Topography of cholinergic afferents from the nucleus basalis of meynert to representational areas of sensorimotor cortices in the rat

We investigated (1) the topography of projection neurons in the nucleus basalis of Meynert (NBM) with efferents to restricted regions of the primary somatosensory (SI), the second somatosensory (SII), and the primary motor (MI) cortices in the rat; (2) the percentage of these NBM projection neurons that were cholinergic; and (3) the collateralization, if any, of single NBM neurons to different subdivisions within SI, to homotopic areas of SI and SII, and to homotopic areas of SI and MI. Retrograde single-and double-labeling techniques were used to study NBM projections to electrophysiologically identified subdivisions of SI and to homotopic representational areas of SI and SII, and of SI and MI. Choline acetyltransferase immunocytochemistry was done to identify cholinergic NBM neurons. Of the retrogradely labeled NBM neurons that projected to selective subdivisions of SI, SII, and MI, 89%, 87%, and 88%, respectively, were cholinergic. We found a rostral-to-caudal progression of retrogradely labeled NBM neurons following a medial-to-lateral sequence of injections into subdivisions of SI. Overlapping groups of single-labeled NBM neurons were observed after injections of different tracers into adjacent subdivisions within SI or homotopic areas of SI and SII, and of SI and MI. We conclude that NBM innervation to SI, SII, and MI is mostly cholinergic in the rat, that each cortical area receives cholinergic afferents from neurons widely distributed within the NBM, and that each NBM neuron projects to a restricted cortical area without significant collateralization to adjacent subdivisions within SI or to homotopic areas of SI and SII, or SI and MI. © 1993 Wiley-Liss, Inc.     

Primary somatosensory cortex is actively involved in pain processing in human

We recorded somatosensory evoked magnetic fields (SEFs) by a whole head magnetometer to elucidate cortical receptive areas involved in pain processing, focusing on the primary somatosensory cortex (SI), following painful CO(2) laser stimulation of the dorsum of the left hand in 12 healthy human subjects. In seven subjects, three spatially segregated cortical areas (contralateral SI and bilateral second (SII) somatosensory cortices) were simultaneously activated at around 210 ms after the stimulus, suggesting parallel processing of pain information in SI and SII. Equivalent current dipole (ECD) in SI pointed anteriorly in three subjects whereas posteriorly in the remaining four. We also recorded SEFs following electric stimulation of the left median nerve at wrist in three subjects. ECD of CO(2) laser stimulation was located medial-superior to that of electric stimulation in all three subjects. In addition, by direct recording of somatosensory evoked potentials (SEPs) from peri-Rolandic cortex by subdural electrodes in an epilepsy patient, we identified a response to the laser stimulation over the contralateral SI with the peak latency of 220 ms. Its distribution was similar to, but slightly wider than, that of P25 of electric SEPs. Taken together, it is postulated that the pain impulse is received in the crown of the postcentral gyrus in human.     

Somatotopic organization of the second somatosensory cortical area after lesions of the primary somatosensory area in infant and adult cats

The somatotopic organization of the second somatosensory cortical area (SII) and receptive fields of multineuron responses to cutaneous stimulation were studied in cats 6–16 months after lesions of the forelimb representation in the primary somatosensory area (SI) at 4 days, 4 weeks of age or in adults. No change was detected in SII. The results contrast with findings of alterations in SII of macaque monkeys following similar ablations of SI.     

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.     

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.     

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.     

Cortical brain responses during passive nonpainful median nerve stimulation at low frequencies (0.5-4 Hz): an fMRI study

Previous findings have shown that the human somatosensory cortical systems that are activated by passive nonpainful electrical stimulation include the contralateral primary somatosensory area (SI), bilateral secondary somatosensory area (SII), and bilateral insula. The present study tested the hypothesis that these areas have different sensitivities to stimulation frequency in the condition of passive stimulation. Functional MRI (fMRI) was recorded in 24 normal volunteers during nonpainful electrical median nerve stimulations at 0.5, 1, 2, and 4 Hz repetition rates in separate recording blocks in pseudorandom order. Results of the blood oxygen level-dependent (BOLD) effect showed that the contralateral SI, the bilateral SII, and the bilateral insula were active during these stimulations. As a major finding, only the contralateral SI increased its activation with the increase of the stimulus frequency at the mentioned range. The fact that nonpainful median-nerve electrical stimuli at 4 Hz induces a larger BOLD response is of interest both for basic research and clinical applications in subjects unable to perform cognitive tasks in the fMRI scanner.     

