Somatosensory and motor representations following bilateral transplants of the hands: A 6-year longitudinal case report on the first pediatric bilateral hand transplant patient.

A vascularized composite tissue allotransplantation (VCA) was performed at the Children's Hospital of Philadelphia (CHOP), on an 8-year-old patient in 2015, six years after bilateral hand and foot amputation. Hand VCA resulted in reafferentation of the medial, ulnar, and radial nerves serving hand somatosensation and motor function. We used magnetoencephalography (MEG) to assess somatosensory cortical plasticity following the post-transplantation recovery of the peripheral sensory nerves of the hands. Our 2-year postoperative MEG showed that somatosensory lip representations, initially observed at "hand areas", reverted to canonical, orthotopic lip locations with recovery of post-transplant hand function. Here, we continue the assessment of motor and somatosensory responses up to 6-years post-transplant. Magnetoencephalographic somatosensory responses were recorded eight times over a six-year period following hand transplantation, using a 275-channel MEG system. Somatosensory tactile stimuli were presented to the right lower lip (all 8 visits) as well as right and left index fingers (visits 3-8) and fifth digits (visits 4-8). In addition, left and right-hand motor responses were also recorded for left index finger and right thumb (visit 8 only).During the acute recovery phase (visits 3 and 4), somatosensory responses of the digits were observed to be significantly larger and more phasic (i.e., smoother) than controls. Subsequent measures showed that digit responses maintain this atypical response profile (evoked-response magnitudes typically exceed 1 picoTesla). Orthotopic somatosensory localization of the lip, D2, and D5 was preserved. Motor beta-band desynchrony was age-typical in localization and response magnitude; however, the motor gamma-band response was significantly larger than that observed in a reference population.These novel findings show that the restoration of somatosensory input of the hands resulted in persistent and atypically large cortical responses to digit stimulation, which remain atypically large at 6 years post-transplant; there is no known perceptual correlate, and no reports of phantom pain. Normal somatosensory organization of the lip, D2, and D5 representation remain stable following post-recovery reorganization of the lip's somatosensory response.

Brain activity modifications following spinal cord stimulation for chronic neuropathic pain: A systematic review.

BACKGROUND AND OBJECTIVE: Spinal cord stimulation (SCS) is believed to exert supraspinal effects; however, these mechanisms are still far from fully elucidated. This systematic review aims to assess existing neurophysiological and functional neuroimaging literature to reveal current knowledge regarding the effects of SCS for chronic neuropathic pain on brain activity, to identify gaps in knowledge, and to suggest directions for future research. DATABASES AND DATA TREATMENT: Electronic databases and hand-search of reference lists were employed to identify publications investigating brain activity associated with SCS in patients with chronic neuropathic pain, using neurophysiological and functional neuroimaging techniques (fMRI, PET, MEG, EEG). Studies investigating patients with SCS for chronic neuropathic pain and studying brain activity related to SCS were included. Demographic data (age, gender), study factors (imaging modality, patient diagnoses, pain area, duration of SCS at recording, stimulus used) and brain areas activated were extracted from the included studies. RESULTS: Twenty-four studies were included. Thirteen studies used neuroelectrical imaging techniques, eight studies used haemodynamic imaging techniques, two studies employed both neuroelectrical and haemodynamic techniques separately, and one study investigated cerebral neurobiology. CONCLUSIONS: The limited available evidence regarding supraspinal mechanisms of SCS does not allow us to develop any conclusive theories. However, the studies included appear to show an inhibitory effect of SCS on somatosensory evoked potentials, as well as identifying the thalamus and anterior cingulate cortex as potential mediators of the pain experience. The lack of substantial evidence in this area highlights the need for large-scale controlled studies of this kind.

Psychophysiological responses to visceral and somatic pain in functional chest pain identify clinically relevant pain clusters.

