The projection of muscle afferents from the hand to cerebral cortex in man

Source Localization of Somatosensory Neural Generators in Adults with Attention-Deficit/Hyperactivity Disorder

Attention-Deficit/Hyperactivity Disorder (ADHD) is a neurodevelopmental disorder, where differences are often present relating to the performance of motor skills. Our previous work elucidated unique event-related potential patterns of neural activity in those with ADHD when performing visuomotor and force-matching motor paradigms. The purpose of the current study was to identify whether there were unique neural sources related to somatosensory function and motor performance in those with ADHD. Source localization (sLORETA) software identified areas where neural activity differed between those with ADHD and neurotypical controls when performing a visuomotor tracing task and force-matching task. Median nerve somatosensory evoked potentials (SEPs) were elicited, while whole-head electroencephalography (EEG) was performed. sLORETA localized greater neural activity post-FMT in those with ADHD, when compared with their baseline activity (p < 0.05). Specifically, greater activity was exhibited in BA 31, precuneus, parietal lobe (MNI coordinates: X = -5, Y = -75, and Z = 20) at 156 ms post stimulation. No significant differences were found for any other comparisons. Increased activity within BA 31 in those with ADHD at post-FMT measures may reflect increased activation within the default mode network (DMN) or attentional changes, suggesting a unique neural response to the sensory processing of force and proprioceptive afferent input in those with ADHD when performing motor skills. This may have important functional implications for motor tasks dependent on similar proprioceptive afferent input.

Sensorimotor integration and motor learning during a novel force-matching task in young adults with attention-deficit/hyperactivity disorder

Introduction

Attention-Deficit/Hyperactivity Disorder (ADHD) is a neurodevelopmental disorder that exhibits unique neurological and behavioral characteristics. Those with ADHD often have noted impairments in motor performance and coordination, including during tasks that require force modulation. The present study provides insight into the role of altered neural processing and SMI in response to a motor learning paradigm requiring force modulation and proprioception, that previous literature has suggested to be altered in those with ADHD, which can also inform our understanding of the neurophysiology underlying sensorimotor integration (SMI) in the general population.

Methods

Adults with ADHD (n = 15) and neurotypical controls (n = 15) performed a novel force-matching task, where participants used their right-thumb to match a trace template that varied from 2-12% of their Abductor Pollicis Brevis maximum voluntary contraction. This motor task was completed in pre, acquisition, and post blocks. Participants also completed a retention test 24 h later. Median nerve somatosensory-evoked potentials (SEPs) were collected pre and post motor acquisition. SEPs were stimulated at two frequencies, 2.47 Hz and 4.98 Hz, and 1,000 sweeps were recorded using 64-electrode electroencephalography (EEG) at 2,048 Hz. SEP amplitude changes were normalized to each participant's baseline values for that peak.

Results

Both groups improved at post measures (ADHD: 0.85 ± 0.09; Controls: 0.85 ± 0.10), with improvements maintained at retention (ADHD: 0.82 ± 0.11; Controls: 0.82 ± 0.11). The ADHD group had a decreased N18 post-acquisition (0.87 ± 0.48), while the control N18 increased (1.91 ± 1.43). The N30 increased in both groups, with a small increase in the ADHD group (1.03 ± 0.21) and a more pronounced increase in controls (1.15 ± 0.27).

Discussion

Unique neural differences between groups were found after the acquisition of a novel force-matching motor paradigm, particularly relating to the N18 peak. The N18 differences suggest that those with ADHD have reduced olivary-cerebellar-M1 inhibition when learning a novel motor task dependent on force-modulation, potentially due to difficulties integrating the afferent feedback necessary to perform the task. The results of this work provide evidence that young adults with ADHD have altered proprioceptive processing when learning a novel motor task when compared to neurotypical controls.

Sensorimotor integration and motor learning during a novel visuomotor tracing task in young adults with attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder that has noted alterations to motor performance and coordination, potentially affecting learning processes and the acquisition of motor skills. This work will provide insight into the role of altered neural processing and sensorimotor integration (SMI) while learning a novel visuomotor task in young adults with ADHD. This work compared adults with ADHD (n = 12) to neurotypical controls (n = 16), using a novel visuomotor tracing task, where participants used their right-thumb to trace a sinusoidal waveform that varied in both frequency and amplitude. This learning paradigm was completed in pre, acquisition, and post blocks, where participants additionally returned and completed a retention and transfer test 24 h later. Right median nerve short latency somatosensory-evoked potentials (SEPs) were collected pre and post motor acquisition. Performance accuracy and variability improved at post and retention measures for both groups for both normalized (P < 0.001) and absolute (P < 0.001) performance scores. N18 SEP: increased in the ADHD group post motor learning and decreased in controls (P < 0.05). N20 SEP: increased in both groups post motor learning (P < 0.01). P25: increased in both groups post motor learning (P < 0.001). N24: increased for both groups at post measures (P < 0.05). N30: decreased in the ADHD group and increased in controls (P < 0.05). These findings suggest that there may be differences in cortico-cerebellar and prefrontal processing in response to novel visuomotor tasks in those with ADHD.NEW & NOTEWORTHY Alterations to somatosensory-evoked potentials (SEPs) were present in young adults with attention-deficit/hyperactivity disorder (ADHD), when compared with neurotypical controls. The N18 and N30 SEP peak had differential changes between groups, suggesting alterations to olivary-cerebellar-M1 processing and SMI in those with ADHD when acquiring a novel visuomotor tracing task. This suggests that short-latency SEPs may be a useful biomarker in the assessment of differential responses to motor acquisition in those with ADHD.

