification of peripheral sensorimotor inputs is associated with changes in neural activity at multiple sites of the somatosensory system. Changes in spinal and brainstem structures could contribute to the mechanisms subserving changes in the S1. Changes in synaptic strength and unmasking inputs secondary to disconnection of the normally dominant inputs to the 'median nerve' cortex may be the mechanisms underlying ulnar nerve SEP changes.
TL;DR: The mean distances measured between fMRI activity and the corresponding MEG dipoles may reflect the correlation between electrophysiological and hemodynamic responses due to the different underlying substrates of neurophysiology measured by fMRI and MEG: BOLD contrast vs neuronal biomagnetic activity.
Abstract: Functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) were performed in six subjects during self-paced finger movement performance, tactile somatosensory stimulation and binaural auditory stimulation using identical stimulation paradigms. Both functional imaging modalities localized brain activity in adjacent areas of anatomically correct cortex. The mean distances measured between fMRI activity and the corresponding MEG dipoles were 10.1 mm (motor), 10.7 mm (somatosensory), 13.5 mm (auditory right hemisphere) and 14.3 mm (auditory left hemisphere). The distances found may reflect the correlation between electrophysiological and hemodynamic responses due to the different underlying substrates of neurophysiology measured by fMRI and MEG: BOLD contrast vs neuronal biomagnetic activity.
TL;DR: FMRI results suggest that subtractive or correlative methods may not be sufficiently sensitive to image the activity of nociceptive cells, which are sparsely distributed throughout the somatosensory cortex.
Abstract: In the present study, functional magnetic resonance imaging (fMRI) was used to examine pain perception in humans. Three types of noxious stimuli were presented: electric shock (20.8 mA, 2 Hz), heat (48°C), and mechanical, as well as a control tactile stimulus. The significance of activation at the level of the voxel was determined using correlation analysis. Significant region of interest (ROI) activation was determined by comparing the percentage of active voxels in each ROI to activation in a control ROI in the visual cortex. In response to tactile and shock stimuli, consistent activation was seen in the postcentral gyrus, parietal operculum, and ipsilateral cerebellar cortex. No significant cortical activation was detected in response to noxious heat or mechanical stimulation when compared to nonpainful intensity levels. The data did not indicate adaptation, although further study in this area is necessary. Stationary noxious thermal and mechanical stimulation are “pure” noxious stimuli, while electrical stimulation influenced nociceptive and nonnociceptive receptors. Lack of detectable activation in response to pure noxious stimuli supports the idea that nociceptive and nonnociceptive fibers are interspersed in the somatosensory cortex. Conflicting results from recent functional imaging studies of pain perception regarding cortical activation indicate that it is essential to consider both the tactile and nociceptive components of the stimuli used, the spatial extent of stimulation, and the possibility of adaptation to the response. Furthermore, these results suggest that subtractive or correlative methods may not be sufficiently sensitive to image the activity of nociceptive cells, which are sparsely distributed throughout the somatosensory cortex. Hum. Brain Mapping 6:150–159, 1998. © 1998 Wiley-Liss, Inc.
TL;DR: The findings suggest that the somatosensory change-related positivity is probably generated not by activation of new afferent elements but by a detection of change in a process of comparison with sensory memory.
TL;DR: This study demonstrates the transfer of both motor and sensory functions from one hemisphere to the other in children who had an entire cortical hemisphere surgically removed, suggesting the regionalization of brain plasticity.
TL;DR: Results of stimulation through a microelectrode at microampere thresholds (threshold microstimulation; TMIS) in the region of the human thalamic principal sensory nucleus (ventral caudal; Vc) during operations for treatment of movement disorders or of chronic pain are described.
