Sensorimotor integration in human primary and secondary somatosensory cortices

We measured somatosensory evoked fields (SEFs) to electric median nerve stimuli from eight healthy subjects with a whole-scalp 122-channel neuromagnetometer in two different conditions: (i) ‘rest', with stimuli producing clear tactile sensation without any motor movement, and (ii) ‘contraction' with exactly the same stimuli as in ‘rest', but with the subjects maintaining sub-maximal isometric contraction in thenar muscles of the stimulated hand. The aim was to study the role of the primary (SI) and secondary somatosensory (SII) cortices in sensorimotor integration. The amplitude of the SI response N20m did not change with coincident isometric contraction, whereas P35m was significantly reduced. On the contrary, activation of contra- and ipsilateral SII cortices was significantly enhanced during the contraction. We suggest that isometric contraction facilitates activation of SII cortices to tactile stimuli, possibly by decreasing inhibition from the SI cortex. The enhanced SII activation may be related to tuning of SII neurons towards relevant tactile input arising from the region of the body where the muscle activation occurs.     

Functional Organization of the Human First and Second Somatosensory Cortices: a Neuromagnetic Study

Multichannel neuromagnetic recordings were used to differentiate signals from the human first (SI) and second (SII) somatosensory cortices and to define representations of body surface in them. The responses from contralateral SI, peaking at 20 – 40 ms, arose mainly from area 3b, where representations of the leg, hand, fingers, lips and tongue agreed with earlier animal studies and with neurosurgical stimulations and recordings on convexial cortex in man. Representations of the five fingers were limited to a cortical strip of ∼2 cm in length. Responses from SII peaked 100 – 140 ms after contra- and ipsilateral stimuli and varied considerably from one subject to another. Signs of somatotopical organization were seen also in SII. Responses of SII were not fully recovered at interstimulus intervals of 8 s.     

Characteristics of the human contra- versus ipsilateral SII cortex.

OBJECTIVES: In order to study the interaction between left- and right-sided stimuli on the activation of cortical somatosensory areas, we recorded somatosensory evoked magnetic fields (SEFs) from 8 healthy subjects with a 122 channel whole-scalp SQUID gradiometer. METHODS: Right and left median nerves were stimulated either alternately within the same run, with interstimulus intervals (ISIs) of 1.5 and 3 s, or separately in different runs with a 3 s ISI. In all conditions 4 cortical source areas were activated: the contralateral primary somatosensory cortex (SI), the contra- and ipsilateral secondary somatosensory cortices (SII) and the contralateral posterior parietal cortex (PPC). RESULTS: The earliest activity starting at 20 ms was generated solely in the SI cortex, whereas longer-latency activity was detected from all 4 source areas. The mean peak latencies for SII responses were 86-96 ms for contralateral and 94-97 ms for ipsilateral stimuli. However, the activation of right and left SII areas started at 61+/-3 and 62+/-3 ms to contralateral stimuli and at 66+/-2 and 63+/-2 ms to ipsilateral stimuli, suggesting a simultaneous commencing of activation of the SII areas. PPC sources were activated between 70 and 110 ms in different subjects. The 1.5 s ISI alternating stimuli elicited smaller SII responses than the 3 s ISI non-alternating stimuli, suggesting that a considerable part of the neural population in SII responds both to contra- and ipsilateral stimuli. The earliest SI responses did not differ between the two conditions. There were no significant differences in source locations of SII responses to ipsi- and contralateral stimuli in either hemisphere. Subaverages of the responses in sets of 30 responses revealed that amplitudes of the SII responses gradually attenuated during repetitive stimulation, whereas the amplitudes of the SI responses were not changed. CONCLUSIONS: The present results implicate that ipsi- and contralateral SII receive simultaneous input, and that a large part of SII neurons responds both to contra- and ipsilateral stimulation. The present data also highlight the different behavior of SI and SII cortices to repetitive stimuli.     

