Distribution of mislocalizations of tactile stimuli on the fingers of the human hand

Sensitivity of the fingers of the two hands to faint tactile stimuli were tested in eight healthy subjects with a von Frey hair in a forced choice point localization test. Frequencies of correct responses were higher on the left than on the right hand, consistent with a right hemispheric advantage for spatial processing. Within the hands, stimulations of the ring fingers resulted in the highest percentage of correct localizations and stimulations of the thumbs in the fewest correct responses. This superiority of the ring fingers is probably related to a higher point pressure sensitivity and does not reflect the relative size of the representational area of the different fingers in the somatosensory cortex. Mislocalizations, i.e., stimuli that were not correctly attributed to the stimulation site, were located in the vicinity of the stimulation site within the finger as well as across fingers. The distribution of mislocalization across fingers deviates from a distribution expected by chance, showing a higher frequency of mislocalizations to the neighboring fingers than to more distant fingers. This observation in humans matches well with electrophysiological evidence from animal studies that some primary somatosensory cortex neurons have receptive fields that are not restricted to a single digit, but rather cover neighboring digits.     

The effects of neck flexion on cerebral potentials evoked by visual,auditory and somatosensory stimuli and focal brain blood flow in related sensory cortices

Background

A flexed neck posture leads to non-specific activation of the brain. Sensory evoked cerebral potentials and focal brain blood flow have been used to evaluate the activation of the sensory cortex. We investigated the effects of a flexed neck posture on the cerebral potentials evoked by visual, auditory and somatosensory stimuli and focal brain blood flow in the related sensory cortices.

Methods

Twelve healthy young adults received right visual hemi-field, binaural auditory and left median nerve stimuli while sitting with the neck in a resting and flexed (20° flexion) position. Sensory evoked potentials were recorded from the right occipital region, Cz in accordance with the international 10–20 system, and 2 cm posterior from C4, during visual, auditory and somatosensory stimulations. The oxidative-hemoglobin concentration was measured in the respective sensory cortex using near-infrared spectroscopy.

Results

Latencies of the late component of all sensory evoked potentials significantly shortened, and the amplitude of auditory evoked potentials increased when the neck was in a flexed position. Oxidative-hemoglobin concentrations in the left and right visual cortices were higher during visual stimulation in the flexed neck position. The left visual cortex is responsible for receiving the visual information. In addition, oxidative-hemoglobin concentrations in the bilateral auditory cortex during auditory stimulation, and in the right somatosensory cortex during somatosensory stimulation, were higher in the flexed neck position.

Conclusions

Visual, auditory and somatosensory pathways were activated by neck flexion. The sensory cortices were selectively activated, reflecting the modalities in sensory projection to the cerebral cortex and inter-hemispheric connections.     

Imaging the spatio-temporal dynamics of supragranular activity in the rat somatosensory cortex in response to stimulation of the paws

We employed voltage-sensitive dye (VSD) imaging to investigate the spatio-temporal dynamics of the responses of the supragranular somatosensory cortex to stimulation of the four paws in urethane-anesthetized rats. We obtained the following main results. (1) Stimulation of the contralateral forepaw evoked VSD responses with greater amplitude and smaller latency than stimulation of the contralateral hindpaw, and ipsilateral VSD responses had a lower amplitude and greater latency than contralateral responses. (2) While the contralateral stimulation initially activated only one focus, the ipsilateral stimulation initially activated two foci: one focus was typically medial to the focus activated by contralateral stimulation and was stereotaxically localized in the motor cortex; the other focus was typically posterior to the focus activated by contralateral stimulation and was stereotaxically localized in the somatosensory cortex. (3) Forepaw and hindpaw somatosensory stimuli activated large areas of the sensorimotor cortex, well beyond the forepaw and hindpaw somatosensory areas of classical somatotopic maps, and forepaw stimuli activated larger cortical areas with greater activation velocity than hindpaw stimuli. (4) Stimulation of the forepaw and hindpaw evoked different cortical activation dynamics: forepaw responses displayed a clear medial directionality, whereas hindpaw responses were much more uniform in all directions. In conclusion, this work offers a complete spatio-temporal map of the supragranular VSD cortical activation in response to stimulation of the paws, showing important somatotopic differences between contralateral and ipsilateral maps as well as differences in the spatio-temporal activation dynamics in response to forepaw and hindpaw stimuli.     