Effect of Neck Muscle Fatigue on Hand Muscle Motor Performance and Early Somatosensory Evoked Potentials

Even on pain free days, recurrent neck pain alters sensorimotor integration (SMI) measured via somatosensory evoked potentials (SEPs). Neck muscle fatigue decreases upper limb proprioception, and thus may interfere with upper limb motor task acquisition and SMI. This study aimed to determine the effect of cervical extensor muscle (CEM) fatigue on upper limb motor acquisition and retention; and SMI, measured via early SEPs. Twenty-four healthy right-handed individuals were randomly assigned to control or CEM fatigue. Baseline SEPs were elicited via median nerve stimulation at the wrist. Participants then lay prone on a padded table. The fatigue group supported a 2 kg weight until they could no longer maintain the position. The control group rested their neck in neutral for 5 min. Participants completed pre- and post-motor skill acquisition while seated, SEPs were again collected. Task retention was measured 24 h later. Accuracy improved post acquisition and at retention for both groups (p < 0.001), with controls outperforming the fatigue group (p < 0.05). The fatigue group had significantly greater increases in the N24 (p = 0.017) and N30 (p = 0.007) SEP peaks. CEM fatigue impaired upper limb motor learning outcomes in conjunction with differential changes in SEP peak amplitudes related to SMI.

Cortical inhibition is reduced following short-term training in young and older adults

The purpose of this study was to investigate age-related differences in short-term training adaptations in cortical excitability and inhibition. Thirty young (21.9 ± 3.1 years) and 30 older (72.9 ± 4.6 years) individuals participated in the study. Each participant was randomly assigned to a control (n = 30) or a resistance training (n = 30) group, with equal numbers of young and older subjects in each group. Participants completed 2 days of testing, separated by 2 weeks during which time the training group participated in resistance training of the ankle dorsiflexor muscles three times per week. During each testing session, transcranial magnetic stimulation was used to generate motor evoked potentials (MEPs) and silent periods in the tibialis anterior. Hoffmann reflexes (H-reflexes) and compound muscle action potentials (M-waves) were also evoked via electrical stimulation of the peroneal nerve. At baseline, young subjects had higher maximum voluntary contraction (MVC) force (p = 0.002), larger M-wave amplitude (p < 0.001), and longer duration silent periods (p = 0.01) than older individuals, with no differences in the maximal amplitude of the MEP (p = 0.23) or H-reflex (p = 0.57). In the trained group, MVC increased in both young (17.4 %) and older (19.8 %) participants (p < 0.001), and the duration of the silent period decreased by ~15 and 12 ms, respectively (p < 0.001). Training did not significantly impact MEP (p = 0.69) or H-reflex amplitudes (p = 0.38). There were no significant changes in any measures in the control group (p ≥ 0.19) across the two testing sessions. These results indicate that a reduction in cortical inhibition may be an important neural adaptation in response to training in both young and older adults.

Modulation of human corticospinal excitability by paired associative stimulation

Paired Associative Stimulation (PAS) has come to prominence as a potential therapeutic intervention for the treatment of brain injury/disease, and as an experimental method with which to investigate Hebbian principles of neural plasticity in humans. Prototypically, a single electrical stimulus is directed to a peripheral nerve in advance of transcranial magnetic stimulation (TMS) delivered to the contralateral primary motor cortex (M1). Repeated pairing of the stimuli (i.e., association) over an extended period may increase or decrease the excitability of corticospinal projections from M1, in manner that depends on the interstimulus interval (ISI). It has been suggested that these effects represent a form of associative long-term potentiation (LTP) and depression (LTD) that bears resemblance to spike-timing dependent plasticity (STDP) as it has been elaborated in animal models. With a large body of empirical evidence having emerged since the cardinal features of PAS were first described, and in light of the variations from the original protocols that have been implemented, it is opportune to consider whether the phenomenology of PAS remains consistent with the characteristic features that were initially disclosed. This assessment necessarily has bearing upon interpretation of the effects of PAS in relation to the specific cellular pathways that are putatively engaged, including those that adhere to the rules of STDP. The balance of evidence suggests that the mechanisms that contribute to the LTP- and LTD-type responses to PAS differ depending on the precise nature of the induction protocol that is used. In addition to emphasizing the requirement for additional explanatory models, in the present analysis we highlight the key features of the PAS phenomenology that require interpretation.

The representation of egocentric space in the posterior parietal cortex

The posterior parietal cortex (PPC) is the most likely site where egocentric spatial relationships are represented in the brain. PPC cells receive visual, auditory, somaesthetic, and vestibular sensory inputs; oculomotor, head, limb, and body motor signals; and strong motivational projections from the limbic system. Their discharge increases not only when an animal moves towards a sensory target, but also when it directs its attention to it. PPC lesions have the opposite effect: sensory inattention and neglect. The PPC does not seem to contain a "map" of the location of objects in space but a distributed neural network for transforming one set of sensory vectors into other sensory reference frames or into various motor coordinate systems. Which set of transformation rules is used probably depends on attention, which selectively enhances the synapses needed for making a particular sensory comparison or aiming a particular movement.

Does the nervous system depend on kinesthetic information to control natural limb movements?

