Effects on median nerve SEPs of tactile stimulation applied to adjacent and remote areas of the body surface

Study of the influence of continuous tactile stimulation on somatosensory evoked potentials (SEPs) following electrical stimulation of the median nerve revealed an effect due to interfering input from both adjacent and remote regions of the body surface. The distribution of the effect was demonstrated by subtracting the 'interference' from the 'control' response to derive a 'difference' wave form. Tactile stimulation of the thumb ipsilateral to the stimulated median nerve produced a difference wave form in which a marked phase reversal was apparent between pre- and post-central areas for 2 complexes, at latencies of approximately 20 and 30 msec. It is proposed that this may have been due to partial 'saturation' of a generator in the hand region of area 3b in the primary somatosensory cortex (SI), which was then unable to respond fully to the median nerve impulse. A similar effect was observed when the interfering stimulus was applied to the ipsilateral little finger, possibly reflecting a process of 'surround inhibition.' Tactile stimulation of more remote regions (principally the face and contralateral hand) resulted in consistent difference wave forms in which the early components (less than 30 msec latency) had scalp distributions differing from one another but consistent with influence on generators in the face or hand region of the second somatosensory cortex (SII). Later potentials consistently identifiable in the difference wave forms were similar for all locations of the interfering stimulus apart from the ipsilateral thumb and were distributed in accordance with a proposed generator in the parietal 'association' cortex.     

Movement related slow potentials. II. A contrast between finger and foot movements in left-handed subjects

Finger and foot movement related potentials (MRPs) were recorded over the frontal, central and parietal areas of both hemispheres in 20 left-handed subjects. A unilateral flexion of the index finger and a plantar flexion of the foot were studied on either side. MRPs were larger preceding foot movements than preceding finger movements, their onset being earlier also. Prior to a finger flexion amplitudes were larger over the hemisphere contralateral to the movement than over the ipsilateral hemisphere. Preceding a foot movement, however, amplitudes were larger over the ipsilateral hemisphere. These results indicate differently localized sources of the MRPs in the two kinds of movement, in accordance with data obtained in right-handed subjects. No indication of a hemisphere effect, possibly related to motor dominance, was found in left-handers. This is in contrast to a slight hemisphere effect found with foot movements in right-handed subjects in the former study.     

Study of early somatosensory evoked potentials by stimulation of the median nerve at the wrist in 6 patients with unilateral thalamic lesions

Somatosensory evoked potentials were recorded at Erb's point, over the cervical spine (C7) and over the cortex: parietal and frontal electrodes were contralateral and ipsilateral to the stimulus which was applied on the median nerve at the wrist. The stimulation was performed on 2 control groups, the first consisting of 10 subjects (average age: 33.6 years), the second of 16 subjects (average age: 66.2 years) and on 6 patients presenting unilateral thalamic lesions. These lesions were circumscribed, ischaemic or haemorrhagic and were visualized by a scanner. In 5 of our patients, a diffusion of the P14 wave with normal latency and a delay in the N20 cortical wave was obtained at the parietal electrode contralateral to the stimulus and homolateral to the lesion. Normal latencies were observed for the diffusion of the N18 wave recorded at the frontal electrode contralateral to the stimulus. In the 6th patient, the evoked potentials were normal. The results of the somatosensory evoked potentials observed in our patients are discussed in the context of the anatomical lesions.     

Influence of concurrent tactile stimulation on somatosensory evoked potentials following posterior tibial nerve stimulation in man

A topographical study was made of SEPs following stimulation of the right posterior tibial nerve at the ankle, with and without concurrent tactile stimulation of the soles of either foot or the palm of the right hand. Effects of the interfering stimulus were best demonstrated by subtracting the wave forms to derive "difference' potentials. The majority of SEP components were significantly attenuated by tactile stimulation of the ipsilateral foot, and the difference wave form was of similar morphology to the control response. Components of opposite polarity peaking at 39 msec were consistent with the field of a cortical generator with dipolar properties, situated in the contralateral hemisphere just posterior to the vertex with the positive poles oriented towards the ipsilateral side. By analogy with median SEP findings, these potentials were believed to originate in the foot region of area 3b where neurones are mainly concerned with cutaneous sensory processing. When the tactile stimulus was applied to the contralateral foot, difference potentials maximally recorded just posterior to the vertex were of smaller amplitude but similar morphology to ipsilateral foot difference components. This suggested the possibility that input from the two lower extremities may converge at cortical or subcortical level, the effect being manifested in the response of certain neurones in area 3b. With both contralateral foot and ipsilateral hand stimulation, other difference potentials were present which suggested that there may be cortical regions responding to combinations of sensory stimuli applied to various parts of the body surface.     