Suppression of vibrotactile discrimination by transcranial magnetic stimulation of primary somatosensory cortex

A number of human and animal studies have reported a differential representation of the frequency of vibrotactile stimuli in the somatosensory cortices: neurons in the primary somatosensory cortex (SI) are predominantly responsive to lower frequencies of tactile vibration, and those in the secondary somatosensory cortex (SII) are predominantly responsive to higher frequencies. We employed transcranial magnetic stimulation (TMS) over SI in human subjects to investigate the extent to which the inactivation of SI disrupted the discrimination of vibrotactile stimulation at frequencies that give rise to the tactile sensations of flutter (30 Hz) and vibration (200 Hz). Frequency discrimination around the 30-Hz standard following application of TMS to SI was reduced in seven of the eight subjects, and around the 200-Hz standard was reduced in all eight subjects. The average change in discrimination following TMS was about 20% for both low and high frequencies of vibrotactile stimulation. These data suggest that disruption of SI: (1) has a direct effect on the discrimination of both low and high frequencies of vibrotactile stimuli, consistent with a serial model of processing, or (2) has a direct effect on low-frequency vibrotactile stimuli and an indirect effect on the processing of high-frequency vibrotactile stimuli by SII via cortico-cortical connections between the two regions.     

Seeing touch in the somatosensory cortex: a TMS study of the visual perception of touch

Recent studies suggest the existence of a visuo-tactile mirror system, comprising the primary (SI) and secondary (SII) somatosensory cortices, which matches observed touch with felt touch. Here, repetitive transcranial magnetic stimulation (rTMS) was used to determine whether SI or SII play a functional role in the visual processing of tactile events. Healthy participants performed a visual discrimination task with tactile stimuli (a finger touching a hand) and a control task (a finger moving without touching). During both tasks, rTMS was applied over either SI or SII, and to the occipital cortex. rTMS over SI selectively reduced subject performance for interpreting whether a contralateral visual tactile stimulus contains a tactile event, whereas SII stimulation impaired visual processing regardless of the tactile component. These findings provide evidence for a multimodal sensory-motor system with mirror properties, where somatic and visual properties of action converge. SI, a cortical area traditionally viewed as modality-specific, is selectively implicated in the visual processing of touch. These results are in line with the existence of a sensory mirror system mediating the embodied simulation concept.     

Role of the corpus callosum in the somatosensory activation of the ipsilateral cerebral cortex: an fMRI study of callosotomized patients

To verify whether the activation of the posterior parietal and parietal opercular cortices to tactile stimulation of the ipsilateral hand is mediated by the corpus callosum, a functional magnetic resonance imaging (fMRI, 1.0 tesla) study was performed in 12 control and 12 callosotomized subjects (three with total and nine with partial resection). Eleven patients were also submitted to the tactile naming test. In all subjects, unilateral tactile stimulation provoked a signal increase temporally correlated with the stimulus in three cortical regions of the contralateral hemisphere. One corresponded to the first somatosensory area, the second was in the posterior parietal cortex, and the third in the parietal opercular cortex. In controls, activation was also observed in the ipsilateral posterior parietal and parietal opercular cortices, in regions anatomically corresponding to those activated contralaterally. In callosotomized subjects, activation in the ipsilateral hemisphere was observed only in two patients with splenium and posterior body intact. These two patients and another four with the entire splenium and variable portions of the posterior body unsectioned named objects explored with the right and left hand without errors. This ability was impaired in the other patients. The present physiological and anatomical data indicate that in humans activation of the posterior parietal and parietal opercular cortices in the hemisphere ipsilateral to the stimulated hand is mediated by the corpus callosum, and that the commissural fibres involved probably cross the midline in the posterior third of its body.     

Sensorimotor corticocortical projections from rat barrel cortex have an anisotropic organization that facilitates integration of inputs from whiskers in the same row

We used a dual anterograde-tracing paradigm to characterize the organization of corticocortical projections from primary somatosensory (SI) barrel cortex. In one group of rats, biotinylated dextran amine (BDA) and Fluoro-Ruby (FR) were injected into separate barrel columns that occupied the same row of barrel cortex; in the other group, the tracers were deposited into barrel columns residing in different rows. The labeled corticocortical terminals in the primary motor (MI) and secondary somatosensory (SII) cortices were plotted, and digital reconstructions of these plots were quantitatively analyzed. In all cases, labeled projections from focal tracer deposits in SI barrel cortex terminated in elongated, row-like strips of cortex that corresponded to the whisker representations of the MI or SII cortical areas. When both tracers were injected into separate parts of the same SI barrel row, FR- and BDA-labeled terminals tended to merge into a single strip of labeled MI or SII cortex. By comparison, when the tracers were placed in different SI barrel rows, both MI and SII contained at least two row-like FR- and BDA-labeled strips that formed mirror image representations of the SI injection sites. Quantitative analysis of these labeling patterns revealed three major findings. First, labeled overlap in SII was significantly greater for projections from the same barrel row than for projections from different barrel rows. Second, in the infragranular layers of MI but not in the supragranular layers, labeled overlap was significantly higher for projections from the same SI barrel row. Finally, in all layers of SII and in the infragranular layers of MI, the amount of labeled overlap was proportional to the proximity of the tracer injection sites. These results indicate that SI projections to MI and SII have an anisotropic organization that facilitates the integration of sensory information received from neighboring barrels that represent whiskers in the same row.     