This target article draws together two groups of experimental studies on the control of human movement through peripheral feedback and centrally generated signals of motor commands. First, during natural movement, feedback from muscle, joint, and cutaneous afferents changes; in human subjects these changes have reflex and kinesthetic consequences. Recent psychophysical and microneurographic evidence suggests that joint and even cutaneous afferents may have a proprioceptive role. Second, the role of centrally generated motor commands in the control of normal movements and movements following acute and chronic deafferentation is reviewed. There is increasing evidence that subjects can perceive their motor commands under various conditions, but that this is inadequate for normal movement; deficits in motor performance arise when the reliance on proprioceptive feedback is abolished either experimentally or because of pathology. During natural movement, the CNS appears to have access to functionally useful input from a range of peripheral receptors as well as from internally generated command signals. The unanswered questions that remain suggest a number of avenues for further research.

Implications of neural networks for how we think about brain function

Engineers use neural networks to control systems too complex for conventional engineering solutions. To examine the behavior of individual hidden units would defeat the purpose of this approach because it would be largely uninterpretable. Yet neurophysiologists spend their careers doing just that! Hidden units contain bits and scraps of signals that yield only arcane hints about network function and no information about how its individual units process signals. Most literature on single-unit recordings attests to this grim fact. On the other hand, knowing a system's function and describing it with elegant mathematics tell one very little about what to expect of interneuronal behavior. Examples of simple networks based on neurophysiology are taken from the oculomotor literature to suggest how single-unit interpretability might decrease with increasing task complexity. It is argued that trying to explain how any real neural network works on a cell-by-cell, reductionist basis is futile and we may have to be content with trying to understand the brain at higher levels of organization.

Subcortical Interactions Between Somatosensory Stimuli of Different Modalities and Their Temporal Profile

Interactions between inputs of different sensory modality occur along the sensory pathway, including the thalamus. However, the temporal profile of such interaction has not been fully studied. In eight patients who had been implanted an intrathalamic electrode for deep brain stimulation as symptomatic treatment of tremor, we investigated the interactions between mechanical taps and electrical nerve stimuli. Somatosensory evoked potentials (SEPs) were recorded from Erb's point, cervical spinal cord, nucleus ventrointermedialis of the thalamus, and parietal cortex. A handheld electronic reflex hammer was used to deliver a mechanical tap to the skin overlying the first dorsal interosseous muscle and to trigger an ipsilateral digital median nerve electrical stimulus time-locked to the mechanical tap with a variable delay of 0 to 50 ms. There were significant time-dependent interactions between the two sensory volleys at the subcortical level. Thalamic SEPs were decreased in amplitude at interstimulus intervals (ISIs) from 10 to 40 ms with maximum effect at 20 ms (−42.8 ± 10.5%; P < 0.001). A similar decrease was also seen in the number and frequency of the high-frequency components of thalamic SEPs (−25 ± 4%). A smaller reduction (−18.1 ± 5.8%; P < 0.001) was present in upper cervical response at ISI = 20 ms. There were no changes in peripheral responses. Cortical SEPs were almost completely absent in some subjects at ISIs from 20 to 50 ms. There were no changes in SEP latencies. Our results indicate that significant time-dependent interactions between sensory volleys occur at the subcortical level. These observations provide further insight into the physiological mechanisms underlying afferent gating between sensory volleys of different modality.

Defective temporal discrimination of passive movements in Parkinson's disease

Perception of limb position and motion is abnormal in Parkinson's disease (PD). Despite the fact that the processing of proprioceptive inputs is inherently temporal, most studies have assessed spatial aspects of proprioception in PD patients. Here, we use a recently described method to test whether deficits also exist in temporal discrimination of proprioceptive inputs. We induced index finger abduction or wrist flexion through percutaneous electrical stimulation of the motor point of the first dorsal interosseous muscle (FDI) or the flexor carpii radialis (FCR), respectively. Twelve patients with unilateral bradykinetic-rigid PD and 12 healthy subjects were asked to report whether pairs of stimuli separated by different time intervals produced single or double index finger abduction movement or wrist flexion. The shortest interval at which subjects reported two separated movements was considered as temporal movement discrimination threshold. Results showed that thresholds were significantly higher in PD patients than in control subjects for both FCR and FDI muscle, thus demonstrating for the first time that temporal proprioceptive processing is altered in PD.

Temporal discrimination of two passive movements in writer's cramp

Although movement abnormalities are predominant symptoms of dystonia, patients also have alterations in temporal as well as spatial discrimination of cutaneous inputs. Here, we use a recently described method to test whether deficits also exist in temporal discrimination of proprioceptive inputs. Percutaneous electrical stimulation of the motor point of the first dorsal interosseous (FDI) muscle, and of the flexor carpii radialis (FCR) muscle separately, was used to produce a nonpainful contraction of the muscles that caused index finger abduction and wrist flexion, respectively. In 10 patients with writer's cramp and in 10 healthy subjects, pairs of stimuli separated by different time intervals were given and subjects were asked to report whether they perceived a single or a double index finger abduction movement or wrist flexion. The threshold value was the shortest interval at which the subjects reported two separated movements (temporal discrimination motor threshold [TDMT]). In both writer's cramp patients and controls, TDMTs were higher for FCR than for FDI. But in contrast to the reduced temporal discrimination reported for cutaneous sensation, there was no significant difference in either muscle between TDMT in patients and normal subjects. We conclude that temporal processing of muscle and cutaneous afferents is differentially affected in focal hand dystonia.

