Somatosensory Evoked Responses Research Articles

Cortical responses to tactile stimuli in preterm infants.

The conventional assessment of preterm somatosensory functions using averaged cortical responses to electrical stimulation ignores the characteristic components of preterm somatosensory evoked responses (SERs). Our study aimed to systematically evaluate the occurrence and development of SERs after tactile stimulus in preterm infants. We analysed SERs performed during 45 electroencephalograms (EEGs) from 29 infants at the mean post-menstrual age of 30.7weeks. Altogether 2,087 SERs were identified visually at single-trial level from unfiltered signals capturing also their slowest components. We observed salient SERs with a high-amplitude slow component at a high success rate after hand (95%) and foot (83%) stimuli. There was a clear developmental change in both the slow wave and the higher-frequency components of the SERs. Infants with intraventricular haemorrhage (IVH; eleven infants) had initially normal SERs, but those with bilateral IVH later showed a developmental decrease in the ipsilateral SER occurrence after 30weeks of post-menstrual age. Our study shows that tactile stimulus applied at bedside elicits salient SERs with a large slow component and an overriding fast oscillation, which are specific to the preterm period. Prior experimental research indicates that such SERs allow studying both subplate and cortical functions. Our present findings further suggest that they might offer a window to the emergence of neurodevelopmental sequelae after major structural brain lesions and, hence, an additional tool for both research and clinical neurophysiological evaluation of infants before term age.

Evaluation of acute anodal direct current stimulation-induced effects on somatosensory-evoked responses in the rat

Transcranial direct current stimulation (tDCS) is a non-invasive tool used to treat brain disorders. The DC electric field is thought to modulate neuronal excitability and it has been reported to exert effects within the localized treatment area under the electrode, as well as in diffuse brain regions extending beyond the electrode. However, the manner in which tDCS influences neural transmission in the cortex and modulates neural activity in distant interconnected cortical regions remains unclear. Thus, the present study investigated the effects of anodal DCS (aDCS) on the forelimb-evoked sensory response that initially appears in the primary sensorimotor cortex (S1-M1) and then propagates to the secondary motor cortex (M2). When aDCS application was confined to the S1-M1 region, local field potential (LFP) recordings and voltage-sensitive dye (VSD) imaging revealed that the forelimb-evoked response in the S1-M1 was clearly enhanced. In contrast, the neural response in the M2 remained almost unchanged. On the other hand, aDCS application confined to the M2 increased the forelimb-evoked response in the M2 but not the S1-M1. Taken together, these results suggest that, when applied to the cortex, the aDCS may have intrinsic local effects, influencing afferent neural activity immediately underneath the stimulation site. Thus, the present results indicate that aDCS has less influence on neural activity in distant cortical areas interconnected to the stimulation site than at the stimulation site itself. Therefore, the findings do not support the idea of DCS remote activation via cortico-cortical connections, at least between the S1-M1 and M2 regions in rats.

Somatosensory Evoked Responses Elicited by Haptic Sensations in Midair

The design of human-machine interfaces (HMIs) by virtual haptics is an emerging field of research. So far, the perception of virtual haptic feedback, e.g., generated by focused ultrasound in midair has not been objectively evaluated. This paper demonstrates the feasibility of eliciting somatosensory evoked potentials (SEPs) with ultrasonic stimuli in midair for the first time. The palm was stimulated by short ultrasonic focal points generated by an ultrasound board. The results are compared with a no stimulation condition as well as with the results of a vibro tactile stimulation. The SEPs are analyzed with the wavelet phase synchronization stability (WPSS) and the M-consecutive averaged WPSS. The results indicate a clear SEP waveform elicited by ultrasound. It can be significantly differentiated from a no stimulation condition by the M-consecutive averaged WPSS. These results could enable the possibility of developing an objective evaluation method for virtual haptic feedback in HMIs.