Abstract: Stimulation of the somatosensory system is more likely to evoke pain in patients with chronic pain after nervous system injury than in patients without somatosensory abnormalities. We now describe results of stimulation through a microelectrode at microampere thresholds (threshold microstimulation; TMIS) in the region of the human thalamic principal sensory nucleus (ventral caudal; Vc) during operations for treatment of movement disorders or of chronic pain. Patients were trained preoperatively to use a standard questionnaire to describe the location (projected field) and quality of sensations evoked by TMIS intraoperatively. The region of Vc was divided on the basis of projected fields into areas representing the part of the body where the patients experienced chronic pain (pain affected) or did not experience chronic pain (pain unaffected) and into a control area located in the thalamus of patients with movement disorders and no experience of chronic pain. The region of the Vc was also divided into a core region and a posterior-inferior region. The core was defined as the region above a standard radiologic horizontal line (anterior commissure-posterior commissure line; ACPC line) where the majority of cells responded to innocuous somatosensory stimulation. The posterior-inferior area was a cellular area posterior and inferior to the core. In both the core and the posterior-inferior regions, the proportion of sites where TMIS evoked pain was larger in pain-affected and unaffected areas than in control areas. The number of sites where thermal (warm or cold) sensations were evoked was correspondingly smaller, so that the total of pain-plus-thermal (sensation of warmth or cold) sites was the same in all areas. Therefore, sites pain where stimulation evoked pain in patients with neuropathic pain (i.e., pain following an injury to the nervous system) may correspond to sites where thermal sensations were evoked by stimulation in patients without somatosensory abnormality.
TL;DR: The data show that somatotopically organized lateral striatal neurons stop discharging in response to natural stimulation during anesthesia, and available data indicate they do not reach threshold in Response to depolarizations produced by glutamatergic corticostriatal synaptic transmission projected from the somatosensory cortex.
Abstract: The somatotopic organization of the lateral striatum has been demonstrated by anatomical studies of corticostriatal projections from somatosensory and motor cortices and by single-cell recordings in awake animals. The functional organization in the rat, characterized thus far in the freely moving rat preparation, could be mapped more precisely if a stereotaxic, and possibly an anesthetized, preparation could be used. Because striatal discharges evoked by innocuous somatosensory stimulation are used in mapping, this study tested whether such discharges can be observed during anesthesia, encouraged by responsiveness during anesthesia in somatosensory cortical layers projecting to the striatum. Electrode tracks through lateral striatum of anesthetized rats (pentobarbital or ketamine) revealed spontaneously discharging neurons but no discharges evoked by somatosensory examination (passive manipulation and cutaneous stimulation of 14 body parts). Similar tracks in chronically implanted rats showed evoked firing at numerous sites during wakefulness but not during anesthesia (pentobarbital or urethane). Comparisons of the activity of individual neurons between wakefulness and anesthesia showed that pentobarbital, ketamine, chloral hydrate, urethane, or metofane eliminated evoked firing and suppressed spontaneous firing. Recovery time was greater for neural than for behavioral measures. Thus, mapping as proposed is ruled out, and more importantly, the data show that somatotopically organized lateral striatal neurons stop discharging in response to natural stimulation during anesthesia. Available data indicate they do not reach threshold in response to depolarizations produced by glutamatergic corticostriatal synaptic transmission projected from the somatosensory cortex. These data and demonstrations of anesthetic-induced imbalances in most striatal neurotransmitters emphasize that many results regarding striatal physiology and pharmacology during anesthesia cannot be extrapolated to behavioral conditions, thus indicating the need for more empirical testing in conscious animals.
TL;DR: It is suggested that sensory input can regulate cortical GABAA receptors in adulthood and during ontogeny and may be a compensatory mechanism that serves to disinhibit the reduced sensory input.
Abstract: Both sensory deprivation and blockade of γ-aminobutyric acid A (GABAA) receptors result in signs of cortical disinhibition. To investigate whether down-regulation of GABAA receptors could underlie effects of sensory deprivation, [3H]muscimol binding was assessed in rat whisker barrels after chronic whisker trimming. Vibrissae in row C or rows A,B,D, and E were trimmed during certain developmental periods. When whiskers were trimmed for the first 6 postnatal weeks, [3H]muscimol binding was 8.3% lower in deprived barrel rows than in adjacent nondeprived rows (P < 0.001). The effect may be somewhat selective for GABAA receptors because there was no evident change in N-methyl-D-aspartate (NMDA) receptors as indicated by [3H]MK-801 binding. Ten weeks after whiskers were allowed to regrow, the decrease in [3H]muscimol binding was partly reversed (P < 0.002), leaving a 3.3% decrease (P < 0.001). These declines in GABAA receptors could contribute to persisting electrophysiological signs of reduced inhibition in similarly deprived barrel neurons (Simons and Land [1987] Nature 326:694–697). A 6-week deprivation beginning in adulthood resulted in a 7.7% decrease (P < 0.001), indicating that the effect is not restricted to an early critical period. In rats trimmed for the first 10 postnatal days, [3H]muscimol binding declined 2.3% (P < 0.05), which is a small change compared with the magnitude of the developmental peak; thus, normal whisker input apparently is not required for the developmental increase in GABAA receptors. The present study suggests that sensory input can regulate cortical GABAA receptors in adulthood and during ontogeny. Down-regulation of cortical GABAA receptors may be a compensatory mechanism that serves to disinhibit the reduced sensory input. J. Comp. Neurol. 395:209–216, 1998. © 1998 Wiley-Liss, Inc.