Temporal discrimination of two passive movements in humans: a new psychophysical approach to assessing kinaesthesia

Percutaneous electrical stimulation of the motor point of the first dorsal interosseous muscle (FDI) was used to produce a non-painful contraction of the FDI muscle that caused index finger abduction movement but no radiating cutaneous paraesthesias or sharp sensations localized to joints. Pairs of stimuli separated by different time intervals were given and subjects were asked to report whether they perceived a single or a double index finger abduction movement. The threshold value was the shortest interval for which the subjects reported two separate index finger abduction movements. Temporal discrimination movement thresholds (TDMT) were measured for both right and left hand. To assess the possible role of muscle and cutaneous afferents in temporal discrimination, we investigated the effects of high-frequency (20 Hz) electrical stimulation of the right ulnar and radial nerves on TDMT. In humans, muscle afferents from FDI are supplied by the ulnar nerve whereas the cutaneous territory overlying the muscle and joint is supplied by the radial and median nerves. Threshold values were not significantly different for right (75.1 ms) and left (75.6 ms) hands. During ulnar and to a lesser extent during radial nerve stimulation, TDMT values were significantly increased (119.2 and 93.5 ms, respectively) compared with baseline conditions (78.0 ms) whereas no changes were observed during median nerve stimulation (80.5 ms). These results suggest that muscle, and in part cutaneous, afferents contribute to temporal discrimination of a dual movement. The technique may provide a useful way of measuring temporal discrimination of kinaesthetic inputs in humans.

Response monitoring without sensory feedback

OBJECTIVE

The elicitation of an evoked potential, the 'error negativity' (Ne) when subjects commit errors in speeded tasks, is often taken as an index of response monitoring processes. The presence of a Ne-like wave on purely correct trials challenges the current conceptions about the nature of such a monitoring system. Here, we evaluate the possibility that the Ne-like wave on correct trials is merely due to reafferences, and at the same time, we test directly the general opinion according to which the Ne is generated by an internal signal.

METHODS

We studied the presence of a Ne-like wave in a completely deafferented patient. The patient performed two reaction time (RT) tasks: a two-responses RT task and a go/no-go task.

RESULTS

In this patient, a Ne occurs on errors, on incorrect EMG activations, and on purely correct responses. On errors, the Ne was clearly followed by an error positivity (Pe).

CONCLUSIONS

The Ne and the Ne-like wave are not generated by reafferences. This similarity is a further argument to consider that these two waves are of same nature.

SIGNIFICANCE

The present data demonstrate that sensory information is not mandatory for the brain to monitor and correct ongoing responses.

Rapid modulation of cortical proprioceptive activity induced by transient cutaneous deafferentation: neurophysiological evidence of short-term plasticity across different somatosensory modalities in humans

Single cell recording in non-human primates shows plastic changes of cortical somatic representations across different types of somatic inputs originating from the same peripheral territory. In humans, muscle afferents from first dorsal interosseus are supplied by the ulnar nerve while the cutaneous territory overlying this muscle is supplied by the radial nerve. This peculiar anatomical nervous distribution allowed us to devise an experimental model which provided a unique opportunity to assess, in humans with a non-invasive technique, the functional relationships between cutaneous and muscle afferent inputs originating from the same peripheral territory. We recorded spinal, brainstem and cortical somatosensory potentials evoked by stimulation of muscle afferents of the right first dorsal interosseus before, during and after anaesthetic block of the sensitive branch of the ipsilateral radial nerve. Amplitude of parietal N20 and P27 and frontal N30 somatosensory evoked potential components showed an increase of amplitudes with more profound anaesthesia. Amplitudes returned to pre-anaesthetic values several minutes after anaesthesia. By contrast, spinal N13 and brainstem P14 potentials did not change throughout the experiment. Results show, for the first time in humans, that a transient cutaneous deafferentation may induce rapid modulation of cortical activity evoked by stimulation of muscle afferents originating in the anaesthetic territory.

Mapping dermatomes during selective dorsal rhizotomy: case report and review of the literature

BACKGROUND

Studies suggest that the pattern of dermatomal segmental innervation in any given patient, may differ from the classic dermatomal maps first described in the 1890s. Such variability may limit the effectiveness of selective dorsal rhizotomy for treatment of neurogenic pain.

CASE DESCRIPTION

A 46-year-old male presented with a 27-year history of intractable pain in his left arm after being shot during the Vietnam War; multiple surgical and medical therapeutic modalities failed to produce durable pain relief. The patient underwent selective dorsal rhizotomy, with intraoperative dermatomal and mixed somatosensory evoked potential recordings. Pre- and postrhizotomy recordings were compared, effectively mapping this patient's dermatomal pattern. At 4 years' follow-up, the patient remains pain free.

CONCLUSION

Intraoperative monitoring of somatosensory evoked potentials during dorsal rhizotomy for neurogenic pain can be used to establish the degree to which an individual's pattern of segmental innervation conforms to the traditionally described dermatomes.