Mouse fMRI under ketamine and xylazine anesthesia: Robust contralateral somatosensory cortex activation in response to forepaw stimulation

Mouse fMRI is critically useful to investigate functions of mouse models. Until now, the somatosensory-evoked responses in anesthetized mice are often widespread and inconsistent across reports. Here, we adopted a ketamine and xylazine mixture for mouse fMRI, which is relatively new anesthetics in fMRI experiments. Forepaw stimulation frequency was optimized using cerebral blood volume (CBV)-weighted optical imaging (n = 11) and blood-oxygenation-level dependent (BOLD) fMRI with a gradient-echo time of 16 ms at 9.4 T, and 4 Hz stimulation with 0.5 ms and 0.5 mA pulses induced the highest hemodynamic response. For 20-s 4-Hz unilateral forepaw stimulation, localized BOLD activity was consistently found in the contralateral primary forelimb somatosensory cortex (S1FL), while no significant change was observed in the ipsilateral S1FL. The mean magnitude was 1.44 ± 0.20% SEM (n = 9) in the contralateral S1FL and 0.69 ± 0.10% in the contralateral thalamus. The variability of evoked fMRI responses across sessions was investigated by comparing with resting state fMRI (rsfMRI) functional connectivity (FC). Evoked responses in S1FL were correlated positively with rsfMRI FC between bilateral S1FL (r = 0.63 to 0.69) and negatively with FC between S1FL and the anterior cingulate cortex (r = −0.50 to −0.57), suggesting that rsfMRI FC is a good index of the evoked fMRI response and anesthetized animal condition. Finally, three weekly fMRI scans were performed in 5 mice, and localized activity was reproducibly observed in S1FL, with a success rate of 70–95%. In summary, our developed fMRI protocol can be used for mapping functions of mouse models.

Platform Session – NIBS: Quantifying the stereotypy of TMS-evoked EEG potentials using a cosine similarity metric

Introduction TMS-evoked EEG potentials (TEPs) can be used to probe connectivity and consciousness in healthy and diseased brains. A downside of this technique is that TMS causes neural artifacts, such as auditory and somatosensory-evoked responses, which may mask the true response to cortical stimulation, particularly at longer latencies. To avoid misinterpreting TEPs, methods for detecting neural artifact are required. One potentially helpful assumption is that neural artifacts have stereotyped topographies, while the true response to focal cortical stimulation will be spatially differentiated between stimulation conditions. Based on this assumption we used cosine similarity as a measure of response similarity and compared waveforms from real and sham stimulation at three scalp locations. Methods In 20 healthy volunteers, we delivered 150 active and 50 sham pulses to the right dorsolateral prefrontal cortex, left primary motor cortex, and left parietal cortex at 100% resting motor threshold. Sham stimulation was performed by placing a ∼2 cm spacer between the coil and scalp. TEPs were recorded with a 32-channel TMS-compatible EEG system and preprocessed using Fieldtrip® software. The derivatives of the preprocessed TEPs were calculated and then binarized for every time point at each channel. Cosine similarity at each time-point was calculated between conditions using all channels. Results The cosine similarity metric showed that for each comparison (e.g. motor vs. parietal, real vs. sham, etc.), the early ( ∼ 100 ms across all comparisons, coinciding with the N100 component. These quantitative results closely match qualitative assessment of the whole-brain topographies, lending credibility to our use of the cosine similarity technique as a measure of stereotypy. Conclusion Our results indicate that while early TEP components reflect site-specific responses to direct cortical stimulation, later responses become progressively stereotyped across stimulation sites, suggesting that these late components are neural artifact. Due to the exponential-like decay in differentiation seen in each comparison, we propose that only TEP components occurring earlier than 50–70 ms are safely interpretable as responses to cortical stimulation. Auditory and somatosensory stimulation controls and/or further refinements in signal processing may be necessary to extract true responses to cortical stimulation from later components.

Mirror imaging of finger mechanical stimulation affects secondary somatosensory response.

The aim of this study was to investigate the effect of finger-tip touch on the somatosensory cortex under mirror imaging conditions. Magnetoencephalographic recordings of somatosensory-evoked field responses to mechanical tactile stimulation of the index finger in both hands were obtained for the right-handed healthy participants as part of the following mirror or nonmirror tasks. Both hands were screened and mechanical stimuli were delivered to the right index finger pulp. A mirror was placed in front of the patient with the right hand facing the reflective surface, as if it were the left side (mirror task). Another task comprised the screening of right hands behind a partition, after which mechanical stimuli were delivered to the right index finger pulp without a mirror (nonmirror task). The same tasks for the left hand were also examined. The estimated current dipoles were identified and analyzed. In the measurements following all the tasks, the equivalent current dipoles (ECDs) were estimated in the corresponding contralateral primary somatosensory area (cSI). The other ECDs were estimated in the secondary somatosensory area both contralateral (cSII) and ipsilateral (iSII) to the stimulus side. The amplitudes of ECDs in the cSII area following the left index stimuli in the mirror task were significantly larger than those in the nonmirror task (P=0.043). The mirror effect on the somatosensory cortex in this study may be small, but a positive variation was shown in the form of a response to the actual stimulation in the hand opposite to the hand which was hidden behind the mirror.

