Cutaneous afferents from human plantar sole contribute to body posture awareness

We investigated whether the tactile information from the main supporting areas of the foot are used by the brain for perceptual purposes, namely body posture awareness and body representation in space. We applied various patterns of tactile stimulation to one or both soles of unmoving and blindfolded subjects by a 60 micro-vibrator tactile matrix set in a force platform. The perceptual effects of the stimulation were assessed through a 3D joystick handled by the subjects. All subjects reported illusory perceptions of whole-body leaning. Both orientation and amplitude of these perceptions depended on the stimulation pattern. Additional kinesthetic illusions sometimes occurred along the longitudinal axis of the body. We conclude that foot sole input contributes to the coding and the spatial representation of body posture.     

Cerebral slow waves related to the perception of pain in man

Significant amplitude and temporal augmentation occurred in later time segments of human somatosensory evoked responses (60–700 ms) when percutaneous electrical pulse stimulation, delivered to finger, toe, or lip, indicated subjectively both crossing of a perceptual pain threshold and somatotopic movement associated with a noxious, qualitative change. Sequential pseudorandomizing of stimulus intensity (noxious and nonnoxious) or modality (contingent acoustic clicks) suggested that the waveform changes represented, at least in part, stimulus-specific information due to differential activation of peripheral fiber systems, rather than stimulus-nonspecific processing of event significances. The late waves were localizable, on scalp, to parietal and vertex regions, with insignificant contralateralization for finger stimuli. Their augmentation was related to subjective reports rather than to physical stimulus parameters, confirming previous data on potentially noxious mechanical stimulation of digits and palm, and another laboratory's noxious stimulation of tooth pulp. Subsequent data from a third laboratory, resulting from noxious laser stimulation of forearm, have replicated this late, slow-wave activity.     

Studying the human somatosensory hand area: A new way to compare fMRI and MEG

Valid localization is a prerequisite to study plasticity of the somatosensory cortex in humans. We compared the localizations of left and right thumb and little finger in the primary somatosensory cortex obtained with fMRI and MEG. Representations were investigated in 11 healthy right-handed subjects using echoplanar fMRI and 122-channel MEG together with electric finger stimulation. Activation observed with fMRI was based on an increase in the BOLD signal. Most of the activation clusters (71.1%) were located on the lateral surface of the postcentral gyrus. Representations of thumb and little finger were 17mm apart on average and consistently showed a somatotopic arrangement with the thumb representation inferior, lateral, and anterior to the representation of the little finger. Activation observed with MEG was modelled by equivalent current dipoles. Dipole localization was compatible with an assumed origin of activation within the posterior wall of the central sulcus. The Euclidian distance between corresponding dipoles was 11.5mm on average with deviations from the expected spatial arrangement of 35, 30, and 20% in the x-, y- und z-direction, respectively. Our study demonstrates how relative localization of somatosensory activations can serve as an indicator for localization validity when comparing different methods or studying somatosensory plasticity.     

Functional organization of human supplementary motor cortex studied by electrical stimulation

The presence of somatotopic organization in the human supplementary motor area (SMA) remains a controversial issue. In this study, subdural electrode grids were placed on the medial surface of the cerebral hemispheres in 13 patients with intractable epilepsy undergoing evaluation for surgical treatment. Electrical stimulation mapping with currents below the threshold of afterdischarges showed somatotopic organization of supplementary motor cortex with the lower extremities represented posteriorly, head and face most anteriorly, and the upper extremities between these two regions. Electrical stimulation often elicited synergistic and complex movements involving more than one joint. In transitional areas between neighboring somatotopic representations, stimulation evoked combined movements involving the body parts represented in these adjacent regions. Anterior to the supplementary motor representation of the face, vocalization and speech arrest or slowing of speech were evoked. Various sensations were elicited by electrical stimulation of SMA. In some cases a preliminary sensation of "urge" to perform a movement or anticipation that a movement was about to occur were evoked. Most responses were contralateral to the stimulated hemisphere. Ipsilateral and bilateral responses were elicited almost exclusively from the right (nondominant) hemisphere. These data suggest the presence of combined somatotopic organization and left-right specialization in human supplementary motor cortex.     