Somatosensory evoked potentials and sensory involvement in multiple sclerosis: comparison with clinical findings and quantitative sensory tests

Sensory disturbances are one of the most common findings in patients with multiple sclerosis (MS). However, they are usually assessed at the standard neurological examination only. Quantitative Sensory Tests (QSTs) for temperature and vibratory sense allow a more objective evaluation. In a group of 19 clinically definite MS patients, we compared vibratory and temperature thresholds with sensory symptoms or signs at clinical neurological examination and somatosensory evoked potentials (SEPs) at the four limbs. The frequency of abnormalities of clinical symptoms/signs, vibration threshold and median SEPs were 69%, 33% and 55%, respectively. Correlation between degree of abnormality of SEPs and clinically assessed vibration sense (V) was statistically significant (P<0.007; Spearmann rank coefficient), as well as between SEPs and vibration perception threshold (P<0.02). Clinical evaluation of thermal sense did not show false positive results compared to quantitative thermal threshold, but false negative findings (35%). This study suggests that the combined use of vibration threshold and SEPs allows a better objectivation of sensory function, allowing the detection of subclinical abnormalities and possibly reducing the number of false positive results introduced by clinical assessment. Moreover QSTs are to be preferred to clinical evaluation in the assessment of thermal sense, due to their superior sensitivity.

Peripheral and segmental spinal abnormalities of median and ulnar somatosensory evoked potentials in Hirayama's disease

OBJECTIVES

To investigate the origin of juvenile muscle atrophy of the upper limbs (Hirayama's disease, a type of cervical myelopathy of unknown origin).

SUBJECTS

Eight male patients were studied; data from 10 normal men were used as control.

METHODS

Median and ulnar nerve somatosensory evoked potentials (SEP) were recorded. Brachial plexus potentials at Erb's point (EP), dorsal horn responses (N13), and subcortical (P14) and cortical potentials (N20) were evaluated. Tibial nerve SEP and motor evoked potentials (MEP) were also recorded from scalp and spinal sites to assess posterior column and pyramidal tract conduction, respectively.

RESULTS

The most important SEP findings were: a very substantial attenuation of both the EP potentials and the N13 spinal responses; normal amplitude of the scalp N20; and normal latency of the individual peaks (EP-N9-N13-P14-N20). Although both nerves were involved, abnormalities in response to median nerve stimulation were more significant than those in response to ulnar nerve stimulation. There was little correlation between the degree of alterations observed and the clinical state. Latencies of both spinal and cortical potentials were normal following tibial nerve stimulation. The mean latency of cervical MEP and the central conduction time from the thenar eminence were slightly but significantly longer in patients than in controls.

CONCLUSIONS

The findings support the hypothesis that this disease, which is clinically defined as a focal spinal muscle atrophy of the upper limb, may also involve the sensory system; if traumatic injury caused by stretching plays a role in the pathogenesis, the damage cannot be confined to the anterior horn of the spinal cord.

Modality-related scalp responses after electrical stimulation of cutaneous and muscular upper limb afferents in humans

To elucidate whether the selective electrical stimulation of muscle as well as cutaneous afferents evokes modality-specific responses in somatosensory evoked potentials (SEPs) recorded on the scalp of humans, we compared scalp SEPs to electrical stimuli applied to the median nerve and to the abductor pollicis brevis (APB) motor point. In three subjects, we also recorded SEPs after stimulation of the distal phalanx of the thumb, which selectively involved cutaneous afferents. Motor point and median nerve SEPs showed the same scalp distribution; moreover, very similar dipole models, showing the same dipolar time courses, explained well the SEPs after both types of stimulation. Since the non-natural stimulation of muscle afferents evokes responses also in areas specifically devoted to cutaneous input processing, it is conceivable that, in physiological conditions, muscle afferents are differentially gated in somatosensory cortex. The frontocentral N30 response was absent after purely cutaneous stimulation; by contrast, it was relatively more represented in motor point rather than in mixed nerve SEPs. These data suggest that the N30 response is specifically evoked by proprioceptive inputs.

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.

Thalamic potentials evoked by motor point stimulation

We studied thalamic potentials elicited by stimulation of the extensor digitorum communis muscle motor point during stereotactic surgery for movement disorders. In 6 patients with Parkinson's disease and 1 with cerebral palsy, muscle afferent-evoked thalamic potentials (METPs) were recorded in the ventral intermediate (Vim) nucleus. METPs consisted of three peaks with an average latency of 11.02 +/- 0.80 ms for initial positive peak (PI), 13.04 +/- 0.58 ms for negative (N), and 14.30 +/- 0.87 ms for later positive peak (PII). These METPs were different from those evoked by median nerve stimulation. No peaks corresponding to the N20 component of the somatosensory evoked potential were recorded from C3 or C4 scalp electrodes. The METP amplitudes in the ventrolateral (VL) nucleus were markedly lower than those in the Vim nucleus. These findings suggest that METPs in the Vim nucleus are responses that occur via the fast conducting group Ia afferent fibers rather than somatosensory afferent fibers.

Respiratory-related evoked potential elicited by expiratory occlusion

Respiratory-related evoked potentials (RREPs) have been elicited by inspiratory loads in adults and children. The RREP was recorded over the somatosensory region of the cerebral cortex. It was hypothesized that a RREP could be recorded by using expiratory occlusion. Electroencephalographic activity was recorded in adults from 14 scalp locations, referenced to the linked earlobes. The occlusion was presented as an interruption of expiration. Epochs of electroencephalographic activity and mouth pressure were recorded for each expiratory occlusion presentation. There were two occlusion trials and a control trial of 100 presentations each. The epochs in each trial were averaged and examined for the presence of short-latency, occlusion-related peaks. RREP peaks were observed bilaterally with expiratory occlusion and were absent in control unoccluded averages. A positive peak, P(34), was observed at central and postcentral sites. A negative peak, N(53), was observed at frontal and central sites. A second positive peak, P(95), was observed at frontal and central sites. These results demonstrate that expiratory occlusion elicits a RREP. This suggests that expiratory occlusion-related sensory information activates the cerebral cortex similar to that for inspiratory loads.