Magnetoencephalographic responses to painful impact stimulation

Magnetoencephalographic (MEG) field recordings are unique to detect current dipoles in SI and SII. Few devices are available for painful mechanical stimulation in magnetically shielded MEG rooms. The aim of the present MEG (dual 37-channel biomagnetometer) study was to investigate the location of the cortical generators evoked by painful impact stimuli of different intensities. An airgun was placed outside the shielded MEG room, and small plastic bullets were fired at the arm and trunk of the subjects in the room. The velocity of the bullet was measured and related to the evoked pain intensity. Stimuli were delivered for each of the following three conditions: strong pain intensity elicited from the upper arm and upper trunk; weak pain intensity elicited from the upper trunk. The evoked MEG responses had a major component with the characteristically polarity-reversal deflections indicating a dipole located beneath the coils. The response could be estimated by a single current dipole. When the estimated locations of the dipoles were superimposed on the individual magnetic resonance images (MRIs), consistent bilateral activation of areas corresponding to the secondary sensory cortices (SII) was found.     

Thalamic and corticocortical connections of the second somatic sensory area of the mouse

Thalamic and corticocortical connections of the second somatic sensory area (SII) in the mouse cerebral cortex were investigated by means of the retrograde transport of horseradish peroxidase. Focal injections of the enzyme were made in physiologically determined locations within the parietal cortex. Results show that SII receives substantial inputs from topographically appropriate regions within the ipsilateral ventrobasal nucleus and from the ipsilateral posterior group. The limb representation, which was previously found to be responsive to auditory stimulation, received inputs also from the medial division of the medial geniculate body. The SII face representation, which is largely unresponsive to auditory stimuli, received little or no input from the medial geniculate body. SII injections yielded retrograde labeling in the topographically appropriate region in the first somatic sensory area (SI), and SI injections retrogradely labeled cells in SII in a pattern consistent with previous electrophysiological maps. Homotypical regions within SI and SII therefore appear to be reciprocally interconnected. SII also receives inputs from the ipsilateral motor cortex and from contralateral SI and SII. Finally, injections into the SI paw but not face regions yielded retrograde labeling in the thalamic ventrolateral nucleus. Thus, the distal limb representations in SI and SII each receive inputs from a third major relay nucleus (i.e., medial geniculate to SII, ventrolateral nucleus to SI) whereas the face representations do not. These results indicate a close functional interrelationship between homotypical areas in SI and SII, though the two areas differ in several important respects. It is proposed that SII in mice may complement the function of SI by helping to define the overall sensory context in which detailed tactile discriminations are made.     

Short communication: mapping of somatosensory cortices with functional magnetic resonance imaging in anaesthetized macaque monkeys

Functional magnetic resonance imaging (fMRI) in macaque monkeys is emerging as a potent candidate to bridge the gap between data from human fMRI studies and data from anatomy, electrophysiology and lesion studies in monkeys. The primary (SI) and secondary (SII) somatosensory cortices are the principal regions for somatosensory information processing and contain systematic representations of the body surface map (somatotopy). To examine the functional organization of the somatosensory cortices in anaesthetized macaque monkeys with fMRI, we asked whether focal and differential activation could be observed in SI and SII in response to tactile stimulation with two parameters: body sides (right and left) and body regions (hand and face). We found that changes in stimulus parameters elicited differential focal activation in both SI and SII in two ways. First, the hand and face stimulation activated SI and SII in the contralateral, but not in the ipsilateral, hemisphere. Second, the hand and face stimulation differentially activated two adjacent regions in both SI and SII. These fMRI results appear to correlate with previous mapping studies by other methods in the macaque somatosensory cortices. This study shows the feasibility of fMRI studies in mapping multiple sensory areas in monkeys by which we can distinguish between adjacent functionally distinct regions.     