Both pain and EEG response to cold pressor stimulation occurs faster in fibromyalgia patients than in control subjects

Pain-evoked brain potentials elicited by laser stimulation have been repeatedly shown to be abnormal in fibromyalgia syndrome. However, to our knowledge this is the first study assessing enduring (cold pressor) pain and correlated EEG changes in fibromyalgia. EEG power and subjective pain ratings during the cold pressor test were analyzed and contrasted with tasks not involving sensory stimulation (rest, mental arithmetic and pain imagery) in 20 patients with fibromyalgia and 21 healthy control subjects. Fibromyalgia patients both perceived pain and judged pain as intolerable earlier than control subjects, while pain intensity ratings and EEG power changes during subjective awareness of pain were similar in both groups. In patients and control subjects, pain was correlated with a rise in delta, theta and beta power. EEG power spectra during pain imagery and mental arithmetic were significantly different from those observed during the cold pressor test. In conclusion, fibromyalgia patients seem to process painful stimuli abnormally in a quantitative sense, thus producing both the sensation of pain, as well as the associated EEG patterns, much earlier than control subjects. However, the quality of the pain-associated EEG changes seems similar.     

Human DC scalp potentials during vestibular and optokinetic stimulation: non-specific responses?

In normal subjects transient horizontal body rotation in the dark (vestibular stimulation) elicited a DC potential change with the maximum at Cz. The response appeared to covary with stimulus velocity, which is the most relevant stimulus parameter transferred by the horizontal semicircular canal system. A similar response was obtained during optokinetic stimulation. However, the following findings suggested that the responses are related to 'high' perceptual functions rather than representing a visuo-vestibular evoked cortical potential: (a) the visual response was found independent of whether the subjects perceived, as a task, self-motion in a stationary environment or 'object' motion about the stationary body; its amplitude depended on the subjects' subjective compliance to the task; (b) when presenting various combinations of optokinetic and vestibular stimuli, the response amplitude depended on the subjective and objective intersensory conflict in the combinations; (c) sinusoidal stimulation yielded a negative shift of the DC potential, but the potential was not modulated along with the waxing and waning of stimulus velocity or of the self-motion sensation evoked; (d) patients with loss of vestibular functions showed a similar Cz response during body rotation in the dark.     

Segmentally arranged somatotopy within the face representation of human primary somatosensory cortex

Though somatotypic representation within the face in human primary somatosensory cortex (S1) to innocuous stimuli is controversial; previous work suggests that painful heat is represented based on an "onion-skin" or segmental pattern on the face. The aim of this study was to determine if face somatotopy for brush stimuli in S1 also follows this segmental representation model. Twelve healthy subjects (nine men: three women) underwent functional magnetic resonance imaging to measure blood oxygen level dependent signals during brush (1 Hz, 15 s) applied to their faces. Separate functional scans were collected for brush stimuli repetitively applied to each of five separate stimulation sites on the right side of the face. These sites were arranged in a vertical, horizontal, and circular manner encompassing the three divisions of the trigeminal nerve. To minimize inter-individual morphological differences in the post-central gyrus across subjects, cortical surface-based registration was implemented before group statistical image analysis. Based on activation foci, somatotopic activation in the post-central gyrus was detected for brush, consistent with the segmental face representation model.     

Motor somatotopy of extensor indicis proprius and extensor pollicis longus

After tendon transfer of extensor indicis proprius (EIP) to extensor pollicis longus (EPL), rehabilitation is initiated to enhance motor cortex reorganization. However, patients have been described showing thumb extension immediately after the tendon transfer. At cortical level, no evidence supports either of these assumptions. We noninvasively investigated motor cortical source locations of EIP and EPL muscles. Magnetoencephalography was used to identify motor somatotopic map in healthy right-handed participants, who performed voluntary extension at index metacarpophalangeal joint and thumb interphalangeal joint. Motor cortical representation of EIP was more medial than cortical representation of EPL, with mean Euclidean distance of 15.4±2.7 mm. Motor somatotopic map of EIP/EPL that was obtained by magnetoencephalography supports 'functional somatotopy' representation of the finger in primary motor cortex.     

Somatotopic organization of the human periventricular gray matter.