Effects of tactile interference stimulation on somatosensory evoked magnetic fields following tibial nerve stimulation

We studied the effects of interfering tactile stimulation applied to the foot ipsilateral and contralateral to the stimulation on somatosensory evoked magnetic fields (SEFs) following tibial nerve stimulation at the ankle. Equivalent current dipoles (ECDs) of all 4 components, 1M-4M, in all sessions were estimated to be very close each other, around the foot area of the primary sensory cortex (SI). The 1M, 2M and 4M components were significantly reduced in amplitude by the ipsilateral-foot interference, and we consider that this phenomenon is due mainly to 'saturation' of the neurons in area 3b of the SI. In contrast, the 3M component was significantly enhanced in amplitude by the contralateral-foot interference. We suspect that this result was due to the effects of neuronal activities in areas 2, 5 and/or 7, which receive inputs from both sides of the body, i.e. to 'bilateral function'. Considering the various types of interference effects on SEFs and somatosensory evoked potentials (SEPs) observed in not only the present, but also in the previous studies, we conclude that both SEFs and SEPs following tibial nerve stimulation are generated mainly by ascending signals mediated by cutaneous fibers of the peripheral nerves rather than the muscle afferents.

Cerebral processing of acute skin and muscle pain in humans

The human cerebral processing of noxious input from skin and muscle was compared with the use of positron emission tomography with intravenous H2(15)O to detect changes in regional cerebral blood flow (rCBF) as an indicator of neuronal activity. During each of eight scans, 11 normal subjects rated the intensity of stimuli delivered to the nondominant (left) forearm on a scale ranging from 0 to 100 with 70 as pain threshold. Cutaneous pain was produced with a high-energy CO2 laser stimulator. Muscle pain was elicited with high-intensity intramuscular electrical stimulation. The mean ratings of perceived intensity for innocuous and noxious stimulation were 32.6 +/- 4.5 (SE) and 78.4 +/- 1.7 for cutaneous stimulation and 15.4 +/- 4.2 and 73.5 +/- 1.4 for intramuscular stimulation. The pain intensity ratings and the differences between noxious and innocuous ratings were similar for cutaneous and intramuscular stimuli (P > 0.05). After stereotactic registration, statistical pixel-by-pixel summation (Z score) and volumes-of-interest (VOI) analyses of subtraction images were performed. Significant increases in rCBF to both noxious cutaneous and intramuscular stimulation were found in the contralateral secondary somatosensory cortex (SII) and inferior parietal lobule [Brodmann area (BA) 40]. Comparable levels of rCBF increase were found in the contralateral anterior insular cortex, thalamus, and ipsilateral cerebellum. Noxious cutaneous stimulation caused significant activation in the contralateral lateral prefrontal cortex (BA 10/46) and ipsilateral premotor cortex (BA 4/6). Noxious intramuscular stimulation evoked rCBF increases in the contralateral anterior cingulate cortex (BA 24) and subsignificant responses in the contralateral primary sensorimotor cortex (MI/SI) and lenticular nucleus. These activated cerebral structures may represent those recruited early in nociceptive processing because both forms of stimuli were near pain threshold. Correlation analyses showed a negative relationship between changes in rCBF for thalamus and MI/SI for cutaneous stimulation, and positive relationships between thalamus and anterior insula for both stimulus modalities. Direct statistical comparisons between innocuous cutaneous and intramuscular stimulation with the use of Z scores and VOI analyses showed no reliable differences between these two forms of noxious stimulation, indicating a substantial overlap in brain activation pattern. The comparison of noxious cutaneous and intramuscular stimulation indicated more activation in the premotor cortex, SII, and prefrontal cortex with cutaneous stimulation, but these differences did not reach statistical significance. The similar cerebral activation patterns suggest that the perceived differences between acute skin and muscle pain are mediated by differences in the intensity and temporospatial pattern of neuronal activity within similar sets of forebrain structures.

The methodology and scope of human microneurography

Microneurography was introduced in 1967 and has developed into an invaluable tool for investigating human somatosensory, motor and cardiovascular physiology and pathophysiology. It involves percutaneous insertion of a metal microelectrode into fascicles of limb and facial nerves. This review covers the procedures and equipment necessary for microneurography and provides a current circuit for a preamplifier. Evidence is presented that (i) most recordings from myelinated axons involve an effective penetration of the myelin by the electrode; (ii) based on physiological criteria, microstimulation through the electrode can be used to activate single axons although the probability of this is relatively low and (iii) despite 'micro' lesions caused by the electrode insertion into the nerve and its fascicles, the morbidity with the procedure is acceptably low.

Afferent mechanism of cortical myoclonus studied by proprioception-related SEPs

Proprioception-related somatosensory evoked potentials (SEPs) to passive flexion movement of the middle finger at proximal interphalangeal joint were recorded in 7 patients with myoclonus of cortical origin who demonstrated enlarged electrical SEPs (giant SEPs). In 3 out of the 7 patients, the proprioception-related SEPs were also enlarged. The remaining 4 patients showed giant electrical SEPs without enhancement of proprioception-related SEPs. Long loop electromyographic response was recorded during the resting condition in all of the 3 patients with enlarged proprioception-related SEPs. We have previously reported that proprioception-related SEPs are mainly generated by muscle afferent inputs, though electrical SEPs are thought to reflect mostly cutaneous inputs with some contribution from muscle afferents. Therefore, it is concluded that hyperexcitability of the sensorimotor cortex in cortical myoclonus is modality-specific. Cortical excitability is exaggerated to both cutaneous and deep receptor inputs in some patients, but only to cutaneous input in others.

