Organization Of Primary Somatosensory Cortex Research Articles

Mesoscale hierarchical organization of primary somatosensory cortex captured by resting-state-fMRI in humans

The primary somatosensory cortex (S1) plays a key role in the processing and integration of afferent somatosensory inputs along an anterior-to-posterior axis, contributing towards necessary human function. It is believed that anatomical connectivity can be used to probe hierarchical organization, however direct characterization of this principle in-vivo within humans remains elusive. Here, we use resting-state functional connectivity as a complement to anatomical connectivity to investigate topographical principles of human S1. We employ a novel approach to examine mesoscopic variations of functional connectivity, and demonstrate a topographic organisation spanning the region's hierarchical axis that strongly correlates with underlying microstructure while tracing along architectonic Brodmann areas. Our findings characterize anatomical hierarchy of S1 as a ‘continuous spectrum’ with evidence supporting a functional boundary between areas 3b and 1. The identification of this topography bridges the gap between structure and connectivity, and may be used to help further current understanding of sensorimotor deficits.

Alteration in the response properties of primary somatosensory cortex related to differential aversive Pavlovian conditioning

The effects of differential aversive Pavlovian conditioning on the functional organization of primary somatosensory cortex (SI) were examined in 17 healthy participants. Neuroelectric source imaging from 60 electrodes was employed while nine subjects received an innocuous electric stimulus (conditioned stimulus, CS) to one finger (left or right) that was followed by painful electric shock to the lower back (unconditioned stimulus, US) and an innocuous stimulus to the other finger that was never followed by pain. Eight subjects received a presentation of the innocuous and painful stimuli with equal probability to both fingers (control group). The data included the electromyogram (EMG) from the left m. corrugator, and judgments of intensity, aversiveness, and CS–US contingency. Only the experimental group displayed EMG conditioning, differential contingency judgments, as well as a change of dipole orientation for the CS and an enhanced dipole moment for the US in the electroencephalogram. Intensity and unpleasantness ratings were altered in a more unspecific manner and did not differ between groups and stimulus conditions. The data suggest that SI contributes to memory processes in associative learning. Pavlovian conditioning of tactile responses might be important in the altered processing of painful stimuli in chronic pain patients where enhanced conditioning has been demonstrated.

Organization of somatosensory cortical areas in the naked mole‐rat (Heterocephalus glaber)

Multiunit electrophysiology was combined with histological analysis of cortical sections to investigate the organization of somatosensory areas in the naked mole-rat. We provide new details for the organization of primary somatosensory cortex (S1) and identify cortical modules and barrels that correspond to the representations of different body parts. In addition, details of the location and organization of secondary somatosensory cortex (S2) are reported, and evidence for a third somatosensory representation, likely the parietal ventral area (PV), is provided and discussed. S1 contained a complete and systematic representation of the contralateral body surface and oral structures. The orientation of S1 was inverted, with the lower body represented medially and the face and oral structures located rostrolaterally. The S2 representation was found in caudolateral cortex forming a mirror image of S1. The two areas were joined at the representation of the vibrissae and snout, so that the orientation of S2 formed an upright representation of the body in cortex. Receptive fields for S2 were consistently larger than those in S1. Evidence for the presumptive parietal ventral area, lateral to S2, suggests that this area may be an inverted mirror image of S2. By aligning the electrophysiological maps of body representations with cytochrome oxidase-reacted cortical sections we were able to identify modules related to the buccal pad, chin, vibrissae, forelimb, hindlimb, trunk, tongue, lower incisor, and upper incisors. The orofacial modules in lateral cortex resemble similar modules reported to relate to oral structures previously described in the laboratory rat, owl monkey, and squirrel monkey.

Effects of motor activity on the organization of primary somatosensory cortex

Recent studies have shown that adaptation of representational maps within the primary somatosensory cortex can be induced by task-related motor activity. Here, we explore the relationship between the complexity of the motor task and the extent of task-specific adaptation within the primary somatosensory cortex. We hypothesized that the extent of adaptation increases with the complexity of the motor task. Using neuromagnetic source imaging based on electrical stimulation of the thumb and ring finger, we demonstrate that cortical finger representations are more distant during performance of the pinch finger grip than in a rest condition. Our data suggest that somatosensory cortical maps undergo rapid modulation depending on the task-specific involvement of somatosensory feedback in movements.