The periventricular gray (PVG) matter is an established anatomical target for chronic deep brain stimulation (DBS) in the treatment of certain intractable pain syndromes. Data relating to the representation of pain and other somatosensory modalities within the PVG in humans are negligible. We examined the character and location of somatosensory responses elicited by electrical stimulation along the length of the PVG in a patient who underwent unilateral DBS for intractable nociceptive head pain. Consistent responses were obtained and indicated the presence of a somatotopic representation in this region. The contralateral lower limb was represented cranially, followed by the upper limb and trunk, with the face area located caudally, near the level of the superior colliculi. Bilateral representation was only observed in the forehead and scalp.     

District-related frequency specificity in hand cortical representation: dynamics of regional activation and intra-regional synchronization

The aim of this work was to study the degree of neuronal synchronization occurring within the portion of the somatosensory cortex devoted to hand control during an external sensory stimulation. In this way, we focused on the properties of the sensory cortical representation, rather than the more investigated motor one. To this aim, we collected magnetoencephalograhic data from healthy subjects during separate stimulation of their thumbs and little fingers and analyzed these data by means of a time-dependent 'synchronization index'. The properties of this index within the beta [16-32 Hz] and gamma [36-44 Hz] frequency bands suggest that the hand representation in the human primary cortex follows a frequency coding, in addition to the somatotopic one, for discriminating different districts. Our results showed that the gamma synchronization is higher following stimulation of the thumb than of the little finger and we suggest that the strength of gamma band synchronization works as a code for functional prevalence. In particular, our comparative analysis of the dynamic synchronization index and the signal amplitude suggests that a prevalent district (thumb) recruits a smaller number of higher-synchronic gamma band tuned neurons than a non-prevalent district (little finger).     

Somatotopic representation of the tongue in human secondary somatosensory cortex

OBJECTIVE: To clarify the somatotopic representation of the tongue secondary somatosensory cortex (SII) in humans. METHODS: Somatosensory evoked magnetic fields (SEFs) were recorded from nine subjects after stimulating four body sites, left antero (LA) and postero (LP) lateral margins of the tongue, left median nerve at the wrist (Hand), and left tibial nerve at the ankle (Foot). RESULTS: Clear neural activities were recorded from the bilateral SII in both hemispheres after the four sites were stimulated. The tongue SII for LA and LP was located close to the hand SII and significantly more anterior than the Foot SII. There was no significant difference in the location of dipoles between the LA and LP areas of the tongue SII. The mean peak latencies of the tongue SII for LA and LP were significantly shorter in the hemisphere contralateral to the stimulation than the ipsilateral hemisphere. CONCLUSIONS: The tongue areas are considered to occupy a small region in SII with insufficient spatial separation to differentiate anterior from posterior areas even using magnetoencephalography which has a higher spatial resolution than electroencephalography (EEG). SIGNIFICANCE: This is the first systematical study to clarify the activated regions in SII following stimulation of the tongue.     

fMRI shows multiple somatotopic digit representations in human primary somatosensory cortex

Using electrical finger nerve stimulation in normal human subjects, fMRI detected separate representations for all 5 fingers in the primary somatosensory cortex. Responses were located in the posterior wall of the deep central sulcus (most likely corresponding to Brodmann Area (BA) 3b), and the anterior (BA 1) or posterior crown of the postcentral gyrus (BA 2) with rare activations in BA 3a and 4. In BA 3b we found a regular somatotopic mediolateral digit arrangement for fingers 5 to 1 with a mean Euclidean distance of 16 mm between fingers 1 and 5. In contrast BA 1/2 showed a greater number of adjacent activation foci with significantly more overlap and partly even reversed ordering of neighbouring fingers.     

Correlational methods for determining regional coupling of cerebral glucose metabolism: a pilot study

The current study presents an alternate method of analyzing PET data by means of a correlational approach. Correlational or coupling patterns of regional cerebral glucose metabolism were found for normal subjects and patients with schizophrenia under electric shock stimulation. At two levels (91 mm and 78 mm) significant differences were found between the coupling patterns of normal subjects and patients with schizophrenia. For the normals, the obtained correlational maps were consistent with the sensory input. However, such patterns were not found for the patients with schizophrenia. Therefore it is suggested that the outlined analysis procedure may complement the standard approach of comparing the mean metabolic rates.     

Photoparoxysmal response elicited by flickering dot pattern stimulation to the center and periphery.