Cortical sources of human short-latency somatosensory evoked fields to median and ulnar nerve stimuli

;;;;;õry evoked magnetic fields were measured with a 122-channel whole-scalp neuromagnetometer from seven healthy adults. Electric stimuli, with an intensity above the motor threshold, were delivered once every 0.5 s alternately to the median and ulnar nerves at the wrist; both wrists were stimulated successively within one session. In most subjects, two distinct neural sources were identified at the contralateral primary somatosensory cortex SI for both stimuli. The first source (M20) peaked at 21-22 ms and indicated activation of area 3b in the contralateral SI hand region. The same source peaked with opposite current direction at 32 ms. The second source (M40) was slightly medial to M20 and exhibited two peaks with the same current direction, first at 25 ms and most prominently at 42 ms. M20 was on average 7 mm more lateral along the central sulcus for median than ulnar nerve stimuli, in agreement with the somatotopic organization of the SI cortex; similar organization for M40 was less clear. These results suggest that M20 and M40 to upper limb stimulation represent activation of distinct neuronal populations in hand SI cortex, presumably in area 3b.

A comparison of forepaw and vibrissae somatosensory cortical evoked potentials in the rat

Somatosensory evoked potentials were elicited in anesthetized rats by electrical stimulation of the forepaw (F-SEP) or the vibrissae (V-SEP) and were compared in order to study which of these is a more valid animal model for studying the physiology and pathophysiology of somatosensory evoked potentials (SEPs) that are often recorded in man in a clinical setting. Intensity and rate functions were measured for the two potentials. The V-SEPs had larger amplitudes than the F-SEPs at high stimulus intensity and low stimulus rate. Furthermore, the ratios of the maximal amplitude of the F-SEP to that of the V-SEP (0.66) and of the areas under the curves of the two responses (0.75) reflected the smaller representation of the forepaw in the primary somatosensory cortex of the rat, compared to the vibrissae (ratio of cortical areas about 0.79). The differences should be taken into account when using median nerve SEP in the rat as a model of the human SEP. Study of V-SEPs in rat may provide insight into trigeminal nerve SEPs in man, which are also occasionally used for neurological evaluation.

Changes in movement-related brain activity during transient deafferentation: a neuromagnetic study

Neuromagnetic fields from the left cerebral hemisphere of three healthy, right-handed subjects were investigated preceding and during voluntary index finger movements performed every 8-15 s under two different experimental conditions: before (stage A) and during (stage B) anesthetic block of median and radial nerves at the wrist. The anesthesia caused blocking of cutaneous receptors and some of the proprioreceptors from a wide hand area, including the entire index finger. However, the index finger movements were not impaired because the muscles participating in the task were not anesthetized. The magnetic signals of the brain sources corresponding to the main components of the movement-related neuromagnetic fields (motor field, MF and movement-evoked field I, MEFI) were mapped and localized using a moving dipole model. In the three investigated subjects the MF and MEFI dipole sources were stronger (30% on average) during stage B than during stage A. No significant changes in spatial coordinates of the estimated dipole locations between stages A and B were observed. This was true for both MF and MEFI. The results show that the MEFI reflects not only proprioceptive input from the periphery but cutaneous inputs as well. In this way the results support the view that cutaneous inputs play a specific role in the cortical control of movement.

The effects of stimulus rates upon median, ulnar and radial nerve somatosensory evoked potentials

We examined the effect of stimulus rates on the somatosensory evoked potential (SEP) amplitude following stimulation of the median nerve (MN) and the ulnar nerve (UN) at the elbow or wrist, and the radial nerve (RN) at the wrist in 12 normal subjects. We measured the amplitude of frontal (P14-N18-P22-N30) and parietal peaks (P14-N20-P26-N34) at a stimulus rate of 1.1, 3.5 and 5.7 Hz. The amplitude attenuation was found at frontal P22 and N30 and to a lesser degree at parietal N20 and P26 peaks with an increasing stimulus rate from 1.1 to 5.7 Hz. The amplitude attenuation was greatest at the elbow when compared to the wrist stimulation for both MN and UN. The attenuation was least for wrist stimulation for the RN. The UN block by local anesthesia just distal to the stimulus electrode at the elbow abolished the amplitude attenuation caused by the fast stimulus rate. The observed amplitude attenuation with the faster stimulus rate is probably due, in part, to interference from the "secondary" afferent inputs. The secondary afferent inputs arise from peripheral receptor stimulation (muscle, joint and/or cutaneous) as a subsequent effect of efferent volleys initiated from the point of stimulation. The greater number of peripheral receptors being activated as more proximal sites of stimulation in a mixed nerve would result in greater attenuation of the SEP recorded from scalp electrodes. We postulate that the attenuation of frontal peaks by the fast stimulus rate is due to the frontal projection of interfering "secondary" afferent inputs.