Comparison between SI and SII responses as a function of stimulus intensity

In this MEG study we investigated the differences in responses to somatosensory electrical stimuli between primary (SI) and secondary (SII) sensory cortices using 10 different levels of stimulus intensity, starting from below the sensory threshold up to a weak painful level. SI dipole source linearly increased in amplitude as the stimulus intensity raised up to a strong motor level and then saturated at higher stimulation levels. The contralateral and ipsilateral SII dipole source strengths followed the stimulus intensity growing up to the motor threshold, but showed a decrease at the strong motor level, followed by an increase as the stimulus intensity raised towards the weak painful threshold. These results suggest different responses of SI and SII cortices as the intensity of stimulation rises from non-painful to painful values.     

Primary and secondary somatosensory cortex responses to anticipation and pain: a magnetoencephalography study

Several brain regions, including the primary and secondary somatosensory cortices (SI and SII, respectively), are functionally active during the pain experience. Both of these regions are thought to be involved in the sensory-discriminative processing of pain and recent evidence suggests that SI in particular may also be involved in more affective processing. In this study we used MEG to investigate the hypothesis that frequency-specific oscillatory activity may be differentially associated with the sensory and affective components of pain. In eight healthy participants (four male), MEG was recorded during a visceral pain experiment comprising baseline, anticipation, pain and post-pain phases. Pain was delivered via intraluminal oesophageal balloon distension (four stimuli at 1 Hz). Significant bilateral but asymmetrical changes in neural activity occurred in the β-band within SI and SII. In SI, a continuous increase in neural activity occurred during the anticipation phase (20-30 Hz), which continued during the pain phase but at a lower frequency (10-15 Hz). In SII, oscillatory changes only occurred during the pain phase, predominantly in the 20-30 Hz β band, and were coincident with the stimulus. These data provide novel evidence of functional diversity within SI, indicating a role in attentional and sensory aspects of pain processing. In SII, oscillatory changes were predominantly stimulus-related, indicating a role in encoding the characteristics of the stimulus. We therefore provide objective evidence of functional heterogeneity within SI and functional segregation between SI and SII, and suggest that the temporal and frequency dynamics within cortical regions may offer valuable insights into pain processing.     

The profile of the recovery cycle in human primary and secondary somatosensory cortex: a magnetoencephalography study.

OBJECTIVE: To estimate the lifetime of sensory memory in human primary (SI) and secondary (SII) somatosensory cortex with a view to furthering our understanding of the roles played by these cortices in the processing of tactile information. METHODS: Somatosensory evoked fields (SEFs) were recorded following trains of 5 electrical pulses applied to the right median nerve at the wrist using a whole-head 80 channel magnetoencephalography (MEG) system. Recordings were acquired for trains of pulses with differing interstimulus intervals (ISIs) occurring at 100, 200, 300, 400 and 500 ms. The profile of SEF intensities for the different ISIs provided an estimate of the recovery cycle of evoked neuronal activity, and the time constant of the exponential curve fitted to the recovery cycle was calculated to obtain a putative measure of the lifetime of somatic sensory memory in SI and SII. RESULTS: The estimated time constants were 0.11+/-0.06 s (mean+/-SD) in SI and 0.82+/-0.34 s in SII. The mean time constant in SII was significantly longer than that in SI (Student's paired t test: P=0.021; analysis of variance: F(1,3)=19.7, P=0.021). CONCLUSIONS: These data indicate that the lifetime of somatic sensory memory is of longer duration in higher order cortical areas than in primary sensory cortex in the somatosensory information processing system.     