BACKGROUND: Despite chronic pain being a feature of functional chest pain (FCP) its experience is variable. The factors responsible for this variability remain unresolved. We aimed to address these knowledge gaps, hypothesizing that the psychophysiological profiles of FCP patients will be distinct from healthy subjects. METHODS: 20 Rome III defined FCP patients (nine males, mean age 38.7 years, range 28-59 years) and 20 healthy age-, sex-, and ethnicity-matched controls (nine males, mean 38.2 years, range 24-49) had anxiety, depression, and personality traits measured. Subjects had sympathetic and parasympathetic nervous system parameters measured at baseline and continuously thereafter. Subjects received standardized somatic (nail bed pressure) and visceral (esophageal balloon distension) stimuli to pain tolerance. Venous blood was sampled for cortisol at baseline, post somatic pain and post visceral pain. KEY RESULTS: Patients had higher neuroticism, state and trait anxiety, and depression scores but lower extroversion scores vs controls (all p < 0.005). Patients tolerated less somatic (p < 0.0001) and visceral stimulus (p = 0.009) and had a higher cortisol at baseline, and following pain (all p < 0.001). At baseline, patients had a higher sympathetic tone (p = 0.04), whereas in response to pain they increased their parasympathetic tone (p ≤ 0.008). The amalgamating the data, we identified two psychophysiologically distinct 'pain clusters'. Patients were overrepresented in the cluster characterized by high neuroticism, trait anxiety, baseline cortisol, pain hypersensitivity, and parasympathetic response to pain (all p < 0.03). CONCLUSIONS & INFERENCES: In future, such delineations in FCP populations may facilitate individualization of treatment based on psychophysiological profiling.

Psychological traits influence autonomic nervous system recovery following esophageal intubation in health and functional chest pain.

BACKGROUND: Esophageal intubation is a widely utilized technique for a diverse array of physiological studies, activating a complex physiological response mediated, in part, by the autonomic nervous system (ANS). In order to determine the optimal time period after intubation when physiological observations should be recorded, it is important to know the duration of, and factors that influence, this ANS response, in both health and disease. METHODS: Fifty healthy subjects (27 males, median age 31.9 years, range 20-53 years) and 20 patients with Rome III defined functional chest pain (nine male, median age of 38.7 years, range 28-59 years) had personality traits and anxiety measured. Subjects had heart rate (HR), blood pressure (BP), sympathetic (cardiac sympathetic index, CSI), and parasympathetic nervous system (cardiac vagal tone, CVT) parameters measured at baseline and in response to per nasum intubation with an esophageal catheter. CSI/CVT recovery was measured following esophageal intubation. KEY RESULTS: In all subjects, esophageal intubation caused an elevation in HR, BP, CSI, and skin conductance response (SCR; all p < 0.0001) but concomitant CVT and cardiac sensitivity to the baroreflex (CSB) withdrawal (all p < 0.04). Multiple linear regression analysis demonstrated that longer CVT recovery times were independently associated with higher neuroticism (p < 0.001). Patients had prolonged CSI and CVT recovery times in comparison to healthy subjects (112.5 s vs 46.5 s, p = 0.0001 and 549 s vs 223.5 s, p = 0.0001, respectively). CONCLUSIONS & INFERENCES: Esophageal intubation activates a flight/flight ANS response. Future studies should allow for at least 10 min of recovery time. Consideration should be given to psychological traits and disease status as these can influence recovery.

Gamma oscillatory amplitude encodes stimulus intensity in primary somatosensory cortex.

Gamma oscillations have previously been linked to pain perception and it has been hypothesized that they may have a potential role in encoding pain intensity. Stimulus response experiments have reported an increase in activity in the primary somatosensory cortex (SI) with increasing stimulus intensity, but the specific role of oscillatory dynamics in this change in activation remains unclear. In this study, Magnetoencephalography (MEG) was used to investigate the changes in cortical oscillations during four different intensities of a train of electrical stimuli to the right index finger, ranging from low sensation to strong pain. In those participants showing changes in evoked oscillatory gamma in SI during stimulation, the strength of the gamma power was found to increase with increasing stimulus intensity at both pain and sub-pain thresholds. These results suggest that evoked gamma oscillations in SI are not specific to pain but may have a role in encoding somatosensory stimulus intensity.

Sensory thresholds obtained from MEG data: cortical psychometric functions.

Sensory sensitivity is typically measured using behavioural techniques (psychophysics), which rely on observers responding to very large numbers of stimulus presentations. Psychophysics can be problematic when working with special populations, such as children or clinical patients who may lack the compliance or cognitive skills to perform the behavioural tasks. We used an auditory gap-detection paradigm to develop an accurate measure of sensory threshold derived from passively-recorded magnetoencephalographic (MEG) data. Auditory evoked responses were elicited by silent gaps of varying durations in an on-going noise stimulus. Source modelling was used to spatially filter the MEG data and sigmoidal 'cortical psychometric functions' relating response amplitude to gap duration were obtained for each individual participant. Fitting the functions with a curve and estimating the gap duration at which the amplitude of the evoked response exceeded one standard deviation of the prestimulus brain activity provided an excellent prediction of psychophysical threshold. Accurate sensory thresholds can therefore be reliably extracted from MEG data recorded while participants listen passively to a stimulus. Because our paradigm required no behavioural task, the method is suitable for studies of populations where variations in cognitive skills or vigilance make traditional psychophysics unsuitable.