Pain processing within the primary somatosensory cortex in humans

To investigate the processing of noxious stimuli within the primary somatosensory cortex (SI), we recorded magnetoencephalography following noxious epidermal electrical stimulation (ES) and innocuous transcutaneous electrical stimulation (TS) applied to the dorsum of the left hand. TS activated two sources sequentially within SI: one in the posterior bank of the central sulcus and another in the crown of the postcentral gyrus, corresponding to Brodmann's areas 3b and 1, respectively. Activities from area 3b consisted of 20- and 30-ms responses. Activities from area 1 consisted of three components peaking at 26, 36 and 49 ms. ES activated one source within SI whose location and orientation were similar to those of the TS-activated area 1 source. Activities from this source consisted of three components peaking at 88, 98 and 109 ms, later by 60 ms than the corresponding TS responses. ES and TS subsequently activated a similar region in the upper bank of the sylvian fissure, corresponding to the secondary somatosensory cortex (SII). The onset latency of the SII activity following ES (109 ms) was later by 29 ms than that of the first SI response (80 ms). Likewise, the onset latency of SII activity following TS (52 ms) was later by 35 ms than that of area 1 of SI (17 ms). Therefore, our results showed that the processing of noxious and innocuous stimuli is similar with respect to the source locations and activation timings within SI and SII except that there were no detectable activations within area 3b following noxious stimulation.     

Differential modulation in human primary and secondary somatosensory cortices during the preparatory period of self-initiated finger movement

To elucidate the mechanisms underlying sensorimotor integration, we investigated modulation in the primary (SI) and secondary (SII) somatosensory cortices during the preparatory period of a self-initiated finger extension. Electrical stimulation of the right median nerve was applied continuously, while the subjects performed a self-initiated finger extension and were instructed not to pay attention to the stimulation. The preparatory period was divided into five sub-periods from the onset of the electromyogram to 3000 ms before movement and the magnetoencephalogram signals following stimulation in each sub-period were averaged. Multiple source analysis indicated that the equivalent current dipoles (ECDs) were located in SI and bilateral SII. Although the ECD moment for N 20 m (the upward deflection peaking at around 20 ms) was not significantly changed, that for P 30 m (the downward deflection peaking at around 30 m) was significantly smaller in the 0- to -500-ms sub-period than the -2000- to -3000-ms sub-period. As for SII, the ECD moment for the SII ipsilateral to movement showed no significant change, while that for the contralateral SII was significantly larger in the 0- to -500-ms sub-period than the -1500- to -2000-ms or -2000- to -3000-ms sub-period. The opposite effects of movement on SI and SII cortices indicated that these cortical areas play a different role in the function of the sensorimotor integration and are affected differently by the centrifugal process.     

Functional characterization of human second somatosensory cortex by magnetoencephalography

Magnetoencephalographic (MEG) recordings allow noninvasive monitoring of simultaneously active brain areas with reasonable spatial and excellent temporal resolution. Whole-scalp neuromagnetic recordings show activation of contralateral primary (SI) and bilateral second (SII) somatosensory cortices to unilateral median nerve stimulation. Recent MEG studies on healthy and diseased human subjects have shown some functional characteristics of SII cortex. Besides tactile input, the SII cortex also responds to nociceptive afferents. The SII activation is differentially modulated by isometric muscle contraction of various body parts. Lesions in the SII cortex may disturb the self-perception of body scheme. Moreover, the SI and SII cortices may be sequentially activated within one hemisphere, but the SII cortex may also receive direct peripheral input on the ipsilateral side.     

Mapping of secondary somatosensory cortex activation induced by vibrational stimulation: an fMRI study

Sensory functional MRI was performed in seven normal volunteers at 1. 5 T using a vibratory stimulus applied to the pad of the first finger of the left hand. The data was normalized to a standard atlas, and individual and group statistical parametric maps were computed. Robust bilateral activation was demonstrated in the secondary somatosensory cortex (SII), indicating a bilateral representation of SII in humans. Greater maxima and activation volumes were achieved in contralateral SII as compared to SI. Sensory fMRI can provide a sensitive assay for probing the nature and function of SII in vivo.     

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.     