Sustained Spatial Attention to Vibrotactile Stimulation in the Flutter Range: Relevant Brain Regions and Their Interaction

The present functional magnetic resonance imaging (fMRI) study was designed to get a better understanding of the brain regions involved in sustained spatial attention to tactile events and to ascertain to what extent their activation was correlated. We presented continuous 20 Hz vibrotactile stimuli (range of flutter) concurrently to the left and right index fingers of healthy human volunteers. An arrow cue instructed subjects in a trial-by-trial fashion to attend to the left or right index finger and to detect rare target events that were embedded in the vibrotactile stimulation streams. We found blood oxygen level-dependent (BOLD) attentional modulation in primary somatosensory cortex (SI), mainly covering Brodmann area 1, 2, and 3b, as well as in secondary somatosensory cortex (SII), contralateral to the to-be-attended hand. Furthermore, attention to the right (dominant) hand resulted in additional BOLD modulation in left posterior insula. All of the effects were caused by an increased activation when attention was paid to the contralateral hand, except for the effects in left SI and insula. In left SI, the effect was related to a mixture of both a slight increase in activation when attention was paid to the contralateral hand as well as a slight decrease in activation when attention was paid to the ipsilateral hand (i.e., the tactile distraction condition). In contrast, the effect in left posterior insula was exclusively driven by a relative decrease in activation in the tactile distraction condition, which points to an active inhibition when tactile information is irrelevant. Finally, correlation analyses indicate a linear relationship between attention effects in intrahemispheric somatosensory cortices, since attentional modulation in SI and SII were interrelated within one hemisphere but not across hemispheres. All in all, our results provide a basis for future research on sustained attention to continuous vibrotactile stimulation in the range of flutter.     

Tactile-Vibration-Activated Foci in Insular and Parietal-Opercular Cortex Studied with Positron Emission Tomography: Mapping the Second Somatosensory Area in Humans

Positron emission tomographic measurements were used to study the distribution of focal changes in cerebral blood flow (CBF) induced by vibrotactile stimulation of the hands and feet in 22 normal humans. Subjects received bolus intravenous saline injections containing ～ 60 mCi ¹⁵O-labeled water. Active regions during stimulation were defined relative to resting, nonstimulated states. Scan data from different subjects were averaged after stereotactic standardization. The results identified previously described foci of increased CBF in postrolandic sensory cortex (primary somatosensory cortex) and supplementary motor cortex. New statistical testing procedures provided independent demonstrations of two additional increases in regional CBF, bilaterally, within the sylvian fissure. One site along the parietal operculum corresponded to previous conjectures about a second somatosensory cortical area (SII) in humans. Another site also was found on the insula. No topographic organization was found in either location. The discussion considers these responsive areas to innocuous tactile stimuli in reference to suggestions about a role for SII in the perception of pain.     

The interaction of the somatosensory evoked potentials to simultaneous finger stimuli in the human central nervous system. A study using direct recordings

In order to investigate the interaction of sensory electrophysiologic fields arising from the adjacent second (II) and third (III) fingers and the distant second and fifth (V) fingers, direct recordings of somatosensory evoked potentials (SEPs) were performed from the sensory and motor cortices, the sensory thalamic nucleus (nucleus ventralis caudalis, VC) and the cuneate nucleus in humans during neurosurgical operations. Electrical stimulation was given to the II, III or V fingers individually, and also to pairs of either the II and III fingers or the II and V fingers simultaneously. The interaction ratio OR) was devised as the ratio of amplitude attenuation caused by the simultaneous stimulation to two fingers compared with the amplitude of the arithmetically summed SEPs to the individual stimulation of two fingers. The IRs were calculated on N20 and P25 from the sensory cortex, P22 from the motor cortex, P17_thal from the VC, and N16_cune and P35_cune from the cuneate nucleus.With both stimulations to the II and III fingers and the II and V fingers, P25 showed the greatest IR, followed by P22, then by P17_thal while N16_cune exhibited the smallest IR. N20 and P35_cune showed similar IRs and significantly greater IRs with II and III finger stimulation compared with II and V finger stimulation.These results thus indicate that the interaction of somatosensory impulses occurs in several structures along the sensory pathway in CNS, including the cuneate nucleus, the sensory thalamic nucleus, as well as sensory and motor cortices, with the greatest IRs in the cerebral cortices and the weakest ones in the brain-stem. They also suggest that the receptive fields of the fingers in the cortical area generating N20 are arranged according to the order of the fingers while those in the generating sites for cortical P25 and P22, thalamic P17_thal and cuneate N16_cune tend to be arranged in clusters, while P35_cune is possibly modulated by the somatosensory cortex through a long-loop feedback pathway.     