Dynamic shifts in the organization of primary somatosensory cortex induced by bimanual spatial coupling of motor activity

Previous work has shown that training and learning can induce powerful changes in the homuncular organization of the primary somatosensory cortex (SI). Moreover, a number of studies suggest the existence of short-term adaptation of representational maps in SI. Recently, motor activity has been shown to induce rapid modulation of somatosensory cortical maps. It is hypothesized that there is a task-related influence of motor and premotor areas upon the organization of somatosensory cortex. In order to test this hypothesis, we studied the functional organization of somatosensory cortex by examining coupling effects in a bimanual movement task. Bimanual coupling is known to be related to an activation of the premotor cortex and the supplementary motor area. The functional organization of the somatosensory cortex for known bimanual coupling effects was compared to the organization of the somatosensory cortex during the same movements but with only a small effort in coupling. Topography of the functional organization of the somatosensory cortex was assessed using neuromagnetic source imaging based on tactile stimulation of the first (D1) and fifth digit (D5). We could show that the cortical representations of D1 and D5 moved further apart during the bimanual coupling task in comparison to the same task without coupling and rest. Our data suggest that somatosensory cortical maps undergo fast and dynamic modulation as a result of a task-related influence of motor or premotor areas.

Neuroelectric source imaging of steady-state movement-related cortical potentials in human upper extremity amputees with and without phantom limb pain

Whereas several studies reported a close relationship between changes in the somatotopic organization of primary somatosensory cortex and phantom limb pain, the relationship between alterations in the motor cortex and amputation-related phenomena has not yet been explored in detail. This study used steady-state movement-related cortical potentials (MRCPs) combined with neuroelectric source imaging to assess the relationship of changes in motor cortex and amputation-related phenomena such as painful and non-painful phantom and residual limb sensations, telescoping, and prosthesis use. Eight upper limb amputees were investigated. A significant positive relationship between reorganization of the motor cortex (distance of the MRCP source location from the mirrored source for hand movement) and phantom limb pain was found. Non-painful phantom sensations as well as painful and non-painful residual limb sensations were unrelated to motor cortical reorganization. A higher amount of motor reorganization was associated with less daily prosthesis use, which also tended to be related to more severe phantom limb pain. These results extend previous findings of a positive relationship between somatosensory reorganization and phantom limb pain to the motor domain and suggest a potential positive effect of prosthesis use on phantom limb pain and cortical reorganization.

Task-specific plasticity of somatosensory cortex in patients with writer's cramp

Focal dystonias such as writer's cramp are characterized by muscular cramps that accompany the execution of specific motor tasks. Until now, the pathophysiology of focal dystonia remains incompletely understood. Recent studies suggest that the development of writer's cramp is related to abnormal organization of primary somatosensory cortex (SI), which in turn leads to impaired motor function. To explore contributions of SI on mechanisms of task specificity in focal dystonia, we investigated dynamic alterations in the functional organization of SI as well as sensory-motor gating for rest, left- and right-handed writing and brushing in writer's cramp patients and healthy controls. The functional organization of somatosensory cortex was assessed by neuromagnetic source imaging (151 channel whole-head MEG). In accordance with previous reports, distances between cortical representations of thumb and little finger of the affected hand were smaller in patients compared to healthy subjects. However, similar to healthy controls, patients showed normal modulation of the functional organization of SI as induced by the execution of different motor tasks. Both in the control subjects and patients, cortical distances between representations of thumb and little finger increased when writing and brushing compared to the resting condition. Although, cramps only occured during writing, no differences in the organization of SI were seen among motor tasks. Our data suggest that despite alterations in the organization of primary somatosensory cortex in writer's cramp, the capability of SI to adapt dynamically to different tasks is not impaired.