Depicting the inner and outer nose: The representation of the nose and the nasal mucosa on the human primary somatosensory cortex (SI)

The nose is important not only for breathing, filtering air, and perceiving olfactory stimuli. Although the face and hands have been mapped, the representation of the internal and external surface of the nose on the primary somatosensory cortex (SI) is still poorly understood. To fill this gap functional magnetic resonance imaging (fMRI) was used to localize the nose and the nasal mucosa in the Brodman areas (BAs) 3b, 1, and 2 of the human postcentral gyrus (PG). Tactile stimulation during fMRI was applied via a customized pneumatically driven device to six stimulation sites: the alar wing of the nose, the lateral nasal mucosa, and the hand (serving as a reference area) on the left and right side of the body. Individual representations could be discriminated for the left and right hand, for the left nasal mucosa and left alar wing of the nose in BA 3b and BA 1 by comparing mean activation maxima and Euclidean distances. Right‐sided nasal conditions and conditions in BA 2 could further be separated by different Euclidean distances. Regarding the alar wing of the nose, the results concurred with the classic sensory homunculus proposed by Penfield and colleagues. The nasal mucosa was not only determined an individual and bilateral representation, its position on the somatosensory cortex is also situated closer to the caudal end of the PG compared to that of the alar wing of the nose and the hand. As SI is commonly activated during the perception of odors, these findings underscore the importance of the knowledge of the representation of the nasal mucosa on the primary somatosensory cortex, especially for interpretation of results of functional imaging studies about the sense of smell. Hum Brain Mapp 35:4751–4766, 2014. © 2014 Wiley Periodicals, Inc .     

Posterior corpus callosum and interhemispheric transfer of somatosensory information: an fMRI and neuropsychological study of a partially callosotomized patient.

Interhemispheric somatosensory transfer was studied by functional magnetic resonance imaging (fMRI) and neuropsychological tests in a patient who underwent resection of the corpus callosum (CC) for drug-resistant epilepsy in two stages. The first resection involved the anterior half of the body of CC and the second, its posterior half and the splenium. For the fMRI study, the hand was stimulated with a rough sponge. The neuropsychological tests included: Tactile Naming Test (TNT), Same-Different Recognition Test (SDRT), and Tactile Finger Localization Test (intra- and intermanual tasks, TFLT). The patient was studied 1 week before and then 6 months and 1 year after the second surgery. Before this operation, unilateral tactile stimulation of either hand activated contralaterally the first (SI) and second (SII) somatosensory areas and the posterior parietal (PP) cortex, and SII and PP cortex ipsilaterally. All three tests were performed without errors. In both postoperative sessions, somatosensory activation was observed in contralateral SI, SII, and PP cortex, but not in ipsilateral SII and PP cortex. Performance was 100% correct in the TNT for the right hand, but below chance for the left; in the other tests, it was below chance except for TFLT in the intramanual task. This case provides the direct demonstration that activation of SII and PP cortex to stimulation of the ipsilateral hand and normal interhemispheric transfer of tactile information require the integrity of the posterior body of the CC.     

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.     

Different cortical organization of visceral and somatic sensation in humans

Sensory stimuli from the visceral domain exhibit perceptual characteristics different from stimuli applied to the body surface. Compared with somatosensation there is not much known about the cortical projection and functional organization of visceral sensation in humans. In this study, we determined the cortical areas activated by non-painful electrical stimulation of visceral afferents in the distal oesophagus, and somatosensory afferents in the median nerve and the lip in seven healthy volunteers using whole-head magnetoencephalography. Stimulation of somatosensory afferents elicited short-latency responses (≈ 20–60 ms) in the primary somatosensory cortex (SI) contralateral (median nerve) or bilateral (lip) to the stimulated side, and long-latency responses (≈ 60–160 ms) bilaterally in the second somatosensory cortex (SII). In contrast, stimulation of visceral oesophageal afferents did not evoke discernible responses in SI but well reproducible bilateral SII responses (≈ 70–190 ms) in close vicinity to long-latency SII responses following median nerve and lip stimuli. Psychophysically, temporal discrimination of successive stimuli became worse with increasing stimulus repetition rates (0.25 Hz, 0.5 Hz, 1 Hz, 2 Hz) only for visceral oesophageal, but not for somatosensory median nerve stimuli. Correspondingly, amplitudes of the first cortical response to oesophageal stimulation emerging in the SII cortex declined with increasing stimulus repetition rates whereas the earliest cortical response elicited by median nerve stimuli (20 ms SI response) remained unaffected by the stimulus frequency. Our results indicate that visceral afferents from the oesophagus primarily project to the SII cortex and, unlike somatosensory afferents, lack a significant SI representation. We propose that this cortical projection pattern forms the neurophysiological basis of the low temporal and spatial resolution of conscious visceral sensation.