Interaction of influences from the auditory and somatosensory afferent systems on neurons of the primary auditory cortex

In experiments on anesthetized cats, 80 neurons of the primary auditory cortex (A1) were studied. Within the examined neuronal population, 66 cells (or 82.5%) were monosensory units, i.e., they responded only to acoustic stimulations (sound clicks and tones); 8 (10.1%) neurons responded to acoustic stimulation and electrocutaneous stimulation (ECS); the rest of the units (7.4%) were either trisensory (responded also to visual stimulation) or responded only to non-acoustic stimulations. In the A1 area, neurons responding to ECS with rather short latencies (15.6–17.0 msec) were found. ECS usually suppressed the impulse neuronal responses evoked by sound clicks. It is concluded that somatosensory afferent signals cause predominantly an inhibitory effect on transmission of an acoustic afferent volley to the auditory cortex at a subcortical level; however, rare cases of excitatory convergence of acoustic and somatosensory inputs toA1 neurons were observed.     

Stimulus-Rate Sensitivity Discerns Area 3b of the Human Primary Somatosensory Cortex

Previous studies have shown that the hemodynamic response of the primary somatosensory cortex (SI) to electrical median nerve stimulation doubles in strength when the stimulus rate (SR) increases from 1 to 5 Hz. Here we investigated whether such sensitivity to SR is homogenous within the functionally different subareas of the SI cortex, and whether SR sensitivity would help discern area 3b among the other SI subareas. We acquired 3-tesla functional magnetic resonance imaging (fMRI) data from nine healthy adults who received pneumotactile stimuli in 25-s blocks to three right-hand fingers, either at 1, 4, or 10 Hz. The main contrast (all stimulations pooled vs. baseline), applied to the whole brain, first limited the search to the whole SI cortex. The conjunction of SR-sensitive contrasts [4 Hz − 1 Hz] > 0 and [10 Hz − 1 Hz] > 0 ([4Hz − 1Hz] + [10Hz − 1Hz] > 0), applied to the SI cluster, then revealed an anterior-ventral subcluster that reacted more strongly to both 10-Hz and 4-Hz stimuli than to the 1-Hz stimuli. No other SR-sensitive clusters were found at the group-level in the whole-brain analysis. The site of the SR-sensitive SI subcluster corresponds to the canonical position of area 3b; such differentiation was also possible at the individual level in 5 out of 9 subjects. Thus the SR sensitivity of the BOLD response appears to discern area 3b among other subareas of the human SI cortex.     

“面口合谷收”的脑机制

"面口合谷收"是上千年来祖国医学在医疗实践中的经验总结,指位于手阳明大肠经的"合谷穴"能有效治疗大肠经远端循行所过部位"面口部"的疾患(如牙痛、面神经麻痹等).本研究采用单电极和阵列电极电生理技术探讨来自口面部和手部的传入在恒河猴感觉皮层神经元的位域关系,探讨"面口合谷收"的脑机制.在3b区可以记录到外周感受野分布在合谷穴区和口面部相互毗邻的神经元,也记录到合谷-下唇双感受野的会聚神经元.电生理学绘制3b皮层位域图的结果表明,这些神经元确实是在3b皮层并不重合但相互接壤.结扎正中神经和桡神经3个月后3b皮层拇指-口面交接部神经元能够发生可塑性变化.口面部刺激引起反应皮层位域明显扩大深入到拇指的皮层1~2 mm处,表明拇指皮层位域发生了明显的功能重组.本研究表明,"面口合谷收"的脑机制是相互间的接壤关系,并在神经损伤情况下会发生相互"入侵"的脑功能重组的可塑性变化.     