Movement-related slow potentials. I. A contrast between finger and foot movements in right-handed subjects

Movement-related potentials ( MRPs ) preceding a finger flexion and a plantar flexion of the foot on either side were compared over the frontal, central and parietal areas of both hemispheres. MRP amplitudes were larger preceding foot than preceding finger movements. In the first case their onset was earlier and their presence in the frontal area was more marked. Prior to a finger flexion amplitudes over the hemisphere contralateral to the movement side were larger than those recorded over the ipsilateral hemisphere. On the contrary, prior to a plantar flexion of the foot, amplitudes were larger over the hemisphere ipsilateral to the movement. These findings point to differently localized sources of the MRPs in the two cases. In other experiments larger amplitudes preceding foot movements were found near the midline. It is suggested that the ipsilateral preponderance prior to foot movements is caused by a contralateral source in the depth near the longitudinal fissure. The dipoles are presumably directed obliquely to the median plane. The ipsilateral preponderance is present both prior to and following the plantar flexion. This suggests comparable directions of the dipoles in the motor and somatosensory areas.     

An AER analysis of contralateral advantage in the transmission of auditory information.

Average electroencephalic responses (AERs) to clicks were recorded from scalp electrodes over the left and right parietal areas (C₃, C₄) of 12 subjects. Measures of AER latency for monaural click and dichotic click—sentence conditions indicated that during dichotic presentation, the contralateral hemisphere in man responds significantly faster to the click than the ipsilateral hemisphere. These results are consistent with Kimura's (1961a) proposed contralateral-pathway advantage during dichotic listening in man.     

Distribution of scalp somatosensory potentials evoked by stimulation of the tibial nerve in man

The scalp distribution of the response to stimulation of the tibial nerve at the medial malleolus was systematically analysed. The somatosensory evoked potential (SEP) was recorded with electrodes placed in a transversal line over the ipsilateral and contralateral postcentral gyri and in a sagittal line over the longitudinal brain fissure. The SEPs recorded over the ipsilateral hemisphere and along the sagittal line were similar to the F response (the response over the foot primary somatosensory region). Over the contralateral hemisphere the waveform of the responses changed obviously from point F to the point C (contralateral hand primary somatosensory region). The C response started with N37, P40 had a longer latency, N50 was not present and the subsequent waves were also considerably different. Mathematical simulation of the responses recorded from the electrodes between points F and C has shown that they represent an electrical algebraic summation of the activity over points F and C. Although the F and C responses may be 2 potentials arising from the opposite sides of a single dipole generator which is located in the medial fissure, it is more probable that the somatosensory evoked potential on tibial nerve stimulation reflects the activity of 2 separate generators.     

An 'interference" approach to the study of somatosensory evoked potentials in man

Somatosensory evoked potentials (SEPs) were recorded from the shoulders, neck and scalp in response to electrical stimulation of the median nerve, with and without various interfering stimuli delivered to the ipsilateral hand. Vibration applied to the fingertips, like active movement of the fingers, caused a reduction in the amplitude of peripheral and cervical evoked potentials, with a more marked attenuation of the initial negativity and positivity (N18 and P21) recorded over the somatosensory cortical hand area. Light touch on the palm and first three digits had little effect on peripheral and cervical potentials, but caused attenuation of N18 and P21 and marked enhancement of a negative wave at 25-35 msec latency. This was shown to be a composite effect due to wave form alterations at the hand area electrode and the midfrontal reference site. Similar changes occurred to a lesser degree when the interfering stimulus was applied to other parts of the ipsilateral arm and the face, but not with stimulation of the contralateral arm. The altered wave form did not resemble that which is recorded at low median nerve stimulus intensities, but was similar to that recorded at high stimulation frequencies. The absence of any interfering effect of painful or thermal stimulation supports the hypothesis that these changes may reflect build-up of inhibition at synapses of the dorsal column/medial lemniscal pathway, due to input from cutaneous touch receptors and proprioceptors.     