P 105 Spatio-temporal characteristics of somatosensory-evoked high-gamma activity of the EEG

Introduction There is enlarging interest in the cortical function of high gamma band (HGB) changes related to different sensory modalities, movement tasks, and higher cognitive processes. EEG experiments have shown visually-evoked, auditory-evoked, and movement-related HGB changes, including the range of 50–70 Hz ( Tallon-Baudry et al., 1997 , Ball et al., 2008 ). While HGB activity in this frequency range associated to peripheral somatosensory stimulation was reported for intracranial recordings in neurological patients ( Fukuda et al., 2008 ), it has received little attention in EEG studies. Here the focus was on very high frequency somatosensory responses above 600 Hz ( Curio et al., 1994 ). In the present study, we implemented an experimental setup, optimized to measure high frequencies in the EEG, to investigate if robust 50–70 Hz HGB activity in noninvasive EEG could be identified during median nerve SEP stimulation and what the spatio-temporal characteristics of these changes are. Methods In an electromagnetically-shielded cabin, EEG data of 15 healthy subjects was obtained with 128 Ag/AgCl scalp electrodes and with NeurOne amplifiers (24-bit digital resolution and 5 kHz sampling rate). 1200 SEPs per subject were evoked by stimulation of the left median nerve with a SEP-stimulator. Simultaneous eye-tracking data from 7 of these subjects was attained to examine the influence of ocular movements on the neuronal recordings. Data analysis included preprocessing, short-time Fourier transformation algorithm, normalization, calculation of induced and evoked responses and a false discovery rate corrected sign test. Results All analyzed subjects showed a significant spectral power increase in the HGB range at 50–70 Hz after median nerve stimulation ( p Conclusion Using our purpose-build experimental set up, we were able to demonstrate a specific contralateral somatosensory-evoked response pattern in the 50–70-Hz frequency range of the noninvasive EEG. Our results agree with the findings of previous intracranial SEP studies and open up new possibilities to noninvasively study sensory related brain functions, both in healthy subjects and neurological patients.

Panic Disorderの脳波と体性感覚誘発電位 : 過呼吸負荷時の生理学的変化について

Panic Disorderの脳波と体性感覚誘発電位 : 過呼吸負荷時の生理学的変化について

Disinhibitory shift of recovery curve of somatosensory-evoked response in elderly: A magnetoencephalographic study

ObjectiveThe aim of the study was to investigate the functional differences between N20m and P30m components of somatosensory-evoked magnetic cortical field (SEF) in young and senior subjects. MethodsTwenty-nine healthy subjects, 13 younger (mean age: 21.8years) and 16 senior (63.8years), participated. Magnetic fields were measured using a 160-channel, whole head MEG. Single- and paired-pulse stimulations of 200 artifact-free MEG signal epochs were averaged separately. We calculated how aging affects recovery function of SEFs. ResultsThe senior showed a prolonged N20m peak latency compared to the younger, although the P30m peak latency was not significantly different between groups. The N20m ratios at 60 and 80ms in the senior were significantly increased compared to the ratios in the younger (60ms: P<0.05, 80ms: P<0.001). The P30m ratios at inter-stimulus interval (ISI) of 80 and 100ms showed even disinhibition in the senior than in the younger (P<0.05). The younger also showed a significantly negative correlation between P30m and N20m components’ recovery curves (R=0.72, P<0.05). ConclusionAging-related changes that occurred in recovery functioning were the decrease in N20m component suppression and the increase in P30m component recovery, indicating that the N20m and P30m components have different functions in aging-related recovery changes. SignificanceOur results show that the N20m ratio at an ISI of 80ms was significantly increased in the senior group, indicating that the second stimulus-evoked SEF was less inhibited by the initial stimulus at this ISI, suggesting less refractory effect or increased disinhibition.