Effects of Stimulation of the Periaqueductal Gray and <Emphasis Type="Italic">Locus Coeruleus</Emphasis> on Postsynaptic Reactions of Cat Somatosensory Cortex Neurons Activated by Nociceptors

In cats, we studied the influences of stimulation of the periaqueductal gray (PAG) and locus coeruleus (LC) on postsynaptic processes evoked in neurons of the somatosensory cortex by stimulation of nociceptive (intensive stimulation of the tooth pulp) and non-nociceptive (moderate stimulations of the infraorbital nerve and ventroposteromedial nucleus of the thalamus) afferent inputs. Twelve cells activated exclusively by nociceptors and 16 cells activated by both nociceptive and non-nociceptive influences (hereafter, nociceptive and convergent neurons, respectively) were recorded intracellularly. In neurons of both groups, responses to nociceptive stimulation (of sufficient intensity) looked like an EPSP-spike-IPSP (the latter, of significant duration, up to 200 msec) complex. Electrical stimulation of the PAG (which could itself evoke activation of the cortical neurons under study) resulted in long-term suppression of synaptic responses evoked by excitation of nociceptors (inhibition reached its maximum at a test interval of 600 to 800 msec). We observed a certain parallelism between conditioning influences of PAG activation and effects of systemic injections of morphine. Isolated stimulation of LC by a short high-frequency train of stimuli evoked primary excitatory responses (complex EPSPs) in a part of the examined cortical neurons, while in other cells high-amplitude and long-lasting IPSP (up to 120 msec) were observed. Independently of the type of the primary response to PAG stimulation, the latter resulted in long-term (several seconds) suppression of the responses evoked in cortical cells by stimulation of the nociceptive inputs. The mechanisms of modulatory influences coming from opioidergic and noradrenergic brain systems to somatosensory cortex neurons activated due to excitation of high-threshold (nociceptive) afferent inputs are discussed.Neirofiziologiya/Neurophysiology, Vol. 37, No. 1, pp. 61–73, January–February, 2005.     

The human operculo-insular cortex is pain-preferentially but not pain-exclusively activated by trigeminal and olfactory stimuli

Increasing evidence about the central nervous representation of pain in the brain suggests that the operculo-insular cortex is a crucial part of the pain matrix. The pain-specificity of a brain region may be tested by administering nociceptive stimuli while controlling for unspecific activations by administering non-nociceptive stimuli. We applied this paradigm to nasal chemosensation, delivering trigeminal or olfactory stimuli, to verify the pain-specificity of the operculo-insular cortex. In detail, brain activations due to intranasal stimulation induced by non-nociceptive olfactory stimuli of hydrogen sulfide (5 ppm) or vanillin (0.8 ppm) were used to mask brain activations due to somatosensory, clearly nociceptive trigeminal stimulations with gaseous carbon dioxide (75% v/v). Functional magnetic resonance (fMRI) images were recorded from 12 healthy volunteers in a 3T head scanner during stimulus administration using an event-related design. We found that significantly more activations following nociceptive than non-nociceptive stimuli were localized bilaterally in two restricted clusters in the brain containing the primary and secondary somatosensory areas and the insular cortices consistent with the operculo-insular cortex. However, these activations completely disappeared when eliminating activations associated with the administration of olfactory stimuli, which were small but measurable. While the present experiments verify that the operculo-insular cortex plays a role in the processing of nociceptive input, they also show that it is not a pain-exclusive brain region and allow, in the experimental context, for the interpretation that the operculo-insular cortex splay a major role in the detection of and responding to salient events, whether or not these events are nociceptive or painful.     