Changes of somatosensory evoked potentials during writing with the dominant and non-dominant hands

Since close attention and special effort are necessary to perform difficult unskilled movements, particular brain activities underlying such movements could be expected to take place in the primary sensori-motor cortices (SI and MI). In this study we focused on such activities by analyzing the difference in the somatosensory evoked potential (SEP) in presence to the electrical stimulation of the median nerve during writing using the dominant and non-dominant hands in twelve right-handed and eight left-handed normal subjects. By alternately stimulating the right and left median nerves during the writing with either hand, SEPs were recorded from both hemispheres. During the dominant hand writing, the middle latency SEP components, i.e., parietal P25 and N33 and frontal N30, were significantly attenuated only in the hemisphere contralateral to the writing hand, corresponding to the conventional gating effect. During the non-dominant hand writing, not only those components recorded from the hemisphere contralateral to the writing hand, but also those from the hemisphere ipsilateral to the writing hand were significantly attenuated. In addition, N20 in the hemisphere contralateral to the writing hand was also significantly attenuated. There was no significant difference in the attenuation between the right-handed and left-handed subjects. The results indicated that the specific interaction between the signals after electrical stimulation and the sensory cortical activities related to the writing using the non-dominant hand occurred in both hemispheres, while it was recognized only in the hemisphere contralateral to the writing hand during the dominant hand writing. We speculate that the somatosensory cortex was more activated and thus interacted with the applied stimulation during the unskilled movement of the non-dominant hand compared to the movement of the dominant hand.     

Source generators of the early somatosensory evoked potentials to tibial nerve stimulation: an intracerebral and scalp recording study.

OBJECTIVE: To investigate the location of the cerebral generators of the early scalp somatosensory evoked potentials (SEPs) after tibial nerve stimulation. METHODS: Tibial nerve SEPs were recorded in 15 patients, suffering from Parkinson's disease, who underwent implantation of intracerebral (IC) electrodes in the subthalamic nucleus, in the globus pallidum or in the thalamic ventralis intermediate nucleus. SEPs were recorded both from the scalp surface and from the IC leads. RESULTS: The lemniscal P30 response was recorded by all the electrodes. The IC waveforms included a negative N40IC response, followed by a positive (P50IC) and a negative (N60IC) potential. The N40IC, the P50IC and the N60IC potentials did not differ in latency from the P40, the N50 and the P60 responses recorded by the Cz electrode. In 6 patients, in which SEPs were recorded also during the voluntary movement of the stimulated foot (active gating), an amplitude reduction of the SEP components following the P30 potential was observed during movement at the vertex and in the IC traces. Instead, in the contralateral temporal traces the SEP components (N40temp and P50temp) were not modified by active gating, and in the ipsilateral parietal traces only the positive potentials at about 60ms of latency was decreased. CONCLUSIONS: Two differently oriented generators are active in the contralateral hemisphere at both 40 and 50ms of latency after tibial nerve stimulation. One source is oriented perpendicularly to the mesial hemispheric surface and generates the potentials recorded by the contralateral temporal and the ipsilateral parietal leads; the other dipolar source is radial to the hemispheric convexity, and generates the potentials at the vertex and those recorded by the IC electrodes.     

Brain stem auditory evoked potentials: significant latency differences between ipsilateral and contralateral stimulation

A number of electrical potentials can be recorded from the human scalp following acoustic stimulation. The potentials which occur within 10 msec of the stimulus onset have been termed the brain stem auditory evoked potentials (BAEPs). Latency appears to be the most stable measure and in consequence knowledge of the exact limits of normal latency of each wave is important. In this study the effects of ipsilateral and contralateral stimulation on BAEP latencies have been investigated in 23 normal subjects. The exact limits of normal latency of each wave have been established. It has been shown that significant latency differences exist between ipsilateral and contralateral stimulation. Possible hypotheses are put forward to explain the findings which demonstrate that different neural pathways are followed by ipsilateral and contralateral stimuli and that their respective responses can be investigated separately in man using BAEP recordings.     

