Primary Somatic Sensory Cortex Research Articles

The Contribution of the Corpus Callosum to the Symmetrical Representation of Taste in the Human Brain: An fMRI Study of Callosotomized Patients

The present study was designed to establish the contribution of the corpus callosum (CC) to the cortical representation of unilateral taste stimuli in the human primary gustatory area (GI). Unilateral taste stimulation of the tongue was applied to eight patients with partial or total callosal resection by placing a small cotton pad soaked in a salty solution on either side of the tongue. Functional images were acquired with a 1.5 Tesla machine. Diffusion tensor imaging and tractography were also performed. Unilateral taste stimuli evoked bilateral activation of the GI area in all patients, including those with total resection of the CC, with a prevalence in the ipsilateral hemisphere to the stimulated tongue side. Bilateral activation was also observed in the primary somatic sensory cortex (SI) in most patients, which was more intense in the contralateral SI. This report confirms previous functional studies carried out in control subjects and neuropsychological findings in callosotomized patients, showing that gustatory pathways from tongue to cortex are bilaterally distributed, with an ipsilateral predominance. It has been shown that the CC does play a role, although not an exclusive one, in the bilateral symmetrical representation of gustatory sensitivity in the GI area, at least for afferents from one side of the tongue.

PTU-190 Dissociation of upper limb temperature sensation in functional gastrointestinal disorders compared to blood relatives and unrelated controls

<h3>Introduction</h3> Temperature sensations from the upper limbs are transmitted through the contralateral spinothalamic tract in the cord to the ventral posterior lateral nucleus of the thalamus and then the primary somatic sensory cortex. Autonomic dysfunction arising in the central nervous system is known to affect the sympathetic function of skin which can result in a unilateral dissociation of temperature sensation. Autonomic dysfunction is also increasingly being hypothesised as a component of the functional gastrointestinal disorders (FGIDs). <h3>Aims</h3> To assess the bilateral upper limb temperature sensation of a consecutive cohort of FGID patients and compare the incidence of temperature dissociation with controls. <h3>Method</h3> Clinic attenders with a FGID diagnosis were sequentially invited to undertake the temperature sensation test. This comprised holding a bag of ice in both hands for 5 s and stating if they perceived the temperature to be either unilaterally colder or bilaterally equally cold. Unilaterally colder was considered to be evidence of upper limb temperature sensory dissociation. If a relative, spouse or friend was present they were also invited to participate as either a blood relative or un-related control. <h3>Results</h3> 58 FGID patients and 58 controls undertook the temperature sensation test. 42 patients (72.4%) had upper limb temperature dissociation compared with 20 controls (34.4%) [P = 0.005]. 18/22 (81.8%) patients had dissociation compared to 8/22 (36.3%) blood relative controls [P = 0.049], and 24/36 (66.6%) patients had dissociation compared to 12/36 (33.3%) unrelated controls [P = 0.045]. There was no significant difference in the incidence of dissociation between blood relatives and unrelated controls, 36.3% and 33.3% respectively [P = 0.84]. <h3>Conclusion</h3> Upper limb temperature dissociation seems to be more prevalent in patients with FGIDs than their blood relatives or unrelated controls. It may be associated with autonomic dysfunction as a result of a central nervous process. These findings are an interesting observation in our clinic, but do not constitute formal research and are limited by possible biases (selection and assessment) and a relatively small sample size. The observed proportions of temperature dissociation in these groups could be used to calculate an adequate sample size to detect whether this may be true in a formal research study of autonomic dysfunction in FGIDs. <h3>Disclosure of interest</h3> None Declared.