Mechanisms controlling neuronal plasticity in somatosensory cortex

gamma-Aminobutyric acid containing neurons in the somatosensory cortex are the major controlling element determining receptive field size and location. This control of the excitability of cortical neurons can be modulated by activity arising in the basal forebrain. A hypothesis is developed stating that both cholinergic and gamma-aminobutyric acid containing projections from the basal forebrain play important roles in the production of a state in cortex permitting neuronal plasticity to occur. This proposed mechanism involving a simultaneous reduction of inhibition and increased release of acetylcholine allows sensory signals to induce long-term changes in the location and responsiveness of cutaneous receptive fields, thereby changing the somatotopic organization of primary somatosensory cortex.

Mechanisms controlling neuronal plasticity in somatosensory cortex

gamma-Aminobutyric acid containing neurons in the somatosensory cortex are the major controlling element determining receptive field size and location. This control of the excitability of cortical neurons can be modulated by activity arising in the basal forebrain. A hypothesis is developed stating that both cholinergic and gamma-aminobutyric acid containing projections from the basal forebrain play important roles in the production of a state in cortex permitting neuronal plasticity to occur. This proposed mechanism involving a simultaneous reduction of inhibition and increased release of acetylcholine allows sensory signals to induce long-term changes in the location and responsiveness of cutaneous receptive fields, thereby changing the somatotopic organization of primary somatosensory cortex.

Variability and interhemispheric asymmetry of single-whisker functional representations in rat barrel cortex.

1. The rat whisker-to-barrel system was used to investigate the variability and interhemispheric asymmetry in the functional organization of primary somatosensory cortex as assessed with intrinsic signal optical imaging. The areal extent of whisker D1 functional representation was determined for both the left and right barrel cortex of each of 10 adult male rats. The average size of whisker D1 functional representation and the amount of variability away from this average across animals were determined. In addition, interhemispheric asymmetry was addressed at both the population level and the individual level. The degree of side preference for thigmotactic scanning (typical whisker-related rodent behavior) was determined for each rat in an attempt to find a behavioral correlate for the degree of interhemispheric asymmetry in the size of whisker D1 functional representation. 2. The average areal extent of whisker D1 functional representation (defined as area at half-height) was large (1.95 +/- 0.14 mm2, mean +/- SE, N = 10 rats), suggesting that stimulation of a single whisker evokes activity over a large cortical area that includes other whisker representations. 3. The average size of whisker D1 functional representation was not significantly different between the left (1.86 +/- 0.21 mm2) and right (2.04 +/- 0.15 mm2) hemispheric side, suggesting that interhemispheric functional asymmetry of barrel cortex is not systematic toward a specific hemispheric side at the population level. 4. The degree of variability in the size of whisker D1 functional representation from the left hemisphere ranged between 54.6% smaller than to 50.6% larger than the left average areal extent. A large degree of variability was also observed for the right D1 representation, 37.6% smaller than to 34.9% larger than the right average areal extent. Thus it appears that a large variability in the size of unmanipulated single-whisker functional representations exists across animals from the same species and is not exclusive to a particular hemispheric side. 5. In 5 of 10 rats, the size of whisker D1 functional representation between the two hemispheres differed by > or = 25% within an individual animal. Of these five rats, four had a larger representation in their right hemisphere. The degree and direction of behavioral asymmetry was not linearly correlated with the interhemispheric asymmetry in the size of D1 functional representation (r = 0.494). 6. The large size of a single-whisker functional representation as defined with intrinsic signal optical imaging is discussed with respect to previous anatomic and 2-deoxyglucose autoradiography studies, whereas the large variability in this size across animals is discussed with respect to the individuality of each animal. In addition, the results of the present study have implications for projects that plan to investigate relative changes in the size of single-whisker functional representations.