Evaluation of ipsilateral and contralateral brainstem auditory evoked potentials in multiple sclerosis patients

Brainstem auditory evoked potentials (BAEP) were recorded simultaneously between the vertex and the mastoid ipsilateral and contralateral to the ear stimulated in 30 patients with multiple sclerosis (MS) and compared with the responses in a control group of 30 normal hearing adults. The control group showed that significant latency differences exist between ipsilateral and contralateral recording. Definitions of abnormalities were based on interwave separation and the wave V amplitude ratio. No case was found among the MS patients with an abnormal contralateral but normal ipsilateral response.     

Gating of the early components of the frontal and parietal somatosensory evoked potentials in different sensory-motor interference modalities.

Three different interfering conditions were studied during the recording of pre- and postcentral somatosensory evoked potentials (SEPs) following median nerve stimulation at the wrist in 16 normal subjects: active finger movement (MVT), light superficial massage (LSM) and deep muscular massage (DMM) of the hand. Special attention was focused on selective effects on individual SEP components. The frontal N30 component showed the most significant amplitude reduction during the three interfering conditions (76.4% of reduction in MVT, 36.4% in DMM and 32.9% in LSM). In contrast the frontal N23 was not significantly changed and the preceding P22 component was only reduced in the MVT condition. Postcentral N20 was unchanged by the three conditions while P27 was clearly gated by movement but not significantly by LSM and DMM. The three interfering conditions enhanced the parietal N32 and had no significant effect on the parietal P45. An important point was the interindividual variability of these effects and it appeared that group average wave forms would therefore be confusing. The peak latency of some SEP components was changed during the interfering conditions. The most important effect was an increase of postcentral P45 latency which was found to be related to the amplitude enhancement of N32.     

Contributions of the mesial frontal cortex to the premovement potentials associated with intermittent hand movements in humans

Two premovement potentials, the bereitschaftspotential (BP) and negative slope (NS'), can be recorded prior to the execution of self-paced hand movements using back-averaging of scalp electrical recordings. The contributions of the contralateral and ipsilateral primary motor cortex (M1) and the mesial dorsal frontal cortex (MFC) to the generation of the potentials were examined by simultaneously collecting positron emission tomography (PET) scans and scalp recorded electrical activity for dipole source analysis in eight right-handed normal subjects. Subjects performed simple unilateral thumb-finger opposition movements intermittently with an average inter-movement interval of 7.4 s. PET was also collected for the same movement performed repetitively with inter-movement intervals of 0.5 s such that finger movements were nearly continuous. PET studies of the intermittent movement revealed marked activation of the MFC in the region of the rostral supplementary motor area (SMA) and cingulate motor area, contralateral sensorimotor cortex and no activation of the ipsilateral sensorimotor cortex. When the same movements were performed in a continuous repetitive manner, PET revealed strong contralateral sensorimotor and caudal MFC activation, and no ipsilateral sensorimotor or rostral MFC activation. Dipole source solutions of the back-averaged potentials for the intermittent movements were analyzed by testing dipole vectors placed into the regions of PET activation. The premovement potentials were dominated by dipoles in the region of the MFC, with minimal contribution from either the contralateral or ipsilateral M1. Activation in the region of the contralateral M1 began near the onset of muscle activity. The orientation and timing of the MFC dipoles were consistent with both the BP and NS' potentials originating from neurons in the rostral SMA and dorsal tier of the cingulate sulcus and were appropriate for MFC activity to contribute to both the preparation for movement and the descending activation of spinal motor networks. © 1996 Wiley-Liss, Inc.     