Length and Depth of the Central Sulcus: A Postmortem Study

Context: The central sulcus, which is located between the primary motor and primary somatic sensory cortex is an important structure and landmark of the cerebral hemisphere. Morphology of the central sulcus varies with age, sex, manual skill, handedness and in many diseases. This study is carried out to establish a normal standard in different age groups in length and depth of the central sulcus. Materials and Methods: A cross-sectional, analytical type of study was conducted in the department of Anatomy, Dhaka Medical College, on one hundred and forty (140) on human cerebral hemispheres. The samples were divided into four different age groups i.e. Group A (20-29 years), Group B (30-39 years), Group C (40-49 years) and Group D (50-59 years). All the samples were studied morphologically. Results: The mean length of central sulcus was 102.89±3.55 mm to 96.83±3.92 mm in male and 98.00±5.17 mm to 96.29±2.56 mm in female. Depth of the central sulcus was 20.02±2.41 mm to 21.50±2.21 mm in male and 20.50±2.57 mm to 21.36±1.87 mm in female. The difference in mean length of the right central sulcus between male and female was statistically significant in group A (p&lt;0.05) and B (p&lt;0.05). The difference in mean depth of the central sulcus between male and female was not statistically significant in any age groups. No significant difference was found between right and left hemisphere for both length and depth of the central sulcus. The difference in mean length and depth of the central sulcus between different age groups for both left and right hemisphere was not statistically significant in any age group. Conclusion: The length and depth of the central sulcus showed gradual decreasing values with advancing age. DOI: http://dx.doi.org/10.3329/bja.v11i1.20504 Bangladesh Journal of Anatomy, January 2013, Vol. 11 No. 1 pp 19-24

Dipole source analyses of laser evoked potentials obtained from subdural grid recordings from primary somatic sensory cortex

The cortical potentials evoked by cutaneous application of a laser stimulus (laser evoked potentials, LEP) often include potentials in the primary somatic sensory cortex (S1), which may be located within the subdivisions of S1 including Brodmann areas 3A, 3B, 1, and 2. The precise location of the LEP generator may clarify the pattern of activation of human S1 by painful stimuli. We now test the hypothesis that the generators of the LEP are located in human Brodmann area 1 or 3A within S1. Local field potential (LFP) source analysis of the LEP was obtained from subdural grids over sensorimotor cortex in two patients undergoing epilepsy surgery. The relationship of LEP dipoles was compared with dipoles for somatic sensory potentials evoked by median nerve stimulation (SEP) and recorded in area 3B (see Baumgärtner U, Vogel H, Ohara S, Treede RD, Lenz FA. J Neurophysiol 104: 3029-3041, 2010). Both patients had an early radial dipole in S1. The LEP dipole was located medial, anterior, and deep to the SEP dipole, which suggests a nociceptive dipole in area 3A. One patient had a later tangential dipole with positivity posterior, which is opposite to the orientation of the SEP dipole in area 3B. The reversal of orientations between modalities is consistent with the cortical surface negative orientation resulting from superficial termination of thalamocortical neurons that receive inputs from the spinothalamic tract. Therefore, the present results suggest that the LEP may result in a radial dipole consistent with a generator in area 3A and a putative later tangential generator in area 3B.

Studies of properties of "Pain Networks" as predictors of targets of stimulation for treatment of pain.

Two decades of functional imaging studies have demonstrated pain-related activations of primary somatic sensory cortex (S1), parasylvian cortical structures (PS), and medial frontal cortical structures (MF), which are often described as modules in a “pain network.” The directionality and temporal dynamics of interactions between and within the cortical and thalamic modules are uncertain. We now describe our studies of these interactions based upon recordings of local field potentials (LFPs) carried out in an epilepsy monitoring unit over the one week period between the implantation and removal of cortical electrodes during the surgical treatment of epilepsy. These recordings have unprecedented clarity and resolution for the study of LFPs related to the experimental pain induced by cutaneous application of a Thulium YAG laser. We also used attention and distraction as behavioral probes to study the psychophysics and neuroscience of the cortical “pain network.” In these studies, electrical activation of cortex was measured by event-related desynchronization (ERD), over SI, PS, and MF modules, and was more widespread and intense while attending to painful stimuli than while being distracted from them. This difference was particularly prominent over PS. In addition, greater perceived intensity of painful stimuli was associated with more widespread and intense ERD. Connectivity of these modules was then examined for dynamic causal interactions within and between modules by using the Granger causality (GRC). Prior to the laser stimuli, a task involving attention to the painful stimulus consistently increased the number of event-related causality (ERC) pairs both within the SI cortex, and from SI upon PS (SI > PS). After the laser stimulus, attention to a painful stimulus increased the number of ERC pairs from SI > PS, and SI > MF, and within the SI module. LFP at some electrode sites (critical sites) exerted ERC influences upon signals at multiple widespread electrodes, both in other cortical modules and within the module where the critical site was located. In summary, critical sites and SI modules may bind the cortical modules together into a “pain network,” and disruption of that network by stimulation might be used to treat pain. These results in humans may be uniquely useful to design and optimize anatomically based pain therapies, such as stimulation of the S1 or critical sites through transcutaneous magnetic fields or implanted electrodes.