Theta Burst Stimulation Applied over Primary Motor and Somatosensory Cortices Produces Analgesia Unrelated to the Changes in Nociceptive Event-Related Potentials

Continuous theta burst stimulation (cTBS) applied over the primary motor cortex (M1) can alleviate pain although the neural basis of this effect remains largely unknown. Besides, the primary somatosensory cortex (S1) is thought to play a pivotal role in the sensori-discriminative aspects of pain perception but the analgesic effect of cTBS applied over S1 remains controversial. To investigate cTBS-induced analgesia we characterized, in two separate experiments, the effect of cTBS applied either over M1 or S1 on the event-related brain potentials (ERPs) and perception elicited by nociceptive (CO₂ laser stimulation) and non-nociceptive (transcutaneous electrical stimulation) somatosensory stimuli. All stimuli were delivered to the ipsilateral and contralateral hand. We found that both cTBS applied over M1 and cTBS applied over S1 significantly reduced the percept elicited by nociceptive stimuli delivered to the contralateral hand as compared to similar stimulation of the ipsilateral hand. In contrast, cTBS did not modulate the perception of non-nociceptive stimuli. Surprisingly, this side-dependent analgesic effect of cTBS was not reflected in the amplitude modulation of nociceptive ERPs. Indeed, both nociceptive (N160, N240 and P360 waves) and late-latency non-nociceptive (N140 and P200 waves) ERPs elicited by stimulation of the contralateral and ipsilateral hands were similarly reduced after cTBS, suggesting an unspecific effect, possibly due to habituation or reduced alertness. In conclusion, cTBS applied over M1 and S1 reduces similarly the perception of nociceptive inputs originating from the contralateral hand, but this analgesic effect is not reflected in the magnitude of nociceptive ERPs.     

Effects of stimulus duration on neuronal response properties in the somatosensory cortex of the star-nosed mole

Star-nosed moles have a series of mechanosensory appendages surrounding each nostril. Each appendage is covered with sensory organs (Eimer's organs) containing both rapidly adapting and slowly adapting mechanoreceptors and each appendage is represented in primary somatosensory cortex (S1) by a single cortical module. When the skin surface of an appendage is depressed, neurons in the corresponding module in S1 respond in either a transient or sustained fashion. The aim of this study was to characterize and compare the responses of these two classes of neurons to both short (5 or 20 ms) and long (500 ms) mechanosensory stimulation. Activity from neurons in the representation of appendage 11, the somatosensory fovea, was recorded while delivering mechanosensory stimuli to the corresponding skin surface. Transient and sustained neurons had different levels of spontaneous activity and different responses to both short and long mechanosensory stimulation. Neurons with sustained responses had a significantly higher spontaneous firing rate than neurons with transient responses. Transient neurons responded to a 5 ms stimulus with excitation followed by suppression of discharge whereas sustained neurons did not exhibit post-excitatory suppression. Rather, responses of sustained neurons to 5 ms stimuli lasted several hundred milliseconds. Consequently sustained responses contained significantly more spikes than transient responses. These experiments suggest contact to the appendages causes two distinct firing patterns in cortex regardless of the duration of the stimulus. The sustained and transient responses could reflect either the activity of fundamentally different classes of neurons or activity in distinct subcortical and cortical networks.     

Excitability cycles of direct cortical responses during elaboration of conditioned defense reflexes in dogs]

Direct cortical responses (DCRs) to paired stimuli were studied in chronic experiments in dogs during elaboration of classical and instrumental defensive conditioned reflexes. The DCRs were recorded with 20 to 250 ms intervals between stimuli. Paired and single electrical stimulations of the middle suprasylvian gyrus given with a frequency of one per second were used as conditioned stimuli and were reinforced in a similar way. During electrical cutaneous stimulation of the dog's paw and to an even greater extent during isolated action of the conditioned stimulus the initial negativity of the testing DCR became shorter and the degree of its depression diminished. In the case of a following period of facilitation, its degree became greater. It was higher at a distance of 4 to 5 mm from the point of stimulation than at a distance of 2 to 3 mm. During isolated action of the conditioned stimulus, the degree of facilitation was higher than at the period of the possible action of the unconditioned stimulus. The greatest shorterning of the DCR excitability cycle was observed immediately before and during the conditioned lifting of the dog's paw. Excitability cycles of DCR, and possibly of other evoked potentials as well, are a more sensitive indicator of the function state of the cerebral cortex than responses to single stimuli. For this reason it appears promising to use them in studying conditioned reflexes.     