The role of GABAergic modulation in motor function related neuronal network activity.

At rest, the primary motor cortex (M1) exhibits spontaneous neuronal network oscillations in the beta (15-30 Hz) frequency range, mediated by inhibitory interneuron drive via GABA-A receptors. However, questions remain regarding the neuropharmacological basis of movement related oscillatory phenomena, such as movement related beta desynchronisation (MRBD), post-movement beta rebound (PMBR) and movement related gamma synchronisation (MRGS). To address this, we used magnetoencephalography (MEG) to study the movement related oscillatory changes in M1 cortex of eight healthy participants, following administration of the GABA-A modulator diazepam. Results demonstrate that, contrary to initial hypotheses, neither MRGS nor PMBR appear to be GABA-A dependent, whilst the MRBD is facilitated by increased GABAergic drive. These data demonstrate that while movement-related beta changes appear to be dependent upon spontaneous beta oscillations, they occur independently of one other. Crucially, MRBD is a GABA-A mediated process, offering a possible mechanism by which motor function may be modulated. However, in contrast, the transient increase in synchronous power observed in PMBR and MRGS appears to be generated by a non-GABA-A receptor mediated process; the elucidation of which may offer important insights into motor processes.

Effective electromagnetic noise cancellation with beamformers and synthetic gradiometry in shielded and partly shielded environments.

The major challenge of MEG, the inverse problem, is to estimate the very weak primary neuronal currents from the measurements of extracranial magnetic fields. The non-uniqueness of this inverse solution is compounded by the fact that MEG signals contain large environmental and physiological noise that further complicates the problem. In this paper, we evaluate the effectiveness of magnetic noise cancellation by synthetic gradiometers and the beamformer analysis method of synthetic aperture magnetometry (SAM) for source localisation in the presence of large stimulus-generated noise. We demonstrate that activation of primary somatosensory cortex can be accurately identified using SAM despite the presence of significant stimulus-related magnetic interference. This interference was generated by a contact heat evoked potential stimulator (CHEPS), recently developed for thermal pain research, but which to date has not been used in a MEG environment. We also show that in a reduced shielding environment the use of higher order synthetic gradiometry is sufficient to obtain signal-to-noise ratios (SNRs) that allow for accurate localisation of cortical sensory function.

Oesophageal afferent pathway sensitivity in non-erosive reflux disease.

Patients with non-erosive reflux disease (NERD) report symptoms which commonly fail to improve on conventional antireflux therapies. Oesophageal visceral hyperalgaesia may contribute to symptom generation in NERD and we explore this hypothesis using oesophageal evoked potentials. Fifteen endoscopically confirmed NERD patients (four female, 29-56 years) plus 15 matched healthy volunteers (four female, 23-56 years) were studied. All patients had oesophageal manometry/24-h pH monitoring and all subjects underwent evoked potential and sensory testing, using electrical stimulation of the distal oesophagus. Cumulatively, NERD patients had higher sensory thresholds and increased evoked potential latencies when compared to controls (P = 0.01). In NERD patients, there was a correlation between pain threshold and acid exposure as determined by DeMeester score (r = 0.63, P = 0.02), with increased oesophageal sensitivity being associated with lower DeMeester score. Reflux negative patients had lower pain thresholds when compared to both reflux positive patients and controls. Evoked potentials were normal in reflux negative patients but significantly delayed in the reflux positive group (P = 0.01). We demonstrate that NERD patients form a continuum of oesophageal afferent sensitivity with a correlation between the degree of acid exposure and oesophageal pain thresholds. We provide objective evidence that increased oesophageal pain sensitivity in reflux negative NERD is associated with heightened afferent sensitivity as normal latency evoked potential responses could be elicited with reduced afferent input. Increased oesophageal afferent pain sensitivity may play an important role in a subset of NERD and could offer an alternate therapeutic target.

Dissociating the spatio-temporal characteristics of cortical neuronal activity associated with human volitional swallowing in the healthy adult brain.