Altered somatosensory processing in trigeminal neuralgia

Trigeminal neuralgia (TN) is a pain state characterized by intermittent unilateral pain attacks in one or several facial areas innervated by the trigeminal nerve. The somatosensory cortex is heavily involved in the perception of sensory features of pain, but it is also the primary target for thalamic input of nonpainful somatosensory information. Thus, pain and somatosensory processing are accomplished in overlapping cortical structures raising the question whether pain states are associated with alteration of somatosensory function itself. To test this hypothesis, we used functional magnetic resonance imaging to assess activation of primary (SI) and secondary (SII) somatosensory cortices upon nonpainful tactile stimulation of lips and fingers in 18 patients with TN and 10 patients with TN relieved from pain after successful neurosurgical intervention in comparison with 13 healthy subjects. We found that SI and SII activations in patients did neither depend on the affected side of TN nor differ between operated and nonoperated patients. However, SI and SII activations, but not thalamic activations, were significantly reduced in patients as compared to controls. These differences were most prominent for finger stimulation, an area not associated with TN. For lip stimulation SI and SII activations were reduced in patients with TN on the contra‐ but not on the ipsilateral side to the stimulus. These findings suggest a general reduction of SI and SII processing in patients with TN, indicating a long‐term modulation of somatosensory function and pointing to an attempt of cortical adaptation to potentially painful stimuli. Hum Brain Mapp, 2009. © 2009 Wiley‐Liss, Inc.     

Magnetoencephalographic study of event‐related fields and cortical oscillatory changes during cutaneous warmth processing

Thermoreception is an important cutaneous sense, which plays a role in the maintenance of our body temperature and in the detection of potential noxious heat stimulation. In this study, we investigated event‐related fields (ERFs) and neural oscillatory activities, which were modulated by warmth stimulation. We developed a warmth stimulator that could elicit a warmth sensation, without pain or tactile sensation, by using a deep‐penetrating 980‐nm diode laser. The index finger of each participant (n = 24) was irradiated with the laser warmth stimulus, and the cortical responses were measured using magnetoencephalography (MEG). The ERFs and oscillatory responses had late latencies (～1.3 s and 1.0–1.5 s for ERFs and oscillatory responses, respectively), which could be explained by a slow conduction velocity of warmth‐specific C‐fibers. Cortical sources of warmth‐related ERFs were seen in the bilateral primary and secondary somatosensory cortices (SI and SII), posterior part of the anterior cingulate cortex (pACC), ipsilateral primary motor, and premotor cortex. Thus, we suggested that SI, SII, and pACC play a role in processing the warmth sensation. Time–frequency analysis demonstrated the suppression of the alpha (8–13 Hz) and beta (18–23 Hz) band power in the bilateral sensorimotor cortex. We proposed that the suppressions in alpha and beta band power are involved in the automatic response to the input of warmth stimulation and sensorimotor interactions. The delta band power (1–4 Hz) increased in the frontal, temporal, and cingulate cortices. The power changes in delta band might be related with the attentional processes during the warmth stimulation.     

Somatosensory cortex responses to median nerve stimulation: fMRI effects of current amplitude and selective attention.

OBJECTIVES: The aim of this study was to localize and to investigate response properties of the primary (SI) and the secondary (SII) somatosensory cortex upon median nerve electrical stimulation. METHODS: Functional magnetic resonance imaging (fMRI) was used to quantify brain activation under different paradigms using electrical median nerve stimulation in healthy right-handed volunteers. In total 11 subjects were studied using two different stimulus current values in the right hand: at motor threshold (I(max)) and at I(min) (1/2 I(max)). In 7 of these 11 subjects a parametric study was then conducted using 4 stimulus intensities (6/6, 5/6, 4/6 and 3/6 I(max)). Finally, in 10 subjects an attention paradigm in which they had to perform a counting task during stimulation with I(min) was done. RESULTS: SI activation increased with current amplitude. SI did not show significant activation during stimulation at I(min). SII activation did not depend on current amplitude. Also the posterior parietal cortex appeared to be activated at I(min). The I(min) response in SII significantly increased by selective attention compared to I(min) without attention. At I(max) significant SI activity was observed only in the contralateral hemisphere, the ipsilateral cerebellum, while other areas possibly showed bilateral activation. CONCLUSIONS: Distributed activation in the human somatosensory cortical system due to median nerve stimulation was observed using fMRI. SI, in contrast to SII, appears to be exclusively activated on the contralateral side of the stimulated hand at I(max), in agreement with the concept of SI's important role in processing of proprioceptive input. Only SII remains significantly activated in case of lower current values, which are likely to exclusively stimulate the sensible fibres mediating cutaneous receptor input. Selective attention only enhances SII activity, indicating a higher-order role for SII in the processing of somatosensory input.     