Prenatal exposure to benzo( a)pyrene impairs later-life cortical neuronal function

Prenatal exposure to environmental contaminants, such as benzo( a)pyrene [B( a)P] has been shown to impair brain development. The overarching hypothesis of our work is that glutamate receptor subunit expression is crucial for cortical evoked responses and that prenatal B( a)P exposure modulates the temporal developmental expression of glutamatergic receptor subunits in the somatosensory cortex. To characterize prenatal B( a)P exposure on the development of cortical function, pregnant Long Evans rats were exposed to low-level B( a)P (300 μg/kg BW) by oral gavage on gestational days 14–17. At this exposure dose, there was no significant effect of B( a)P on (1) the number of pups born per litter, (2) the pre-weaning growth curves and (3) initial and final brain to body weight ratios. Control and B( a)P-exposed offspring were profiled for B( a)P metabolites in plasma and whole brain during the pre-weaning period. No detectable levels of metabolites were found in the control offspring. However, a time-dependent decrease in total metabolite concentration was observed in B( a)P-exposed offspring. On PND100–120, cerebrocortical mRNA expression was determined for the glutamatergic NMDA receptor subunit (NR2B) in control and B( a)P-exposed offspring. Neural activity was also recorded from neurons in primary somatic sensory (barrel) cortex. Semiquantitative PCR from B( a)P-exposed offspring revealed a significant 50% reduction in NR2B mRNA expression in B( a)P-exposed offspring relative to controls. Recordings from B( a)P-exposed offspring revealed that N-methyl- d-aspartate (NMDA) receptor-dependent neuronal activity in barrel cortex evoked by whisker stimulation was also significantly reduced (70%) as compared to controls. Analysis showed that the greatest deficit in cortical neuronal responses occurred in the shorter latency epochs from 5 to 20 ms post-stimulus. The results suggest that in utero exposure to benzo( a)pyrene results in diminished mRNA expression of the NMDA NR2B receptor subunit to result in late life deficits in cortical neuronal activity in the offspring. The findings from this study lead to a strong prediction that in utero exposure to benzo( a)pyrene at a time when synapses are first formed and adjusted in strength by activity in the sensory pathways will produce a strong negative effect on brain function in offspring progeny.

Optic nerve transection affects development and use-dependent plasticity in neocortex of the rat: Quantitative acetylcholinesterase imaging

We investigated the effects of neonatal optic nerve transection on cortical acetylcholinesterase (AChE) activity in hooded rats during postnatal development and following behavioral manipulation after weaning. AChE reaction product was quantified on digitized images of histochemically stained sections in layer IV of primary somatic sensory, primary visual and visual association cortex. Rats with optic nerve transection were compared to sham-operated littermates. In all cortical regions of both types of animal, AChE reaction product was increased to peak 2 weeks after birth and decreased thereafter, reaching adult levels at the end of the third postnatal week. During postnatal development, reaction product in primary visual cortex was lower in rats deprived of retinal input than in sham-operated littermates and the area delineated by reaction product was smaller. However, optic nerve transection did not modify the time course of postnatal development or statistically significantly diminish adult levels of AChE activity. Behavioral manipulations after weaning statistically significantly increased enzyme activity in sham-operated rats in all cortical areas examined. Compared with cage rearing, training in a discrimination task with food reward had a greater impact than environmental enrichment. By contrast, in the rats with optic nerve transection enrichment and training resulted in statistically significantly increased AChE activity only in lateral visual association cortex. Our findings provide evidence for intra- and supramodal influences of the neonatal removal of retinal input on neural activity- and use-dependent modifications of cortical AChE activity. The laminar distribution of the AChE reaction product suggests that the observed changes in AChE activity were mainly related to cholinergic basal forebrain afferents. These afferents may facilitate the stabilization of transient connections between the somatic sensory and the visual pathway.