Human swallowing represents a complex highly coordinated sensorimotor function whose functional neuroanatomy remains incompletely understood. Specifically, previous studies have failed to delineate the temporo-spatial sequence of those cerebral loci active during the differing phases of swallowing. We therefore sought to define the temporal characteristics of cortical activity associated with human swallowing behaviour using a novel application of magnetoencephalography (MEG). In healthy volunteers (n = 8, aged 28-45), 151-channel whole cortex MEG was recorded during the conditions of oral water infusion, volitional wet swallowing (5 ml bolus), tongue thrust or rest. Each condition lasted for 5 s and was repeated 20 times. Synthetic aperture magnetometry (SAM) analysis was performed on each active epoch and compared to rest. Temporal sequencing of brain activations utilised time-frequency wavelet plots of regions selected using virtual electrodes. Following SAM analysis, water infusion preferentially activated the caudolateral sensorimotor cortex, whereas during volitional swallowing and tongue movement, the superior sensorimotor cortex was more strongly active. Time-frequency wavelet analysis indicated that sensory input from the tongue simultaneously activated caudolateral sensorimotor and primary gustatory cortex, which appeared to prime the superior sensory and motor cortical areas, involved in the volitional phase of swallowing. Our data support the existence of a temporal synchrony across the whole cortical swallowing network, with sensory input from the tongue being critical. Thus, the ability to non-invasively image this network, with intra-individual and high temporal resolution, provides new insights into the brain processing of human swallowing.

Co-registration of magnetoencephalography with magnetic resonance imaging using bite-bar-based fiducials and surface-matching.

OBJECTIVE: To introduce a new technique for co-registration of Magnetoencephalography (MEG) with magnetic resonance imaging (MRI). We compare the accuracy of a new bite-bar with fixed fiducials to a previous technique whereby fiducial coils were attached proximal to landmarks on the skull. METHODS: A bite-bar with fixed fiducial coils is used to determine the position of the head in the MEG co-ordinate system. Co-registration is performed by a surface-matching technique. The advantage of fixing the coils is that the co-ordinate system is not based upon arbitrary and operator dependent fiducial points that are attached to landmarks (e.g. nasion and the preauricular points), but rather on those that are permanently fixed in relation to the skull. RESULTS: As a consequence of minimizing coil movement during digitization, errors in localization of the coils are significantly reduced, as shown by a randomization test. Displacement of the bite-bar caused by removal and repositioning between MEG recordings is minimal ( approximately 0.5 mm), and dipole localization accuracy of a somatosensory mapping paradigm shows a repeatability of approximately 5 mm. The overall accuracy of the new procedure is greatly improved compared to the previous technique. CONCLUSIONS: The test-retest reliability and accuracy of target localization with the new design is superior to techniques that incorporate anatomical-based fiducial points or coils placed on the circumference of the head.

Central neural mechanisms mediating human visceral hypersensitivity.

Although visceral hypersensitivity is thought to be important in generating symptoms in functional gastrointestinal disorders, the neural mechanisms involved are poorly understood. We recently showed that central sensitization (hyperexcitability of spinal cord sensory neurones) may play an important role. In this study, we demonstrate that after a 30-min infusion of 0.15 M HCl acid into the healthy human distal esophagus, we see a reduction in the pain threshold to electrical stimulation of the non-acid-exposed proximal esophagus (9.6 +/- 2.4 mA) and a concurrent reduction in the latency of the N1 and P2 components of the esophageal evoked potentials (EEP) from this region (10.4 +/- 2.3 and 15.8 +/- 5.3 ms, respectively). This reduced EEP latency indicates a central increase in afferent pathway velocity and therefore suggests that hyperexcitability within the central visceral pain pathway contributes to the hypersensitivity within the proximal, non-acid-exposed esophagus (secondary hyperalgesia/allodynia). These findings provide the first electrophysiological evidence that central sensitization contributes to human visceral hypersensitivity.

Comparison of cortical potentials evoked by mechanical and electrical stimulation of the rectum.