Brain generators of laser-evoked potentials: from dipoles to functional significance

In this work we review data on cortical generators of laser-evoked potentials (LEPs) in humans, as inferred from dipolar modelling of scalp EEG/MEG results, as well as from intracranial data recorded with subdural grids or intracortical electrodes. The cortical regions most consistently tagged as sources of scalp LERs are the suprasylvian region (parietal operculum, SII) and the anterior cingulate cortex (ACC). Variability in opercular sources across studies appear mainly in the anterior-posterior direction, where sources tend to follow the axis of the Sylvian fissure. As compared with parasylvian activation described in functional pain imaging studies, LEP opercular sources tended to cluster at more superior sites and not to involve the insula. The existence of suprasylvian opercular LEPs has been confirmed by both epicortical (subdural) and intracortical recordings. In dipole-modelling studies, these sources appear to become active less than 150 ms post-stimulus, and remain in action for longer than opercular responses recorded intracortically, thus suggesting that modelled opercular dipoles reflect a "lumped" activation of several sources in the suprasylvian region, including both the operculum and the insula. Participation of SI sources to explain LEP scalp distribution remains controversial, but evidence is emerging that both SI and opercular sources may be concomitantly activated by laser pulses, with very similar time courses. Should these data be confirmed, it would suggest that a parallel processing in SI and SII has remained functional in humans for noxious inputs, whereas hierarchical processing from SI toward SII has emerged for other somatosensory sub-modalities. The ACC has been described as a source of LEPs by virtually all EEG studies so far, with activation times roughly corresponding to scalp P2. Activation is generally confined to area 24 in the caudal ACC, and has been confirmed by subdural and intracortical recordings. The inability of most MEG studies to disclose such ACC activity may be due to the radial orientation of ACC currents relative to scalp. ACC dipole sources have been consistently located between the VAC and VPC lines of Talairach's space, near to the cingulate subsections activated by motor tasks involving control of the hand. Together with the fact that scalp activities at this latency are very sensitive to arousal and attention, this supports the hypothesis that laser-evoked ACC activity may underlie orienting reactions tightly coupled with limb withdrawal (or control of withdrawal). With much less consistency than the above-mentioned areas, posterior parietal, medial temporal and anterior insular regions have been occasionally tagged as possible contributors to LEPs. Dipoles ascribed to medial temporal lobe may be in some cases re-interpreted as being located at or near the insular cortex. This would make sense as the insular region has been shown to respond to thermal pain stimuli in both functional imaging and intracranial EEG studies.     

Effects of distraction on magnetoencephalographic responses ascending through C-fibers in humans.

OBJECTIVE: Using magnetoencephalography (MEG), we evaluated the cerebral regions relating to second pain perception ascending through C-fibers and investigated the effect of distraction on each region. METHODS: Thirteen normal subjects participated in this study. CO2 laser pulses were delivered to the dorsum of the left hand to selectively activate C-fibers. The MEG responses were analyzed using a multi-dipole model. RESULTS: (1) primary somatosensory cortex (SI), and (2) secondary somatosensory cortex (SII)--insula were the main generators for the primary component, 1M, whose mean peak latency was 744 ms. In addition to (1) and (2), (3) cingulate cortex and (4) medial temporal area (MT) were also activated for the subsequent component, 2M, whose mean peak latency was 947 ms. During a mental calculation task (Distraction), all 6 sources were significantly reduced in amplitude, but the SII-insula (P < 0.01) and cingulate cortex (P < 0.001) were more sensitive than the SI (P < 0.05) and MT (P < 0.05). CONCLUSIONS: We confirmed that SI in the contralateral hemisphere and SII-insula, cingulate cortex and MT in bilateral hemispheres play a major role in second pain perception, and all sites were much affected by a change of attention, indicating that these regions are related to the cognitive aspect of second pain perception. SIGNIFICANCE: The SI, SII, cingulate and MT were activated during the C-fiber-related MEG response, and responses in these regions were significantly diminished during mental distraction.     