Stimulus Frequency Processing in Awake Rat Barrel Cortex

In awake rats, we examined the relationship between neural spiking activity in primary somatic sensory cortex and the frequency of whisker stimulation. Neural responses were recorded extracellularly in barrel cortex while single whiskers were deflected with 0.5-18 air puffs per second (apps), a range that includes the whisk rates observed when rats explore their environment and discriminate surfaces with their whiskers. Twenty-nine neurons in layers III and IV were isolated in three rats (23 in barrel columns and 6 in septum columns). At < or = 9 apps, cortical neurons responded with one to two spikes per stimulus, whereas at > 9 apps, the response efficacy was reduced to only 0.2-0.4 spikes per stimulus. Several mechanisms are discussed that could account for the decrement in responsiveness. Despite this adaptation, neural spike rates increased in direct proportion with stimulus frequency when cast on logarithmic scales. At > 9 apps, however, this relationship deteriorated in barrel columns in which the response approximately halved. In contrast, septum column cells continued to increase their spike rates linearly up to 18 apps, although they responded at lower magnitude than the barrel column cells. Our findings suggest that septum column neurons are potential candidates to encode stimulus frequency using spike rate across the entire frequency range relevant to rats' whisking behavior.

Gestational 2,3,7,8-tetrachlorodibenzo- p-dioxin exposure effects on sensory cortex function

Gestational exposure to environmental contaminants such as 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD) poses a significant threat to normal growth and differentiation of the developing brain. To characterize the impact of gestational TCDD exposure on subsequent cortical function, pregnant Long Evans rats were exposed to a single acute dose (100 or 700 ng/kg b.w. via gavage) on gestational day 15. This dosing regimen had no significant effect on birth index. After the TCDD-exposed animals were born and reached maturity, neural activity was recorded under urethane anesthesia from neurons in primary somatic sensory cortex. Spontaneous activity was reduced by ∼50% in barrel cortex compared to corn oil vehicle controls. The magnitude of neuronal response to sensory (whisker) stimuli also was significantly reduced, and responses did not achieve control levels at any stimulus intensity. The greatest deficit was in the short latency component of the cortical responses. These decrements in cortical responsiveness were present in young F1 generation TCDD-exposed animals and persisted for up to 180 days. Because glutamate receptors are crucial to the evoked responses and show developmental regulation, selected iontotropic glutamate receptor subunits (NMDA NR2A + NR2B and GluR1) were profiled for RNA levels in the cortex of F1 generation rats. The expression of NR2B (NMDA receptor) and GluR1 (AMPA receptor) subunits was significantly reduced in the TCDD-exposed F1 generation animals compared to vehicle controls. The results indicate that gestational TCDD exposure results in cortical deficits that are paralled by diminished expression of certain NMDA and AMPA receptor subunits at a time when synapses are being formed for the first time in cortex.

Rate code and temporal code for frequency of whisker stimulation in rat primary and secondary somatic sensory cortex

We recorded responses to frequencies of whisker stimulation from 479 neurons in primary (S1) and secondary (S2) somatic sensory cortex of 26 urethane-anesthetized rats. Five whiskers on the right side of the snout were deflected with air puffs at seven frequencies between 1 and 18/s. In left S1 (barrels and septa) and S2, subsets of neurons (5%) responded to whisker stimulation across the entire range of frequencies with > or = 1 electrical discharges/ten stimuli (full responders). In contrast, 60% of the recorded cells responded above threshold only at stimulus frequencies below 6/s and 35% remained subthreshold at all frequencies tested. Thus, the full responders are unique in that they were always responsive and appeared particularly suited to facilitate a dynamic, broadband processing of stimulus frequency. Full responders were most responsive at 1 stimulus/s, and showed greatest synchrony with whisker motion at 18 stimuli/s. The barrel cells responded with the greatest temporal accuracy between 3 and 15 stimuli/s. The septum cells responded less accurately, but maintained their accuracy at all frequencies. Only septum cells continued to increase their discharge rate with increasing stimulus frequency. The S2 cells discharged with lowest temporal accuracy modulated only by stimulus frequencies < or = 6/s and exhibited the steepest decrease in discharge/stimulus with increasing stimulus frequency. Our observations suggest that full responders in the septa are well suited to encode high frequencies of whisker stimulation in timing and rate of discharge. The barrel cells, in contrast, showed the strongest temporal coding at stimulus frequencies in the middle range, and S2 cells were most sensitive to differences in low frequencies. The ubiquitous decline in discharge/stimulus in S1 and S2 may explain the decrease in blood flow observed at increasing stimulus frequency with functional imaging.