Patients with irritable bowel syndrome have heightened perception of gut sensation. The mechanisms responsible for this remain unknown, due to current poor knowledge of the central processing of gut sensation. Cortical evoked potentials (CEPs) have been recorded following both electrical rectal stimulation (ERS) and mechanical rectal stimulation (MRS). Because of the lack of a direct comparison of these two methods, their robustness for future clinical use remains unknown. The aim of our study was to compare the characteristics of CEPs following ERS and MRS. CEPs were recorded from the vertex in 14 healthy volunteers following ERS with bipolar ring electrodes, and MRS by repeated rectal distension. CEPs were recorded in all subjects following electrical stimulation, but only in 11 subjects following mechanical stimulation. In comparison with electrical stimulation, mechanical stimulation produced CEPs with a smaller amplitude and longer latency. However, the morphology of CEPs following electrical and mechanical rectal stimulation was similar, with no difference in the interpeak latencies. In conclusion, we have demonstrated that electrical rectal stimulation is a more reliable stimulus for recording CEPs. The similarity of the morphology and interpeak latencies of the CEPs suggests that both stimuli are activating a similar network of cortical neurones.

A cortical evoked potential study of afferents mediating human esophageal sensation.

The aim of this study was to compare the characteristics of esophageal cortical evoked potentials (CEP) following electrical and mechanical stimulation in healthy subjects to evaluate the afferents involved in mediating esophageal sensation. Similarities in morphology and interpeak latencies of the CEP to electrical and mechanical stimulation suggest that they are mediated via similar pathways. Conduction velocity of CEP to either electrical or mechanical stimulation was 7.9-8.6 m/s, suggesting mediation via thinly myelinated Adelta-fibers. Amplitudes of CEP components to mechanical stimulation were significantly smaller than to electrical stimulation at the same levels of perception, implying that electrical stimulation activates a larger number of afferents. The latency delay of approximately 50 ms for each mechanical CEP component compared with the corresponding electrical CEP component is consistent with the time delay for the mechanical stimulus to distend the esophageal wall sufficiently to trigger the afferent volley. In conclusion, because the mechanical and electrical stimulation intensities needed to obtain esophageal CEP are similar and clearly perceived, it is likely that both spinal and vagal pathways mediate esophageal CEP. Esophageal CEP to both modalities of stimulation are mediated by myelinated Adelta-fibers and produce equally robust CEP responses. Both techniques may have important roles in the assessment of esophageal sensory processing in health and disease.

Identification of the optimal parameters for recording cortical potentials evoked by mechanical stimulation of the human oesophagus.

Cortical evoked potentials (CEP) have been recorded in response to both electrical stimulation (ES) and mechanical stimulation (MS) of the oesophagus. While the optimal parameters for recording reproducible oesophageal CEP to ES have recently been established, they have not yet been determined for MS, and reported CEP to MS show considerable variability. This study aimed to identify the optimal parameters required to record reproducible MS induced CEP. CEP were recorded from the vertex (Cz) in six subjects (one female; age range 23-47 years). MS was performed 5 cm above the lower oesophageal sphincter by rapidly inflating a 2-cm long silicone balloon at a frequency of 0.2 Hz. The rise time to maximum inflation was 165 ms. In order to determine the minimum number of stimuli required to produce optimal signal-to-noise quality, we acquired data in runs of 25, 50, 100 and 300 stimuli and to determine the stimulation intensity that produced the shortest latency and the largest amplitude CEP, we averaged four runs of 50 stimuli at five different intensities ranging from sensory threshold to pain. CEP reproducibility was then assessed in three subjects on three separate occasions using parameters determined from these measurements. We found that optimal signal-to-noise quality was achieved by averaging four runs of 50 stimuli; that P1 latency was shortest and P1-N1 amplitude largest at intensities of 75% and pain threshold and that highly reproducible CEP were obtained in all individuals. We conclude that it is possible to obtain highly reproducible oesophageal CEP to MS which can now be compared to those obtained by ES in order to identify which is most suitable for clinical studies.

Identification of the optimal parameters for recording cortical evoked potentials to human oesophageal electrical stimulation.