Evoked magnetic fields from primary and secondary somatosensory cortices: A reliable tool for assessment of cortical processing in the neonatal period

Objective

To determine interhemispheric differences and effect of postmenstrual age (PMA), height, and gender on somatosensory evoked magnetic fields (SEFs) from the primary (SI) and secondary (SII) somatosensory cortices in healthy newborns.

Methods

We recorded SEFs to stimulation of the contralateral index finger (right in 46 and left in 12) healthy fullterm newborns and analyzed the magnetic responses with equivalent current dipoles.

Results

Activity from both the SI and SII was consistently detectable in the contralateral hemisphere of the newborns during quiet sleep. No significant interhemispheric differences existed in SI or SII response peak latencies, source strengths, or location (n = 8, quiet sleep). SI or SII response peak latency or source strength were not significantly affected by PMA, height, or gender.

Conclusions

During the neonatal period (PMA 37–44 weeks), activity from the contralateral SI and SII can be reliably evaluated with MEG. The somatosensory responses are similar in the left and right hemispheres and no corrections for exact PMA, height, or gender are necessary for interpreting the results. However, the evaluation should be conducted in quiet sleep.

Significance

The reproducibility of the magnetic SI and SII responses suggests clinical applicability of the presented MEG method.     

Activation of cat SII cortex by flutter stimulation of contralateral vs. ipsilateral forepaws

A distinguishing feature of SII cortex is that it receives substantial input from skin mechanoreceptors located on both sides of the body. It remains uncertain, however, if integration of bilateral inputs occurs mainly in those regions of SII that represent near-midline body regions or also occurs to a significant extent in those regions of SII that represent the distal extremities. This issue was addressed using extracellular microelectrode recordings in cat SII in combination with the method of optical intrinsic signal (OIS) imaging. Stimulation of the central pad of either the contra- or ipsilateral forepaw with a 25-Hz sinusoidal vertical skin displacement ("skin flutter") stimulus evoked a prominent OIS response ("activation") in an extensive anteroposterior sector of SII. In the anteriorly located SII region that yielded the maximal OIS response to stimulation of the contralateral central pad, neurons consistently possessed receptive fields that included the stimulated skin site. This "forepaw" SII region also exhibited significant although 75% weaker OIS activation in response to stimulation of the ipsilateral central pad. Stimulation of the central pads of either contra- or ipsilateral forepaws also evoked OIS activation in the posteriorly located 'hindlimb' region of SII--defined as the SII region comprised of neurons with receptive fields on the contralateral hindlimb. The OIS response to ipsilateral central pad stimulation was strongest in the posterior SII region that borders the suprasylvian fringe--a region in which neurons have very large, and frequently bilateral, cutaneous receptive fields. The results indicate that widespread regions within cat SII receive cutaneous inputs from the ipsilateral distal forelimb. It is suggested that the functional role of these ipsilateral inputs may be different in different SII regions.     

Enlarged SI and SII somatosensory evoked responses in the CLN5 form of neuronal ceroid lipofuscinosis.

OBJECTIVES: To examine in detail the activation of the primary (SI) and secondary (SII) somatosensory cortex in CLN5, the Finnish variant of late infantile neuronal ceroid lipofuscinoses (NCL). METHODS: Somatory evoked magnetic fields were recorded with a 122-channel planar gradiometer in response to median nerve stimulation in 5 CLN5 patients (aged 8.8-16.7 years) and in 10 healthy age-matched controls. RESULTS: The first two responses from contralateral SI, N20m and P35m, were 6-20 times stronger in the patients than in the controls. The morphology of the subsequent deflections from SI was abnormal in the patients: a prominent N45m was detected, while the normally present P60m deflection was missing. In 4 patients the contra- and in two patients the ipsilateral SII responses were also enlarged. Furthermore, the SII activation was detected at shorter latency in patients than in controls. CONCLUSIONS: At SI, CLN5 is associated with a selective enhancement of the early cortical responses. We propose that the enlargement of N20m most likely reflects increased synchronous input from thalamus, whereas the altered morphology of the following responses may reflect defective interneuronal inhibition at the cortex. The enlargement of SII responses shows that the imbalance between excitation and inhibition in CLN5 extends outside the primary somatosensory areas.     