A simple novel technique to induce short‐lasting local brain ischaemia in the rat

Brain ischaemia models are essential to study the pathomechanisms of stroke. Our aim was to investigate the reliability and reproducibility of our novel focal ischaemia-reperfusion model. To induce a cortical transient ischaemic attack, we lifted the distal middle cerebral artery (MCA) with a special hook. The early changes after 2 × 15-min occlusion were observed in the somatosensory evoked responses (SERs). The histological responses to 2 × 15-min MCA occlusion and to 30-, 45- or 60-min ischaemia were examined after a 1-day survival period by 2,3,5-triphenyltetrazolium chloride (TTC) and Fluoro Jade C (FJC) staining. Another group, with 30-min ischaemia, was analysed histologically by FJC, S100 and CD11b labelling after a 5-day survival period. The amplitudes of the SERs decreased immediately at the beginning of the ischaemic period, and remained at a reduced level during the ischaemia. Reperfusion resulted in increasing SER amplitudes, but they never regained the control level. The short-lasting ischaemia did not lead to brain infarction when evaluated with TTC, but intense labelling was found with FJC. The 30-min ischaemia did not result in FJC labelling after 1 day, but marked labelling was observed after 5 days with FJC, S100 and CD11b in the cortical area supplied by the MCA. We present here a novel, readily reproducible method to induce focal brain ischaemia. The ischaemia-reperfusion results in noteworthy changes in the SERs and the appearance of conventional tissue damage markers. This method involves possibilities for precise blood flow regulation, and the setting of the required level of perfusion.

Neuroprotective effect of oxaloacetate in a focal brain ischemic model in the rat.

During an ischemic event, the well-regulated glutamate (Glu) homeostasis is disturbed, which gives rise to extremely high levels of this excitatory neurotransmitter in the brain tissues. It was earlier reported that the administration of oxaloacetate (OxAc) as a Glu scavenger reduces the Glu level in the brain by enhancing the brain-to-blood Glu efflux. Here, we studied the neuroprotective effect of OxAc administration in a new focal ischemic model in rats. Occlusion of the middle cerebral artery resulted in immediate reduction of the somatosensory-evoked responses (SERs), and the amplitudes remained at the reduced level throughout the whole ischemic period. On reperfusion, the SERs started to increase, but never reached the control level. OxAc proved to be protective, since the amplitudes started to recover even during the ischemia, and finally fully regained the control level. The findings of the histological measurements were in accordance with the electrophysiological data. After Fluoro Jade C staining, significantly fewer labeled cells were detected in the OxAc-treated group relative to the control. These results provide new evidence of the neuroprotective effect of OxAc against ischemic injury, which strengthens the likelihood of its future applicability as a novel neuroprotective agent for the treatment of ischemic stroke patients.

FMRI in the awake marmoset: Somatosensory-evoked responses, functional connectivity, and comparison with propofol anesthesia

Functional neuroimaging in animal models is essential for understanding the principles of neurovascular coupling and the physiological basis of fMRI signals that are widely used to study sensory and cognitive processing in the human brain. While hemodynamic responses to sensory stimuli have been characterized in humans, animal studies are able to combine very high resolution imaging with invasive measurements and pharmacological manipulation. To date, most high-resolution studies of neurovascular coupling in small animals have been carried out in anesthetized rodents. Here we report fMRI experiments in conscious, awake common marmosets (Callithrix jacchus), and compare responses to animals anesthetized with propofol. In conscious marmosets, robust BOLD fMRI responses to somatosensory stimulation of the forearm were found in contralateral and ipsilateral regions of the thalamus, primary (SI) and secondary (SII) somatosensory cortex, and the caudate nucleus. These responses were markedly stronger than those in anesthetized marmosets and showed a monotonic increase in the amplitude of the BOLD response with stimulus frequency. On the other hand, anesthesia significantly attenuated responses in thalamus, SI and SII, and abolished responses in caudate and ipsilateral SI. Moreover, anesthesia influenced several other aspects of the fMRI responses, including the shape of the hemodynamic response function and the interareal (SI-SII) spontaneous functional connectivity. Together, these findings demonstrate the value of the conscious, awake marmoset model for studying physiological responses in the somatosensory pathway, in the absence of anesthesia, so that the data can be compared most directly to fMRI in conscious humans.