Postsynaptic Reactions in Somatosensory Cortex Neurons Activated by Stimulation of Nociceptors: Modulation upon Stimulation of the Central Grey, <Emphasis Type="Italic">Locus Coeruleus</Emphasis>, and <Emphasis Type="Italic">Substantia Nigra</Emphasis>

In experiments on cats, we studied the effects of electrical stimulation of the cerebral central grey (CG), locus coeruleus (LC), and substantia nigra (SN) on postsynaptic processes evoked by nociceptive volleys in somatosensory cortex neurons. Nineteen cells activated exclusively by stimulation of nociceptors (intense stimulation of the dental pulp) and 26 cells activated by both nociceptive and non-nociceptive (near-threshold) stimulations of the n. infraorbitalis and thalamic nucl. ventroposteromedialis (VPM) were intracellularly recorded (nociceptive and convergent cortical neurons, respectively). In neurons of both groups, stimulation of both nociceptive afferents and the VPM evoked complex responses having on EPSP-spike-IPSP patterns (duration of IPSPs about 200-300 msec). Electrical stimulation of the СG, which per se could activate the examined cortical neurons, induced prolonged suppression of synaptic responses evoked by stimulation of nociceptors; maximum inhibition was observed at 600- to 800-msec-long conditioning–test intervals. A certain parallelism was observed between the conditioning effects of СG stimulation and effects of systemic introduction of morphine. Isolated stimulations of the LC and SN by short high-frequency pulse series evoked primary complex EPSPs in a part of the examined cortical neurons, while high-amplitude IPSPs (up to 120 msec long) were observed in other units. Independently of the type of the primary response, conditioning stimulations of the LC and SN induced long-lasting (several seconds) suppression of synaptic responses evoked in cortical neurons by stimulation of nociceptive inputs. Mechanisms of modulating influences coming from opioidergic, noradrenergic, and dopaminergic cerebral systems to neurons of the somatosensory cortex activated upon excitation of high-threshold (nociceptive) afferent inputs are discussed.     

Descending influences of periaqueductal gray matter and somatosensory cerebral cortex on neurones in trigeminal brain stem nuclei.

Single relay (to thalamus) and nonrelay neurones that responded to innocuous and/or noxious oral-facial stimuli were located in trigeminal brain stem nuclei oralis and caudalis. The responses of the cells and the digastric muscle to these stimuli were tested with conditioning stimulation of the periaqueductal gray matter (PGM) and somatosensory cerebral cortex in cats. A greater suppression of nociceptive responses with PGM stimulation was noted, and this effect may contribute to the profound analgesic action that has been reported to occur with PGM stimulation.     

Corticofugal effects on the neuronal activity in the emotiogenic/motivational zones of the hypothalamus

Neuronal responses to stimulation of the proreal (field 8) and cingular (field 24) cortices, pyriform lobe (periamygdalar cortex), and hippocampus (CA3) were studied in the lateral (HL) and ventromedial (Hvm) hypothalamus, dorsal hypothalamic region (aHd), and projection region of the medial forelimb bundle (MFB); single and repeated (series of a 6–300 sec⁻¹ frequency) stimuli were used. At single stimulations, the minimum proportion of inhibitory responses with respect to excitatory effects was observed when the neocortex (the proreal gyrus) was stimulated; this proportion became successively greater at stimulations of the intermediate cortex (the cingular gyrus) and paleocortex (the pyriform cortex), while stimulation of the archicortex (the hippocampus) evoked mostly inhibitory responses. At repeated stimulation of the cortical structures, inhibitory responses prevailed in the neurons under study: their total number was nearly four times larger than that of excitatory reactions. The response patterns to single and serial stimulations of the cortical structures allowed us to demonstrate: (i) significant diversity of the influences received by hypothalamic neurons from the cortical structures and (ii) the dependence of the pattern of these influences on the phylogenetic specificity of the above structures.     

Responses of the red nucleus neurons to stimulation of the paw pads of forelimbs before and after cerebellar lesions.