OBJECTIVE: We performed this study to elucidate the roles of stimulus spatial frequency and visual field in eliciting generalized photoparoxysmal response (PPR) by flickering dot pattern (FDP) stimulation. METHODS: We studied 20 photosensitive epilepsy patients, aged 7-48 years (means +/- SD, 21.4+/-11.7 years), producing a 20 Hz FDP stimulation with a strobo-filter. Using dot patterns (0.5, 1, 2, 4 and 6 mm in diameter), we gave FDP stimulation to the center (11x11 degrees ) and periphery (11-30x11-30 degrees ) of subjects with eyes open, and analyzed each PPR appearance latency. RESULTS: Significant results were: latency by 1 (2.1 c/deg) and 0.5 mm (4.9 c/deg) FDP stimulation to the center was shorter than that to the periphery; latency by 4 mm (0.8 c/deg) FDP stimulation to the periphery was shorter than that to the center. As for 2 (1.5 c/deg) and 6 mm (0.5 c/deg) FDP stimuli, no significant difference could be observed. CONCLUSIONS: A higher spatial frequency FDP stimulation to the center elicits PPR more strongly than that to the periphery, whereas a lower spatial frequency FDP stimulation showed the opposite result, with peripheral stimulation being more effective than central stimulation.     

Interactions between amygdaloid and hypothalamic self-stimulation: a re-examination.

The function relating bar-pressing rate to the frequency of cathodal pulses was obtained in rats self-stimulating with amygdaloid (AMY) and lateral hypothalamic (LH) electrodes. The maximum self-stimulation (SS) rates in the AMY was found to be very low, compared to the LH. Concurrent stimulation with pairs of AMY-LH pulses did not shift the rate-frequency functional laterally, indicating the absence of summation of the two rewarding effects. In a second experiment, concurrent AMY-LH stimulation (using sub-threshold intensity LH pulses) facilitated bar-pressing for AMY stimulation (it increased the slope of the AMY rate-frequency function) without shifting this function laterally. In a third experiment, subjects were given a choice between a pulse frequency yielding maximal AMY rate and a series of higher pulse frequencies. Subjects consistently preferred the higher frequency values, attesting that the maximum AMY rates were not constrained by a saturating reinforcing effect. In a fourth experiment, subjects were given a choice between AMY stimulation and concurrent AMY-LH stimulation, using low intensity LH pulses. Subjects showed no preference for either stimulation condition, although rates were higher for the latter condition. These findings suggest that the maximum rate for AMY stimulation was constrained by factors interfering with bar-pressing and that the effect of these factors was attenuated by co-activation of the LH. In a fifth experiment, pre-treatment with phenobarbital mimicked the rate-enhancing effect of concurrent AMY-LH stimulation for 2 of the 4 subjects tested. This finding suggests that the LH pulses contributed to attenuate seizure activity accompanying AMY SS. In a final experiment, AMY SS rates were also increased by co-activation of rewarding sites in the rostral MFB but not the dorsal raphe, suggesting an anatomical specificity of this effect.     

The use of head/eye-centered, hand-centered and allocentric representations for visually guided hand movements and perceptual judgments

Two experiments are reported that were designed to measure the accuracy and reliability of both visually guided hand movements (Exp. 1) and perceptual matching judgments (Exp. 2). The specific procedure for informing subjects of the required response on each trial was manipulated so that some tasks could only be performed using an allocentric representation of the visual target; others could be performed using either an allocentric or hand-centered representation; still others could be performed based on an allocentric, hand-centered or head/eye-centered representation. Both head/eye and hand centered representations are egocentric because they specify visual coordinates with respect to the subject. The results reveal that accuracy and reliability of both motor and perceptual responses are highest when subjects direct their response towards a visible target location, which allows them to rely on a representation of the target in head/eye-centered coordinates. Systematic changes in averages and standard deviations of responses are observed when subjects cannot direct their response towards a visible target location, but have to represent target distance and direction in either hand-centered or allocentric visual coordinates instead. Subjects’ motor and perceptual performance agree quantitatively well. These results strongly suggest that subjects process head/eye-centered representations differently from hand-centered or allocentric representations, but that they process visual information for motor actions and perceptual judgments together.     