Principles Governing Auditory Cortex Connections

Topographic maps are common constituents of the primary auditory, visual, and somatic sensory cortex. However, in most cortical areas, no such maps have yet been identified, posing a conceptual problem for theories of cortical function centered on topography. What principle guides the organization of these other areas? We investigated this issue in cat auditory cortex. The connectional topography of five tonotopic areas and eight non-tonotopic areas was assessed using retrograde tract tracing and quantified by three metrics: clustering, dispersion, and separation. Clustering measures the spatial density of labeled neurons, dispersion provides an index of their spread, and separation serves as a scaling metric. These parameters each show that all auditory cortical regions receive precise and equally topographic connections from thalamic, corticocortical, and commissural sources. This isotropic principle suggests a common substrate for coordinating communication across the cortex and may reflect common mechanisms related to the developmental patterning of connections. This unifying principle extends to auditory and prefrontal cortex, and perhaps to other neocortical areas.

Both oral and caudal parts of the spinal trigeminal nucleus project to the somatosensory thalamus in the rat

Recent evidence has been accumulated that not only spinal trigeminal nucleus caudalis (Sp5C) neurons but also spinal trigeminal nucleus oralis (Sp5O) neurons respond to noxious stimuli. It is unknown, however, whether Sp5O neurons project to supratrigeminal structures implicated in the sensory processing of orofacial nociceptive information. This study used retrograde tracing with Fluorogold in rats to investigate and compare the projections from the Sp5O and Sp5C to two major thalamic nuclei that relay ascending somatosensory information to the primary somatic sensory cortex: the ventroposteromedial thalamic nucleus (VPM) and the posterior thalamic nuclear group (Po). Results not only confirmed the existence of contralateral projections from the Sp5C to the VPM and Po, with retrogradely labelled neurons displaying a specific distribution in laminae I, III and V, they also showed consistent and similar numbers of retrogradely labelled cell bodies in the contralateral Sp5O. In addition, a topographic distribution of VPM projections from Sp5C and Sp5O was found: neurons in the dorsomedial parts of Sp5O and Sp5C projected to the medial VPM, neurons in the ventrolateral Sp5O and Sp5C projected to the lateral VPM, and neurons in intermediate parts of Sp5O and Sp5C projected to the intermediate VPM. All together, these data suggest that not only the Sp5C, but also the Sp5O relay somatosensory orofacial information from the brainstem to the thalamus. Furthermore, trigemino-VPM pathways conserve the somatotopic distribution of primary afferents found in each subnucleus. These results thus improve our understanding of trigeminal somatosensory processing and help to direct future electrophysiological investigations.

Role of primary somatic sensory cortex in the categorization of tactile stimuli: effects of lesions.

We lesioned the right primary somatic sensory (S1) cortex in two monkeys trained to categorize the speed of moving tactile stimuli. Animals performed the task by pressing with the right hand one of two target switches to indicate whether the speed of a probe moving across the glabrous skin of the left hand was low or high. Sensory performance was evaluated with psychometric techniques and motor behavior was monitored by measuring the reaction (RT) and movement (MT) times before the experiment and throughout the 60 days after the ablation of SI cortex. After the lesion, there was a slight increase in the RTs but no change in the MTs, indicating that removal of SI cortex did not affect the animals' capacity to detect the stimuli. However, monkeys lost their ability to categorize the stimulus speeds. This effect was observed from the 1st day after the lesion until the end of the study. We conclude that somatosensory areas outside SI can by themselves process tactile information in a limited way and that the extraction of higher-order features that takes place during the categorization task requires the intervention of SI cortex.

Structure of the human sensorimotor system. I: Morphology and cytoarchitecture of the central sulcus.

We have studied the morphology of the central sulcus and the cytoarchitecture of the primary sensorimotor cortex in 20 human brains obtained at autopsy. Although the surface appearance of the central sulcus varies greatly from brain to brain (and between hemispheres of individual brains), its deep structure is remarkably consistent. The fundus of the central sulcus is divided into medial and lateral limbs by a complex junction midway between the sagittal and Sylvian fissures. Based on functional imaging studies, this junction appears to be a structural hallmark of the sensorimotor representation of the distal upper extremity. We also identified and measured area 4 (primary motor cortex) and area 3 (primary somatic sensory cortex) in Nissl-stained sections cut orthogonal to the course of the central sulcus. Although the positions of the cytoarchitectonic boundaries in the paracentral lobule showed considerable interindividual variation, the locations of the borders of areas 4 and 3 along the course of the sulcus were similar among the 40 hemispheres examined. In addition to describing more thoroughly this portion of the human cerebral cortex, these observations provide a basis for evaluating lateral symmetry of the human primary sensorimotor cortex.