Cerebellar cortex ablation releases deep cerebellar nuclei of monosynaptic inhibition from Purkinje cells. Therefore, it strengthens excitatory influence from Interpositus Nucleus (IN) upon Red Nucleus (RN), which results in much higher facilitation of the rubro-spinal neurons. This causes a big increase of spontaneous discharge rate, and eliminates brakes of discharges from responses generated by somatosensory stimuli. These two changes destroy content and timing of feedback information flowing through the spino-cerebello-rubro-spinal loop. This false bias of the feedback information, very important for fast postural adjustment and coordination of ongoing movements executed by central motor program, may at least in part be responsible for abnormal motor behavior evoked by cerebellar damage. Hemicerebellectomy resulted in dramatically reduced spontaneous activity and responses to limb stimulation because of severing a major input to the red nucleus from deep cerebellar nuclei. Due to direct somatosensory input to magnocellular Red Nucleus (mcRN) from the spinal cord that bypassed the cerebellum, the latency of response to limb stimulation was not changed and the narrower receptive fields were still present.     

Spatial frames of reference and somatosensory processing: a neuropsychological perspective.

In patients with lesions in the right hemisphere, frequently involving the posterior parietal regions, left-sided somatosensory (and visual and motor) deficits not only reflect a disorder of primary sensory processes, but also have a higher-order component related to a defective spatial representation of the body. This additional factor, related to right brain damage, is clinically relevant: contralesional hemianaesthesia (and hemianopia and hemiplegia) is more frequent in right brain-damaged patients than in patients with damage to the left side of the brain. Three main lines of investigation suggest the existence of this higher-order pathological factor. (i) Right brain-damaged patients with left hemineglect may show physiological evidence of preserved processing of somatosensory stimuli, of which they are not aware. Similar results have been obtained in the visual domain. (ii) Direction-specific vestibular, visual optokinetic and somatosensory or proprioceptive stimulations may displace spatial frames of reference in right brain-damaged patients with left hemineglect, reducing or increasing the extent of the patients'' ipsilesional rightward directional error, and bring about similar directional effects in normal subjects. These stimulations, which may improve or worsen a number of manifestations of the neglect syndrome (such as extrapersonal and personal hemineglect), have similar effects on the severity of left somatosensory deficits (defective detection of tactile stimuli, position sense disorders). However, visuospatial hemineglect and the somatosensory deficits improved by these stimulations are independent, albeit related, disorders. (iii) The severity of left somatosensory deficits is affected by the spatial position of body segments, with reference to the midsagittal plane of the trunk. A general implication of these observations is that spatial (non-somatotopic) levels of representation contribute to corporeal awareness. The neural basis of these spatial frames includes the posterior parietal and the premotor frontal regions. These spatial representations could provide perceptual-premotor interfaces for the organization of movements (e.g. pointing, locomotion) directed towards targets in personal and extrapersonal space. In line with this view, there is evidence that the sensory stimulations that modulate left somatosensory deficits affect left motor disorders in a similar, direction-specific, fashion.     

Raphe Stimulation-Evoked Modulation of Postsynaptic Responses by Neurons of the Cat Somatosensory Cortex Activated by Stimulation of Nociceptors

We studied the effects of electrical stimulation of the raphe nuclei (RN) of the cat brain on postsynaptic potentials developing in somatosensory cortex neurons activated by nociceptive influences. Intracellular records were obtained from 15 cells, which were either selectively excited by stimulation of nociceptors (intense electrical stimulation of the dental pulp) or activated by both the above nociceptive and non-nociceptive (moderate stimulations of the infraorbital nerve or thalamic ventroposteromedial nucleus, VPMN) influences. In neurons of both groups, stimulation of both nociceptive afferents and the VPMN evoked complex responses (EPSP–AP–IPSP; IPSPs were 200 to 300 msec long). In some studied cortical neurons, isolated electrical stimulation of the RN (which caused the release of serotonin, 5-HT, in the cortex) resulted in relatively short-latency synaptic excitation, while inhibition was observed in other cells. In the case where stimulation of the RN was used as conditioning influence, such stimulation (independently of the kind of the initial response to RN stimulation) led to long-latency and long-lasting suppression of all components of the synaptic reactions evoked by excitation of nociceptors. The maximum of inhibition was observed at test intervals of 300 to 800 msec. The mechanisms underlying modulatory influences coming from the 5-HT-ergic brainstem system to neurons of the somatosensory cortex, which are activated by excitation of high-threshold (nociceptive) afferent inputs, are discussed.