Oscillatory Entrainment of Primary Somatosensory Cortex Encodes Visual Control of Tactile Processing

Optimal behavior relies on the successful integration of complementary information from multiple senses. The neural mechanisms underlying multisensory interactions are still poorly understood. Here, we demonstrate the critical role of neural network oscillations and direct connectivity between primary sensory cortices in visual-somatosensory interactions. Extracellular recordings from all layers of the barrel field in Brown Norway rats in vivo showed that bimodal stimulation (simultaneous light flash and whisker deflection) augmented the somatosensory-evoked response and changed the power of induced network oscillations by resetting their phase. Anatomical tracing revealed sparse direct connectivity between primary visual (V1) and somatosensory (S1) cortices. Pharmacological silencing of V1 diminished but did not abolish cross-modal effects on S1 oscillatory activity, while leaving the early enhancement of the evoked response unaffected. Thus, visual stimuli seem to impact tactile processing by modulating network oscillations in S1 via corticocortical projections and subcortical feedforward interactions.

Utility of temporally-extended signal space separation algorithm for magnetic noise from vagal nerve stimulators

ObjectiveTo evaluate the utility of a temporally-extended signal space separation algorithm (tSSS) for patients with vagal nerve stimulator (VNS). MethodsWe evaluated median nerve somatosensory evoked responses (SER) of magnetoencephalography (MEG) in 27 VNS patients (48 sides) with/without tSSS processing. We classified SER dipoles as ‘acceptable’ if: (A) the location of the dipole was in the expected location in the central sulcus, and (B) the goodness of fit value (GOF) was greater than 80%. We evaluated (1) the number of sides which produced acceptable dipoles in each dataset (i.e. with/without tSSS processing), and in cases where the both data produced reliable dipoles, (2) compared their GOFs and the 95% confidence volumes (CV) (mm3). Statistical differences in the GOF and CV between with/without tSSS conditions were determined by paired t test. ResultsOnly 11 (23%) responses had reliable dipoles without tSSS processing, while all 48 (100%) had acceptable dipoles under tSSS processing. Additionally, the latter group had significantly higher GOF (increased by 7% on average) and lower CV (mean decrease of 200mm3) than the former (p<0.01). ConclusionsProcessing with tSSS quantitatively improves dipole fitting of known sources in VNS patients. SignificanceThis algorithm permits satisfactory MEG testing in the relatively commonly encountered epilepsy patient with VNS.

Evoking Potentials from the Brain (P04.009)

Objective: To present the history of discovery of evoked potentials by reviewing the original publications of Caton, Beck, Dawson and Jewett. Background Caton9s publications revealed that unstimulated cerebral cortex had a positive charge and stimulation of an animal9s limb could produce waves of negative cortical currents in contralateral motor cortex. Design/Methods: Review of publications of Caton, Beck, Dawson and Jewett. Results: Richard Caton (1842-1926) discovered somatosensory evoked responses in 1875 Caton demonstrated fluctuations of a galvanometer connected to electrodes on exposed surfaces of rabbits9 cerebral cortex in response to stimulation of hind paws. Caton noted negative current fluctuations from occipital cortex after he shined light into retinas of rabbits. Adolph Beck in 1890 claimed primacy of discovery for similar observations. In 1947 Charles Dawson (1912-1983) superimposed amplified multiple cortical somatosensory evoked responses (SER) on a screen of an oscilloscope improving signal-to-noise ratio (SNR) of the SER. In 1954 Dawson developed the signal averager (SA) to further improve SNRs. Dawson9s SA consisted of a rotating switch driven by a motor that fed each individual scalp response to a single median nerve stimulus into 1 of 62 condensers. The SA serially discharged each condenser into an amplifier connected to a storage oscilloscope. Dawson noted that the superimposed SER had a onset latency of 28 ms. Dawson proved that SA could eliminate EEG waves not related to the stimulus. Dawson9s median SERs resembled modern cortical SERs. Conclusions: Caton should be credited as discoverer of evoked responses. Dawson should be considered founder of human evoked potentials research, because his averaging technique extracted the evoked potential. Disclosure: Dr. Fine has received personal compensation for activities with Slocum-Dickson Clinic. Dr. Fine holds stock and/or stock options in Bristol Myers-Squibb, Eli Lilly, Pfizer, and Johnson & Johnson. Dr. Riggins has nothing to disclose. Dr. Farooq has nothing to disclose.