In vivo effects of insulin-like growth factor-I on the development of sensory pathways: analysis of the primary somatic sensory cortex (S1) of transgenic mice.

In the rodent brain, insulin-like growth factor I (IGF-I) messenger RNA is transiently expressed in sensory projection neurons during periods of synaptogenesis and neuronal growth. Transgenic (Tg) mice with brain IGF-I overexpression and ectopic brain expression of IGF-binding protein-1 (IGFBP-1), an inhibitor of IGF-I actions, show changes in brain size and myelination. We used these mouse models to evaluate in vivo IGF-I effects on sensory pathway development by conducting anatomical studies in the S1 barrel field. Brain size, cortical area, and barrel field dimensions were increased in IGF-I and reduced in IGFBP-1 Tg mice compared with those in wild-type (wt) mice. The brain and cerebral cortex of Tg mice with the highest transgene expression were the most altered in size. Cortex and barrel field size changes were not precisely proportional, because in some Tg mice barrels were relatively more affected than the cortex, whereas in others the opposite was observed. Brain IGF-I overexpression increased the average number of neurons per barrel, neuronal cell body cross-sectional area, and barrel neuropil volume, whereas brain expression of IGFBP-1 reduced each. Neuronal density was greatly reduced in IGF-I Tg mice and increased in IGFBP-1 Tg mice. No differences in body weight, whisker pad and follicle areas, and whisker pad innervation density were found among Tg and wt mice. These observations indicate that IGF-I enhances neuronal growth in developing sensory pathways and support the concept that modified availability of local trophic factors, such as IGF-I, changes brain, neocortical, and S1 relative dimensions by altering neuronal survival and neuropil elaboration. Study of the S1 cortex provides an excellent model to probe the in vivo mechanisms of IGF actions.

In vivo effects of insulin-like growth factor-I on the development of sensory pathways: analysis of the primary somatic sensory cortex (S1) of transgenic mice

In vivo effects of insulin-like growth factor-I on the development of sensory pathways: analysis of the primary somatic sensory cortex (S1) of transgenic mice

Congenital hypothyroidism delays the formation and retards the growth of the mouse primary somatic sensory cortex (S1)

Intrinsic neural factors are thought to regulate the growth and formation of specific regions of the neocortex. To determine whether factors extrinsic to the brain are also involved in shaping the cytoarchitecture of the mammalian neocortex, we evaluated the effects of thyroid hormone (TH) on the formation and postnatal growth of the mouse Sl postero-medial barrel subfield (PMBSF). Congenital deficiency of TH, induced by propylthiouracil administration from day 12 of gestation, did not disrupt S1 specification, because no alterations in barrel number or configuration were observed in congenital hypothyroid mice. Barrel formation, however, was delayed by 3 days. In control mice, barrels were first seen at postnatal day (PN) 4, whereas in congenital hypothyroid mice they appeared at PN7. TH deficiency led to reduced adult brain, cortical, and S1 barrel dimensions. Barrel size, however, was relatively more affected than brain and cortical size. Our observations indicate, therefore, that TH does not participate in S1 specification, but in timing its formation. Our findings also indicate that TH regulates the relative size of the S l barrel field by modulating the developmental timing of areal specification and brain growth.

Changes in glutamate immunoreactivity in the somatic sensory cortex of adult monkeys induced by nerve cuts.

Antibodies to glutamate (Glu) were used to study the effects of reduced afferent input on excitatory neurons in the somatic sensory cortex of adult monkeys. In each monkey, immunocytochemical staining was compared to thionin and cytochrome oxidase (CO) staining in adjacent sections. In the cervical spinal cord, dorsal column nuclei, ventroposterior thalamus, and primary somatic sensory cortex (SI), Glu immunoreactivity (Glu-ir) was analogous to that described in normal animals; regions with reduced or absent Glu-ir were never observed and no appreciable differences were noted between the experimental and normal side. There were also no differences in CO or thionin-stained sections from the affected hemisphere. In the insuloparietal operculum, sections in the hemisphere contralateral to the nerve cut showed that most cortical fields had a normal pattern of Glu-ir (pattern a), some exhibited a reduction of Glu-ir (pattern b), and that in the central portion of the upper bank of the central sulcus, which corresponds to the general location of the hand representation of the second somatic sensory cortex (SII), Glu-ir had virtually disappeared (pattern c). Adjacent sections processed for CO or stained with thionin showed that in the regions corresponding to those characterized by pattern c, CO was slightly decreased and that glial cells had increased in number. In the regions of SII characterized by pattern c, small intensely stained glial cells displayed Glu-ir. These findings indicate that Glu-ir is regulated by afferent activity and suggest that changes in Glu levels in neurons as well as in glial cells may trigger the biochemical processes underlying the functional and structural changes occurring during a slow phase of reorganizational plasticity in the cerebral cortex of adult monkeys.