Temporal dynamics of ipsilateral and contralateral motor activity during voluntary finger movement

The role of motor activity ipsilateral to movement remains a matter of debate, due in part to discrepancies among studies in the localization of this activity, when observed, and uncertainty about its time course. The present study used magnetoencephalography (MEG) to investigate the spatial localization and temporal dynamics of contralateral and ipsilateral motor activity during the preparation of unilateral finger movements. Eight right-handed normal subjects carried out self-paced finger-lifting movements with either their dominant or nondominant hand during MEG recordings. The Multi-Start Spatial Temporal multi-dipole method was used to analyze MEG responses recorded during the movement preparation and early execution stage (-800 msec to +30 msec) of movement. Three sources were localized consistently, including a source in the contralateral primary motor area (M1) and in the supplementary motor area (SMA). A third source ipsilateral to movement was located significantly anterior, inferior, and lateral to M1, in the premotor area (PMA) (Brodmann area [BA] 6). Peak latency of the SMA and the ipsilateral PMA sources significantly preceded the peak latency of the contralateral M1 source by 60 msec and 52 msec, respectively. Peak dipole strengths of both the SMA and ipsilateral PMA sources were significantly weaker than was the contralateral M1 source, but did not differ from each other. Altogether, the results indicated that the ipsilateral motor activity was associated with premotor function, rather than activity in M1. The time courses of activation in SMA and ipsilateral PMA were consistent with their purported roles in planning movements.     

Changes of muscle excitability of the hand contralateral to a task performing one: a transcranial magnetic stimulation study]

It used to be considered that unilateral movements of distal limb parts are associated only with contralateral motor cortical activity. Recent neuroimaging studies, however, suggest that the motor cortex ipsilateral to a task-performing hand is also activated, and that motor patterns in one hand affect the degree of the activity of the ipsilateral motor cortex. If so, muscles of the hand contralateral to a task-performing one may change those excitability depending on types of tasks. We studied eight subjects who performed three different finger tasks by one hand: (a) pinch, (b) sequential finger opposition, and (c) tactile discrimination. Transcranial magnetic stimulation was delivered by a figure eight coil over the hemisphere ipsilateral to a task-performing hand. Motor evoked potentials and background electromyographic activities were recorded from the opponens pollicis muscle contralateral to the stimulated hemisphere. On average, the motor evoked potentials were larger during tactile discrimination task than those at rest in either hand (p < 0.01). Background electromyographic activities in the left hand increased significantly during right hand tactile discrimination task (p < 0.01), whilst those in the right hand did not change during the left hand performance (p > 0.05). These findings suggest the followings: (1) the hand muscle contralateral to a task performing one changes its excitability depending on types of tasks; and (2) increment of excitability of the left hand muscle associated with right hand tactile discrimination is greater than that of the right hand one in association with the same task by the left hand, thus supporting the idea that there is a functional asymmetry between the right and left motor cortex in respect of motor performance.     

Non-invasive evaluation of input-output characteristics of sensorimotor cerebral areas in healthy humans

The topography of scalp SEPs to mixed and sensory median nerve (MN) and to musculocutaneous nerve stimulation was examined in 20 healthy subjects through multichannel (12-36) recording in a 50 msec post-stimulus epoch. MN-SEPs in both frontal leads were characterized by an N18, P20, N24, P28 complex showing maximal amplitude at contralateral parasagittal sites. This was sometimes partly obscured by a wide wave N30 having a fixed latency, but a steep amplitude gradient moving toward the scalp vertex. A P40 component followed, having longer peak latencies, moving the recording sites from contralateral medial parietal toward the vertex and frontal ipsilateral positions. MN-SEPs in contralateral parietal leads contained a widespread N20 with a maximum source posterior to the Cz-ear line. The following P25 enveloped two subcomponents - early and late P25 - having different distributions. The late P25 showed a maximum - coincident with that of wave N20 - which was localized more posteriorly than that of the early P25. An inconstant wave N33 with progressively longer peak latencies from sagittal toward lateral positions was then recorded. MN-SEPs in contralateral central positions showed a well-localized P22 wave in which both the parietal early P25 and the frontal P20 were vanishing. Common or separate generators for frontal, central and parietal SEPs were discriminated by evaluating the influence of stimulus rate and intensity, as well as of general anesthesia and transient CBF deficits, investigated in 7 patients undergoing carotid endarterectomy. Unifocal anodal threshold shocks were separately delivered to each of the scalp electrodes and motor action potentials were recorded from the target muscle in order to delineate the scalp representation of the motor strip for the upper limb and, consequently, to monitor, through SEP tracings, the short-latency sensory input to the motor cortex for hand and shoulder muscles. This was characterized by a boundary zone separating the parietal N20-early P25 complex, from the fronto-central N18-P22 one. This zone had an oblique direction strongly resembling that of the central sulcus.     