Hypothyroidism: Clinical Features

Hypothyroidism: Clinical Features

An Easy and Practical Method for Routine, Bedside Testing of Somatosensory Systems in Extremely Low Birth Weight Infants

This study was set out to develop and describe a novel, simple, and safe method for routine bedside testing of somatosensory system in very early preterm infants. We recorded electroencephalogram (EEG) activity after tactile stimulation of hand (palm) and foot (sole) by a soft hairbrush stimulator in extremely low birth weight infants (n = 10; GA, 24-28, recording at conceptional age 30-32 wk) and compared with the raw EEG responses to those seen by one- or two-channel brain monitors. In every subject, single tactile stimuli produced prominent (100-350 microV) somatosensory evoked responses (SERs) that were readily identified in the ongoing EEG signal. The maximal SER was in the contralateral hemisphere at around the corresponding somatosensory representation areas. Conventional EEG filtering did significantly reduce the SERs, but they could still be identified in the routine brain monitor setting widely available in NICUs. The method described here is directly applicable to assessment of integrity of somatosensory system in the early preterm period. It needs minimal training and requires an EEG system or a brain monitor device that is available in most units. Thus, the technique is likely to open a novel window to neurologic assessment of these babies.

Increasing top-down suppression from prefrontal cortex facilitates tactile working memory

Navigated transcranial magnetic stimulation (TMS) combined with diffusion-weighted magnetic resonance imaging (DW-MRI) and tractography allows investigating functional anatomy of the human brain with high precision. Here we demonstrate that working memory (WM) processing of tactile temporal information is facilitated by delivering a single TMS pulse to the middle frontal gyrus (MFG) during memory maintenance. Facilitation was obtained only with a TMS pulse applied to a location of the MFG with anatomical connectivity to the primary somatosensory cortex (S1). TMS improved tactile WM also when distractive tactile stimuli interfered with memory maintenance. Moreover, TMS to the same MFG site attenuated somatosensory evoked responses (SEPs). The results suggest that the TMS-induced memory improvement is explained by increased top-down suppression of interfering sensory processing in S1 via the MFG–S1 link. These results demonstrate an anatomical and functional network that is involved in maintenance of tactile temporal WM.

Long-Term Plasticity in Mouse Sensorimotor Circuits after Rhythmic Whisker Stimulation

Mice actively explore their environment by rhythmically sweeping their whiskers. As a consequence, neuronal activity in somatosensory pathways is modulated by the frequency of whisker movement. The potential role of rhythmic neuronal activity for the integration and consolidation of sensory signals, however, remains unexplored. Here, we show that a brief period of rhythmic whisker stimulation in anesthetized mice resulted in a frequency-specific long-lasting increase in the amplitude of somatosensory-evoked potentials in the contralateral primary somatosensory (barrel) cortex. Mapping of evoked potentials and intracortical recordings revealed that, in addition to potentiation in layers IV and II/III of the barrel cortex, rhythmic whisker stimulation induced a decrease of somatosensory-evoked responses in the supragranular layers of the motor cortex. To assess whether rhythmic sensory input-based plasticity might arise in natural settings, we exposed mice to environmental enrichment. We found that it resulted in somatosensory-evoked responses of increased amplitude, highlighting the influence of previous sensory experience in shaping sensory responses. Importantly, environmental enrichment-induced plasticity occluded further potentiation by rhythmic stimulation, indicating that both phenomena share common mechanisms. Overall, our results suggest that natural, rhythmic patterns of whisker activity can modify the cerebral processing of sensory information, providing a possible mechanism for learning during sensory perception.

N30 somatosensory evoked potentials in patients with unilateral Parkinson's disease.

The N30 component of the somatosensory evoked responses (SEP) was reported as absent or abnormally low in various basal ganglia disorders, including Parkinson's disease (PD), but its relationship to the more affected side in asymmetric disease has not been assessed systematically. In 14 patients with unilateral PD and 10 controls, SEP were performed by stimulating each upper limb and recording from the parietal and frontal contralateral cortex. N30 was symmetric in 4 patients and 4 controls; it was asymmetric in 2 controls and in 8 patients (in 4 the responses were of lower amplitude over the affected hemisphere); no response was elicited in 2 patients and 4 controls. In all patients SEP were recorded off medication; in 9 of them the test was repeated following administration of L-dopa and did not show any significant changes. In conclusion, the N30 was asymmetric in a similar proportion of PD patients and controls. No correlation was found between the affected side and N30 latencies or amplitudes; no change was seen following L-dopa administration as well. The motor deficits and SEP abnormalities in PD probably reflect pathologies in different anatomical structures or functional circuits.