Motor-related cortical dynamics to intact movements in tetraplegics as revealed by high-resolution EEG

We explored the cortical dynamics during movements of an unaffected body part in tetraplegic subjects with chronic spinal cord injury (SCI). The aims were to find out whether the intact movements were associated with a physiological time-varying pattern of activity in the motor-related cortical areas and whether the primary motor area (MI) activation followed a somatotopic distribution. Event-related potentials to self-initiated lip movements were analyzed by means of cortical source imaging of EEG recorded from seven tetraplegic subjects and seven control subjects. Regions of interest (ROIs) were selected on individual MRI and the time-varying electrophysiologic activity (cortical current density, CCD) was estimated on these ROIs and subjected to across-subject analysis. A significant, bilateral movement-related pattern of MI activation was detected during motor task execution in SCI patients as well as in controls. The site of local maxima activation displayed a symmetrical discrete distribution within MI, consistently with a putative somatotopic lip representation, in all the subjects. The supplementary motor area proper (SMAp) was always coactivated with MI and coactivation was characterized by a time course with typical premotion and motion phases over both motor areas. A clear-cut temporal delay between the SMAp and MI activation did not occur either in SCI patients or in controls. These findings obtained with noninvasive neuroelectrical source imaging document that in chronic SCI subjects "executive" motor areas are engaged with a preserved temporal and spatial pattern during preparation and execution of intact movements.     

Autonomic and subjective responses to real and sham acupuncture stimulation

This study compared verum acupuncture (VA) and sham acupuncture (SA) stimulation by assessing autonomic and subjective responses. Autonomic responses such as skin conductance response (SCR) and heart rate (HR) were measured. Subjective pain ratings were collected and evaluated. A correlation analysis was performed for SCR and HR changes and subjective pain ratings. In both VA and SA sessions, SCR increased, and HR decreased. Subjective responses were different for VA and SA. The SCR changes correlated with subjective responses for VA, but not SA. The present results suggest that VA and SA do not fundamentally differ in their autonomic response patterns.     

GABAergic mechanisms gate tactile discrimination learning

In contrast to mechanisms mediating synaptic plasticity, the pharmacological basis of perceptual learning remains to be clarified. Here we report that a specific form of perceptual learning is influenced by GABAergic mechanisms. We induced perceptual learning by Hebbian co-activation of the skin of the tip of the right index fingers in human subjects. Under placebo conditions, tactile 2-point discrimination was improved on the co-activated, but not on the left, index finger. This augmentation was completely eliminated by lorazepam, a GABAA receptor agonist. No drug effects were found on the left index finger indicating that the drugs had no effect per se on performance. The results demonstrate that perceptual learning is subject to pharmacological gating by basic mechanisms known to mediate and modulate synaptic plasticity.     

Differences in cortical coding of heat evoked pain beyond the perceived intensity: An fMRI and EEG study

Imaging studies have identified a wide network of brain areas activated by nociceptive stimuli and revealed differences in somatotopic representation of highly distinct stimulation sites (foot vs. hand) in the primary (S1) and secondary (S2) somatosensory cortices. Somatotopic organization between adjacent dermatomes and differences in cortical coding of similarly perceived nociceptive stimulation are less well studied. Here, cortical processing following contact heat nociceptive stimulation of cervical (C4, C6, and C8) and trunk (T10) dermatomes were recorded in 20 healthy subjects using functional magnetic resonance imaging (fMRI) and electroencephalography (EEG). Stimulation of T10 compared with the C6 and C8 revealed significant higher response intensity in the left S1 (contralateral) and the right S2 (ipsilateral) even when the perceived pain was equal between stimulation sites. Accordingly, contact heat evoked potentials following stimulation of T10 showed significantly higher N2P2 amplitudes compared to C6 and C8. Adjacent dermatomes did not reveal a distinct somatotopical representation. Within the assessed cervical and trunk dermatomes, nociceptive cortical processing to heat differs significantly in magnitude even when controlling for pain perception. This study provides evidence that controlling for pain perception is not sufficient to compare directly the magnitude of cortical processing [blood oxygen level dependence (BOLD) response and amplitude of evoked potentials] between body sites. Hum Brain Mapp 35:1379–1389, 2014. © 2013 Wiley Periodicals, Inc.