Neurophysiology and functional neuroanatomy of pain perception.

The traditional view that the cerebral cortex is not involved in pain processing has been abandoned during the past decades based on anatomic and physiologic investigations in animals, and lesion, functional neuroimaging, and neurophysiologic studies in humans. These studies have revealed an extensive central network associated with nociception that consistently includes the thalamus, the primary (SI) and secondary (SII) somatosensory cortices, the insula, and the anterior cingulate cortex (ACC). Anatomic and electrophysiologic data show that these cortical regions receive direct nociceptive thalamic input. From the results of human studies there is growing evidence that these different cortical structures contribute to different dimensions of pain experience. The SI cortex appears to be mainly involved in sensory-discriminative aspects of pain. The SII cortex seems to have an important role in recognition, learning, and memory of painful events. The insula has been proposed to be involved in autonomic reactions to noxious stimuli and in affective aspects of pain-related learning and memory. The ACC is closely related to pain unpleasantness and may subserve the integration of general affect, cognition, and response selection. The authors review the evidence on which the proposed relationship between cortical areas, pain-related neural activations, and components of pain perception is based.     

Topography of the secondary somatosensory cortex in humans: a magnetoencephalo-graphic study

The topography of the secondary somatosensory cortex (SII) responses to somatosensory stimulation applied to various parts of the body of normal volunteers was analyzed using magnetoencephalography (MEG). Although there were large inter-individual differences, the following orders of a location of equivalent current dipoles (ECDs) were found; (1) Anterior-posterior direction: lower lip-upper lip-thumb-middle finger-foot, (2) Medial-lateral direction: foot-middle finger-thumb-upper lip-lower lip, and (3) Lower-upper direction: lower lip-upper-lip-thumb-middle finger-foot. In general, these findings are similar to those obtained in studies of monkeys. However, the differentiation was not as clear as that seen in the homunculus in the primary somatosensory cortex (SI). The auditory cortex is located at a site more posterior, lateral and lower than the SII.     

Corticofugal influence upon cat thalamic ventrobasal complex

The influence of somatosensory cortex upon transmission through its specific thalamic relay nucleus, the ventrobasal complex (VB), was studied in the paralyzed, unanesthetized cat. The medial lemniscus was electrically stimulated, and evoked responses were recorded from the thalamic radiations projecting respectively to the first and second somatosensory cortex (SITR) and SIITR) and from the pial surface of SI and SII. Cortical influence was assessed by cooling so as to produce a functional and reversible ablation. This technique avoided the ambiguity usually associated with direct electrical stimulation of cortex. Such stimulation, as used by several other authors, may lead to uncontrolled transsynaptic effects upon VB neurons via antidromic activation of thalamocortical fibers and resultant invasion of VB recurrent collaterals.Cooling of SI and SII together resulted in greatly augmented evoked activity in thalamocortical projection fibers concurrent with cessation of cortical EEG at an intracortical temperature of 21 °C. This is interpreted to mean that under normal conditions the somatosensory cortex exhibits a net tonic inhibitory influence upon VB transmission. The same results were obtained in thecerveau isolé preparation; thus, the net cortical inhibitory influence could not be mediated by the brain stem reticular formation, but must be a direct corticofugal influence exerted upon VB. Antidromic activation of ML terminals in VB was unaltered by cooling of somatosensory cortex. This suggests that the corticofugal inhibition is mediated via a postsynaptic mechanism, rather than a presynaptic one.Cooling SI alone resulted in increased responses in SITR but not in SIITR. On the other hand, separate cooling of SII resulted in increased responses in both SITR and SIITR. This suggests that each somatosensory receiving area exerts inhibitory control over its own thalamic input but that, in addition, SII exerts control over SI input.