Color imaging of parietal and frontal somatosensory potential fields evoked by stimulation of median or posterior tibial nerve in man

Somatosensory evoked potentials (SEPs) to median or fingers or posterior tibial nerve stimulation were recorded with earlobe reference in normal young adults. A system of 16 electrodes on the scalp served to create bit-mapped images of the potential fields at 1 msec intervals. The P14 (median SEP) or P30 (tibial SEP) far fields thought to reflect the afferent volley in the medial lemniscus produced widespread positivity over the scalp. Subsequent components had a characteristic focal distribution suggesting that they reflected one or more generators in cortical areas. For the median SEP, the parietal N20 and the prerolandic P22 showed differences in onset and offset times as well as distribution that precluded their being related to the same generator. While N20 was contralateral, P22 extended ipsilaterally. P22 may be generated in the motor area 4 and the supplementary motor area. P22 was also distinct from the P27 field restricted to the contralateral parietal region. The frontal N30 had a bilateral distribution and the P45 presented variable features. For the tibial SEP, no phase reversal was confirmed between the parietal P38 (midline-ipsilateral focus) and N33 (contralateral focus). N37 over the contralateral prerolandic region might reflect a generator in the motor region. P58 was more symmetrically distributed than P38, possibly because it reflected generators more posteriorly on the parietal convexity. N75 had a widespread field with focus on the ipsilateral side of midline.     

Scalp topography and dipolar source modelling of potentials evoked by CO2 laser stimulation of the hand

CO2 laser evoked potentials to hand stimulation recorded using a scalp 19-channel montage in 11 normal subjects consistently showed early N1/P1 dipolar field distribution peaking at a mean latency of 159 ms. The N1 negativity was distributed in the temporoparietal region contralateral to stimulation and the P1 positivity in the frontal region. The N1/P1 response was followed by 3 distinct components: (1) N2a reaching its maximal amplitude at the vertex and ipsilaterally to the stimulated hand, (2) N2b mostly distributed in the frontal region, and (3) P2 with a mid-central topography. Brain electrical source analysis showed that this sequence was explained, with a residual variance below 5%, by a model including two dipoles in the upper bank of the Sylvian fissure of each hemisphere, a frontal dipole close to the midline, and two anterior medial temporal dipoles, thus suggesting a sequential activation of the two second somatosensory areas, anterior cingulate gyrus and the amygdalar nuclei or the hippocampal formations, respectively. This model fitted well with the scalp field topography of grand average responses to stimulation of left and right hand obtained across all subjects as well as when applied to individual data. Our findings suggest that the second somatosensory area contralateral to the stimulation is the first involved in the building of pain-related responses, followed by ipsilateral second somatosensory area and limbic areas receiving noxious inputs from the periphery.     

Cortical potentials following voluntary and passive finger movements

In order to clarify the time relationship and functional significance of post-motion components of the movement-related cortical potential, averaged cortical potentials associated with voluntary and passive movements were compared mainly with respect to their scalp topography. Fourteen channels of scalp EEG, together with EOG and EMG, were simultaneously recorded in 7 healthy adult subjects while the subject was either repeating a self-paced brisk extension of a middle finger or while the experimenter was extending the middle finger by pulling up a string attached to the finger. Potentials associated with the movement were averaged opisthochronically in relation to a trigger actuated by the finger interrupting a beam of light. Seven peaks were identified in the passive movement-evoked potential. A sharp negative peak occurred over the contralateral precentral region 16 msec after the photometer trigger (N15). Another negative component (N70) formed a composite of double-peaked negativity with N15 and was seen over the frontal region with a contralateral predominance. A positive peak (P65) was recorded over the contralateral parietal region with a similar latency to N70. This N70/P65 complex has some marked similarities in terms of wave form and spatial relationship with the N + 50/P + 90 complex recorded with voluntary movement of the same finger. It is postulated that these components may be the projected potential fields from a dipole source within the central sulcus and may represent a kinesthetic feedback from the muscle afferents.