Age-dependent differences in reorganization of primary somatosensory cortex following low thoracic (T12) spinal cord transection in cats

The organization of primary somatosensory cortex was examined in chronic spinal cats that had sustained cord transection at T12 at 3 ages: 2 and 6 weeks of age, and as adults. Five months to 1 yr following transection, the deprived cortex was mapped electrophysiologically (multiunit recordings). The topographical organization found at each age was compared to that present in normal adults to study effects of developmental age on the ability of the somatosensory system to adjust to changes in afferent input. Cortical responses to deprivation of somatosensory input were age dependent. In animals cord transected at 2 weeks of age, the remaining somatic afferent input excited both its normal cortical area and the area normally reserved for the hindlimb. This resulted in 2 somatotopic maps of the rostral trunk and forelimb. In contrast, in cats spinalized at 6 weeks of age, there was only 1 map for the remaining somatosensory input that was distributed across the mediolateral axis of the primary somatosensory cortex. As a result, the remaining somatosensory input was shifted medially from its normal position and was narrower with respect to the rostrocaudal area driven by light tactile input. The amount of cortex that each body region could excite was essentially the same as in normal animals. In adults, a third response was observed; regions normally devoted to forelimb and trunk appeared to be unchanged, and the region previously serving the hindlimb responded only to a limited extent, and only to tactile stimulation of the trunk. In all cases, however, some sites in the cortex could be excited by parts of the body that in normal animals were served by cortical regions from 3 to 10 mm away, a distance much in excess of the maximum extent of reported thalamocortical overlap. We suggest that the various patterns of cortical organization observed at different ages reflect different developmental processes that are active at the time of transection. Further, we hypothesize that often, in major denervations such as spinal cord transection, a significant component of the reorganization occurs at synaptic levels below the cortex in young animals.

Organization of primary somatosensory cortex in the cat.

1. Multi-unit recordings were made from SI cortex of barbiturte-anesthetized cats. In four cats, multiple vertical penetrations were made at closely spaced intervals. In 12 cats, long surface-parallel penetrations were made in the rostrocaudal or the lateromedial directions with observations taken every 100 micron. 2. Evidence is presented suggesting that cytoarchitectonic area 3a receives input from deep receptors and area 3b receives input from cutaneous receptors. 3. Within area 3b there was an abrupt change in submodality such that the rostral portion of 3b was activated by slowly adapting (SA) afferents, while the caudal portion was activated by rapidly adapting (RA) afferents. 4. The change in modality from deep to cutaneous occurred at the 3a/3b border, but the change in submodality occurred within area 3b and there was no obvious anatomical correlate of the latter transition. 5. These data suggest that there are modality- and submodality-specific bands in register with the bands of cytoarchitecture that extend across the mediolateral dimension of primary somatosensory cortex (SI). 6. A particular receptor population (or populations) from all regions of the body delivers information to each functionally specific band--one map is found in area 3a and two are in area 3b. If this pattern holds for the rest of cat SI, then there must be additional maps of the body in cytoarchitectonic areas 1 and 2.

The representation of the body surface in somatosensory area I of the grey squirrel

Microelectrode mapping methods were used to determine the organization of primary somatosensory cortex, SmI, in grey squirrels. A systematic representation of the contralateral body surface was found within somatic konicortex. This primary representation differs from maps of SmI in other mammals in at least two significant ways. The first way in which SmI of squirrels differs from the organization reported for other mammals is that SmI of squirrels contains a double representation of the hand and parts of the forearm. The glabrous skin of the digits is represented twice in a mirror image fashion joined at the finger tips. The hairy skin of the digits, wrist, and parts of the forearm are also represented twice, once on each side of the joined representations of the glabrous skin. A second unique feature of SmI of squirrels is that there is a small region of cortex completely surrounded by SmI that was unresponsive to light cutaneous stimuli under our recording conditions. This unresponsive zone is easily identified in brain sections by architectonic features that deviate from sensory koniocortex and approach motor cortex. A third significant finding was that the back is rostral to the belly in the representation of the trunk in SmI of squirrels. This is the reverse of the orientation reported elsewhere for SmI of mammals, but corresponds to the orientation of the trunk representation in Area 3b of owl monkeys (Kaas et al., '78; Merzenich et al., '78). This similarity supports an earlier contention that the representation of the body in Area 3b of primates is the homolog of SmI in other mammals (Merzenich et al., '78).