Spinal somatosensory evoked potentials in mice and their developmental changes

Spinal somatosensory evoked potentials (SEP) were recorded in 58 normal mice (C3H strain) divided into 4 groups according to age (3-, 6-, 9- and 12 weeks). Monopolar recordings of spinal SEP were made by subdermal needle electrodes from 3 vertebral levels, "low-lumbar", "high-lumbar" and "mid-thoracic", by stimulating the tibial nerve bilaterally at the ankle. Three negative peaks, NI, NII and NIII, presumably due to conduction through muscle afferents, cutaneous afferents (in the dorsal root or dorsal white column) and spinocerebellar tract, respectively, were recorded at the high-lumbar level in the 12-week-old mouse. Besides the NI and NII peaks, a small ventral root potential was also occasionally recorded at the low-lumbar level. At the mid-thoracic level, only NI and NIII were recordable. At both the high-lumbar and mid-thoracic levels, the negative peaks were superimposed over long duration "summation potentials" of opposite polarities. Well-defined standing potentials were also recorded at these two levels. The standing potentials could be the "entry potential" due to the entry of S1 root into the spinal cord at the T13 vertebral level. The summation potential presumably is due to a fixed generator located between the T7 and T12 vertebral levels resulting from intense synaptic activity at this level. In 3- and 6-week-old mice, the entry point potential was recorded in the low-lumbar SEP also, possibly due to less axial growth of the vertebral column at this stage of development.(ABSTRACT TRUNCATED AT 250 WORDS)

Tactile interference differentiates sub-components of N20, P20 and P29 in the human cortical surface somatosensory evoked potential

Somatosensory evoked potentials (SEPs) to median nerve stimulation were recorded from up to 64 locations on the exposed cortical surface in 19 patients undergoing intracranial surgery for epilepsy and/or tumour removal. In view of previously described 'interference' effects on scalp SEPs, a continuous light tactile stimulus was applied to the palm and the first 3 digits of the stimulated hand in order to try to differentiate components due to input from cutaneous and other sensory receptors. The first cortically generated potentials, N20 at postcentral locations and P20 precentrally, could each be resolved into 2 subcomponents separated by about 2.5 msec. The later subcomponent was consistently the more attenuated by the interfering stimulus and is postulated to be due to input from rapidly adapting cutaneous mechanoreceptors. The earlier subcomponent could be due to input from muscle afferents or from slowly adapting cutaneous receptors which the interfering stimulus would have activated to a lesser degree. In 2 cases the P29 potentials recorded from regions of the postcentral gyrus were dissociated. In one case the potentials recorded at adjacent electrodes were attenuated to differing degrees, and in the other the effect was maximal at different locations when the thumb, index and middle fingers were stimulated separately. The method therefore appears capable of distinguishing regions of the postcentral gyrus concerned with cutaneous input from different parts of the hand.

Cortical somatosensory evoked potentials in spinal cord injury patients

The amplitude and latency of somatosensory evoked potentials (SEPs) in healthy subjects depend on intensity of stimulation. The effect of this parameter on SEPs in patients with neurologic disorders has not been systematically studied, although it could have a profound impact if SEPs are to be used for prognostication. We have compared the latency and amplitude of SEPs in healthy subjects and patients with spinal cord injury (SCI). Stimulation intensity was standardized at two different biologically calibrated levels. Cortical SEPs in patients with SCI showed greater decrease in latency and increase in amplitude with increased intensity of stimulation in comparison to healthy subjects. These phenomena were observed in the majority of patients with incomplete SCI who subsequently showed improvement in cortical SEPs. We observed situations in which the SEP was absent with the usual intensity of stimulation and present only with the stronger stimulation intensity. Furthermore, SEP latencies often changed dramatically with different intensities of stimulation, potentially making any calculation of central conduction velocity meaningless without precise standardization of stimulation. These findings demonstrate a necessity for a biological calibration of stimulation intensity to improve the repeatability of SEPs. We suggest the use of two different standardized intensities of stimulation for SEP studies in SCI patients, one of which should be stronger than the intensity presently recommended.

SEPs to finger joint input lack the N20-P20 response that is evoked by tactile inputs: contrast between cortical generators in areas 3b and 2 in humans

A method using a DC servo motor is described to produce brisk angular movements at finger interphalangeal joints in humans. Small passive flexions of 2 degrees elicited sizable somatosensory evoked potentials (SEPs) starting with a contralateral positive P34 parietal response thought to reflect activation of a radial equivalent dipole generator in area 2 which receives joint inputs. By contrast, electric stimulation of tactile (non-joint) inputs from the distal phalanx evoked the usual contralateral negative N20 reflecting a tangential equivalent dipole generator in area 3b. Finger joint inputs also evoked a precentral positivity equivalent to the P22 of motor area 4, and a large frontal negativity equivalent to N30. It is suggested that natural stimulation allows human SEP components to be differentiated in conjunction with distinct cortical somatotopic projections.

Somatosensory evoked potentials in patients with selective impairment of position sense versus vibration sense

Ten patients with selective impairment of either position sense or vibration sense were studied with somatosensory evoked potential (SEP). Five patients with spinal cord lesion (three with MS, one with spinal cord tumor and one with spinal cord injury) lost the vibration sense below the iliac crests without impairment of the position sense. However, five patients with cerebral vascular lesions involving thalamus unilaterally showed severe impairment of position sense, though there was no asymmetry as to the vibration sense. In all these cases with spinal and cerebral lesions, SEPs showed abnormalities in the distributions where the position sense was impaired and were not related to the impairment of vibration sense. Our study indicates that SEP is much better correlated with the position sense than with the vibration sense at any lesion level.