TL;DR: Somaticatosensory evoked magnetic fields to left median nerve electric stimulation from seven healthy subjects implicate differential roles for various somatosensary cortices in intensity coding.
Abstract: We recorded somatosensory evoked magnetic fields (SEFs) to left median nerve electric stimulation from seven healthy subjects. The stimulus intensity was varied in three sessions: sensory stimuli evoked a clear tactile sensation without any movement, weak motor stimuli exceeded the motor threshold, and strong motor stimuli caused a vigorous movement. Responses were modelled with sources in the contralateral primary somatosensory cortex (SI), the contralateral and ipsilateral secondary somatosensory cortices (SIIs) and the contralateral posterior parietal cortex (PPC). The amplitude of the 20 ms response from the SI cortex and the subjective magnitude estimations followed the stimulus intensity whereas signals from the three other areas saturated already at the level of the motor threshold. The results implicate differential roles for various somatosensory cortices in intensity coding.
TL;DR: Compared cortical topography many months after combined median and ulnar nerve transection with “complete” reorganization evident at relatively short survival times, it is hypothesized that the early crudely topographic maps reflect all available inputs while the refined map is the outcome of an extraction process where only the most useful subset of available inputs is expressed.
Abstract: It has previously been demonstrated that the central somatosensory topographic reorganization within deprived cortex that follows peripheral nerve injury in adult monkeys occurs in at least two stages: an immediate unmasking period; and a more prolonged period where deprived areas of cortex come to express new receptive fields in a topographically arranged manner. In the present experiments, we have compared cortical topography many months after combined median and ulnar nerve transection with "complete" reorganization evident at relatively short (i.e., 2-5 months) survival times. We find further reorganizational changes in cortical topography with longer survival times. That is, the roughly somatotopic, generally multiple-digit receptive fields frequently observed at the shorter survival times are generally sharpened to more distinct, single-digit receptive fields at longer survival times. We hypothesize that the early crudely topographic maps reflect all available inputs while the refined map is the outcome of an extraction process where only the most useful subset of available inputs is expressed. It is further suggested that this distillation process is a use-dependent phenomenon.
TL;DR: It appears that the reflex pathways of the SBR and R2 converge within the brainstem and compete with each other, presumably by presynaptic inhibition at the premotor level, before entering the common blink-reflex pathway.
Abstract: The somatosensory-evoked blink response (SBR) is a newly identified blink reflex elicited by electrical stimulation of peripheral nerves. The present study was performed to investigate the physiological mechanism underlying the SBR elicited by median nerve stimulation in normal subjects. The peripheral afferents responsible for the SBR included low-threshold cutaneous fibres. In the SBR-positive subjects, the late (R2) component of the blink reflex elicited by supraorbital nerve stimulation and the SBR facilitated each other when both responses were induced at the same time, but they each caused long-lasting inhibition in the other when one stimulus was given as a conditioning stimulus. The extent of inhibition was correlated with the size of the preceding SBR. In the SBR-negative subjects, simultaneous inhibition of R2 was observed when median nerve stimulation was applied as a conditioning stimulus. Brainstem excitability, as evaluated by blink-reflex recovery studies, did not differ between SBR-positive and SBR-negative subjects. Therefore, based on anatomical and physiological findings, it appears that the reflex pathways of the SBR and R2 converge within the brainstem and compete with each other, presumably by presynaptic inhibition at the premotor level, before entering the common blink-reflex pathway. The influence of median nerve stimulation upon tonic contraction of the orbicularis oculi muscle was studied to detect the latent SBR. There was not only a facilitatory period corresponding to the SBR but also an active inhibitory period (exteroceptive suppression), suggesting that the mechanism generating the SBR is not only influenced by blink-reflex volleys but also by active exteroceptive suppression. Thus, the SBR may appear as a result of integration of facilitatory and inhibitory mechanisms within the brainstem.