Cortical evoked potentials in response to stimulation of the oesophagus may prove to be a powerful technique for assessing the oesophageal afferent pathway in health and disease. However, in order to maximize the potential of this technique it is essential that the optimal parameters for recording oesophageal CEP are established. The aim was to determine the optimal parameters required to record reproducible CEP. CEP were recorded from the vertex in eight subjects (age range 23-44 years). Electrical stimulation was performed 5 cm above the lower oesophageal sphincter using a bipolar ring electrode at 0.2 Hz. Protocol 1: to determine the stimulation intensity which generates the largest amplitude and shortest latency, two runs of 50 stimuli were applied at increasing intensities. Protocol 2: to determine the number of stimuli for optimal signal to noise ratio, 10 runs of 50 stimuli were recorded. Individual runs were averaged. Protocol 3: to determine the optimal inter-run interval, CEP evoked by 200 stimuli were averaged using randomly chosen inter-run intervals. Protocol 4: CEP reproducibility using parameters determined from Protocols 1-3 was assessed in three subjects on three separate occasions. The results were as follows: Protocol 1; P1 latency was shortest and P1-N1 amplitude largest at an intensity of 75% above threshold. Protocol 2; optimal signal-to-noise was achieved by averaging four runs of 50 stimuli. Protocol 3; the optimal interstudy interval was 10 min. Protocol 4; highly reproducible CEP were obtained in all individuals. Using these optimal parameters, it is possible to obtain highly reproducible oesophageal CEP to ES which can now be used for clinical study.

Cortical localisation of magnetic fields evoked by oesophageal distension.

Magnetoencephalographic source localisation techniques were used to measure oesophageal evoked magnetic fields from the cerebral cortex in 3 subjects. By using rapid balloon distension as a stimulus, a comparison of proximal and distal oesophageal cortical representation was made. The distal oesophagus was represented bilaterally in the insular cortex and SII as well as the inferior aspect of SI. The proximal oesophagus was represented unilaterally in superior and inferior SI, insular cortex and SII. Significantly, the superior portion of SI was consistently activated in subjects following stimulation of the proximal oesophagus, but similar activation was not found in response to distal stimulation. This may reflect the contribution from somatic afferent fibres in the striate muscle of the proximal segment. In conclusion, vagal afferents appear to contribute more to cortical activation following stimulation of the distal rather than the proximal oesophagus, while spinal afferents appear to be activated by both proximal and distal oesophageal stimulation.

Evaluation of MRI-MEG/EEG co-registration strategies using Monte Carlo simulation.

We present a Monte Carlo analysis method for evaluating MRI-MEG/EEG co-registration techniques. The method estimates the error in co-registration as a function of position within the brain. Using this analysis technique, we demonstrate the limitations of conventional head-based fiducial point methods, and propose a new strategy utilising a dental bite-bar incorporating accurately machined fiducial markers. Results presented demonstrate the improved accuracy of MEG/EEG to MRI co-registration using the bite-bar.

Topographic mapping of cortical potentials evoked by distension of the human proximal and distal oesophagus.

We describe cortical potentials evoked by balloon distension of the proximal and distal oesophagus in 8 healthy right handed volunteers. Oesophageal stimulation was performed using a pump which rapidly inflated a 2 cm silicone balloon positioned either 3 cm distal to the upper oesophageal sphincter or 5 cm proximal to the lower oesophageal sphincter, at a frequency of 0.2 Hz, using inflation volumes which produced a definite but not painful sensation. Oesophageal evoked cortical potentials were recorded in all subjects with an initial negative and positive component (N1 and P1), followed by a second negative and positive component (N2 and P2) in 6 subjects. The morphology and the scalp topography of the N1 component elicited by proximal and distal oesophageal stimulation suggests activation of the primary somatosensory cortex and/or the insular. There was also evidence for hemispheric dominance for the N1 potential which was independent of handedness. The frontal emphasis of the proximal oesophageal N1 component, in contrast to the central emphasis of the distal oesophageal N1 component, suggests that different neuronal populations were activated by stimulation of the two sites. The frontal emphasis of the ensuing P1 component from both oesophageal sites suggests that it originates in a separate precentral source. The topography of the N2 components obtained by stimulation of either oesophageal site was similar to that of the N1 component, suggesting that they originate in similar areas of the cortex. The P2 component evoked by stimulation of both oesophageal sites was localised at the vertex. The inter- and intra-subject variation in the morphology of the N2 and P2 components suggests that secondary cortical processes related to cognition may be involved in their generation.

Augmented P22-N31 SEP component in a patient with a unilateral space occupying lesion.

A patient presented with a right rolandic space occupying lesion resulting in a decrease of position sense, touch and stereognosis in the left upper limb. SEPs revealed an augmentation of the right hemisphere P22 component co-existing with relative attenuation of all other right hemisphere components. The augmented P22-N31 complex represented a 'giant' potential in relation to a control group (> 2.5 S.D.). The data provide further evidence that the P22-N31 complex has separate generators from those responsible for the N20-P27-N33 components parietally and P20-N30 components frontally. The focal nature of the lesion and symptomatology are of interest.