Categorization of somaesthetic stimuli

We used psychometric techniques to study the sensorimotor performance of four monkeys trained to classify the speed of moving tactile stimuli. Animals performed the task by pressing one of two target switches to indicate whether the speed of probe movement across the glabrous skin of the hand was low or high. Psychometric curves indicated that animals classified the stimulus speeds irrespective of which finger was stimulated, traverse distance and direction. The mean values of the reaction (RT) and movement (MT) times during the correct categorization of low and high stimulus speeds were similar. However, a slight increase was detected in the mean values of the RT during the incorrect categorization but not in the MT. During the task, activity of single neurones (n = 45) was recorded in primary somatic sensory (SI) cortex. The results indicate that a class of neurones (n = 12) of SI cortex increased their impulse rates as a function of the stimulus speeds. However, the magnitude of their responses was similar during the correct and incorrect categorizations of stimuli. The same neurones also responded when the same set of stimuli used in the categorization task were delivered passively. Neurones of SI cortex responded with a latency of 25.8 +/- 0.6 ms (+/- s.e.m.) relative to the beginning of the moving tactile stimuli during the categorization task. The same neurones (n = 17) also responded with a similar latency (24.6 +/- 4.0 ms) when the stimuli were delivered passively. These results may suggest that, although this evoked neuronal activity may be important for the perception of the moving tactile stimuli, more central structures associated with SI cortex may determine the performance of this learned somaesthetic task.

Representation of moving tactile stimuli in the somatic sensory cortex of awake monkeys

1. We have studied how neurons of primary somatic sensory (SI) cortex encode the direction and speed of moving tactile stimuli delivered to the glabrous skin of the contralateral hand. 2. From a total of 178 neurons recorded in SI cortex of 3 awake Macaca mulatta monkeys, 103 were selected for quantitative analysis. Forty-six neurons had slowly adapting (SA) responses, 43 quickly adapting (QA), and 14 mixed SA-QA properties. All possessed cutaneous receptive fields on the distal segments of digits 2, 3, or 4. Receptive fields were scanned with a metal probe (2 mm diam, hemispheric tip) in four different directions (0, 90, 180, and 270 degrees), over a fixed traverse distance of 6 mm, at a variety of speeds (4-100 mm/s), and with a static normal force of 20 g by means of a computer-controlled tactile stimulator. 3. Most neurons gave statistically significant differences in mean impulse rate during the moving stimuli (Wilcoxon, P < 0.01), in at least one of the four directions, compared with the control (nonstimulus) period. The Kruskal-Wallis test (P < 0.01) and the direction index (DI > 35%) determined that about one-half of SI cortical neurons showed significant differences in mean impulse rates associated with the direction of the stimuli at the speeds of 23, 50, and 100 mm/s, and about one-third at 4 mm/s. 4. We determined how the temporal covariance of the neural activity was associated with the parameters of the moving stimuli by calculating the coefficients of the Karhunen-Loève (KL) transform for each set of stimulus responses. Decomposition of the neural activity into principal components indicated that approximately 85% of the impulse train variance during the stimulus responses was contained in the 1st 10 coefficients of the KL transform for the speeds of 23, 50, and 100 mm/s, and approximately 75% at the speed of 4 mm/s. The line spectra calculated from the coefficients of the KL transform showed that the variance contained in the impulse trains in about one-half of the neurons is related to the stimuli. 5. We investigated how the temporal covariance of the neuronal activity was correlated with the direction of the stimulus, by fitting the first coefficient of the KL transform to a weighting function model. This analysis showed that the first coefficient of the KL transform varied as an orderly function of the direction of the moving stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)