Ipsilateral median somatosensory evoked potentials recorded from human somatosensory cortex

Somatosensory evoked potentials (SEP) to ipsilateral and contralateral median nerve stimulations were recorded from subdural electrode grids over the perirolandic areas in 41 patients with medically refractory focal epilepsies who underwent evaluation for epilepsy surgery. All patients showed clearly defined, high-amplitude contralateral median SEPs. In addition, four patients showed ipsilateral SEPs. Compared with the contralateral SEPs, ipsilateral SEPs were very localized, had a different spatial distribution, were of considerably lower amplitude, had a longer latency (1.2–17.8 ms), did not show an initial negativity, and were markedly attenuated during sleep. Stimulation of the subdural electrodes overlying the sensory hand area was associated with contralateral hand paresthesias, but no ipsilateral hand paresthesias occurred. It was concluded that subdurally recorded cortical SEPs to ipsilateral stimulation of the median nerve (M) reflect unconscious sensory input from the hand possibly serving fast bimanual hand control. The anatomical pathway of these ipsilateral short-latency MSEPs is not yet known. Transcallosal transmission seems unlikely because of the short delay between the ipsilateral and contralateral responses in selected cases. The infrequent occurrence of ipsilateral subdurally recorded SEPs and their low amplitude and limited distribution suggest that they contribute very little to the short-latency ipsilateral median SEPs recorded on the scalp.     

Origin and distribution of brain-stem somatosensory evoked potentials in humans

The distribution of somatosensory evoked potentials (SEPs) recorded from the brain-stem surface was studied to investigate their generator sources in 14 patients during surgical exploration of the posterior fossa. Two distinct SEPs of different morphologies and electrical orientation were obtained by median nerve stimulation. A small positive-large negative-late prolonged positive wave was recorded from the cuneate nucleus and its vicinity. There was a phase-reversal between the cuneate nucleus and the ventral surface of the medulla, depicting a dipole for dorso-ventral organization. From the pons and midbrain, triphasic waves with predominant negativity were obtained. This type of SEP had identical wave forms between dorsal, lateral and ventral surface of the pons and midbrain. It showed an increase in negative peak latency as the recording sites moved rostrally, suggesting an ascending axial orientation. In a patient with pontine hemorrhage, the killed end potential, a large monophasic positive potential was obtained from the lesion. This potential occurs when an impulse approaches but never passes beyond the recording electrode. Therefore, the triphasic SEP from the pons and midbrain reflects an axonal potential generated in the medial lemniscal pathway.     

Short latency somatosensory evoked potentials recorded around the human upper brain-stem

We analyzed the intracranial spatiotermporal distributions of the N18 component of short median nerve somatosensosry evoked potentials (SSEPs) in 3 patients with epilepsy. In these patients, depth electrodes were implanted bilaterally into the frontal and temporal lobes, with targets including the amygdala and hippocampus; the latter two targets are close to the upper pons and midbrain.In this study N18 was divided into the initial negative peak (N18a) and the following prolonged negativity (N18b). Mapping around the upper pons and midbrain showed that: (1) the amplitude of the first negativity, which coincided with scalp N18a, was larger contralateral to the side of stimulation, but showed no polarity change around the upper brain-stem; and (2) the second negativity, which was similar to scalp N18b, did show an amplitude difference or a polarity change. This wave appeared to reflect a positive-negative dipole directed in a dorso-ventral as well as dorso-lateral direction from the midbrain, where positivity arises from the dorsum of the midbrain, contralateral to the side of the stimulation.Recordings from depth electrode derivations oriented in a caudo-rostral direction suggest that N18a and N18b may in part reflect neural activity originating from the upper pons to midbrain region which projects to the rostral subcortical white matter of the frontal lobe as stationary peaks.     

Long sensory tracts (cuneate fascicle) in cervical somatosensory evoked potential after median nerve stimulation

Low amplitude high frequency waves (LHW) were investigated in normal and patient cervical somatosensory evoked potentials after median nerve stimulation (CSEP) in parallel to normal and patient conducted somatosensory evoked potentials (SEP) after tibial nerve stimulation. Normal recordings were obtained in five subjects undergoing dorsal root entry zone (DREZ) coagulation for pain relief. Patient recordings were obtained in 11 subjects suffering from either syringomyelia, spinal cord tumour, or both. All recordings were made intraoperatively from the dorsal spinal cord surface using the subpial recording technique. Normal CSEP showed typical triphasic potential starting with an initial P9, followed by N13 and a final positivity, P1. Numerous LHW were superimposed on slow triphasic potential. To improve the visibility of LHW, slow triphasic potential was removed from the original CSEP. Potentials thus obtained contained only high frequency components of CSEP, i.e. LHW. They were compared with conducted SEP after tibial nerve stimulation. Comparison revealed similarities in high frequency, low amplitude and general wave form, LHW thus showing characteristics of conducted potential. Duration was found to be significantly shorter than normal duration in both patient LHW (Student's t-test, P<0.0005) and patient conducted SEP (Student's t-test, P=0.064). A shorter duration was associated with worsening of configuration in patient LHW and patient conducted SEP. These changes of LHW could not be connected with distortion of N13 seen in patient CSEP. A shorter duration and worsening of configuration in patient LHW were most prominent in cases with a loss of vibration and posture senses, but were also observed in cases where only pain and temperature senses were affected. We therefore concluded that cuneate fascicle is the most likely generator of LHW, although the participation of other cervical long sensory tracts, e.g. spinothalamic tract, cannot be ruled out.     

New techniques in female pudendal somatosensory evoked potential testing

Objective—The primary nerves innervating the female genitalia are the dorsal nerve of the clitoris (DNC) and the perineal nerve, which innervate the clitoris and the external genitalia/distal vagina, respectively. We describe two novel electrodiagnostic techniques for evaluating the integrity of these female genital somatosensory pathways.

Subjects and methods—Seventy-seven healthy women (mean age 29.3 years) were enrolled for this study. We performed DNC somatosensory evoked potentials (SEPs), stimulating through self-adhesive disk electrodes on either side of the clitoris. Perineal nerve SEPs were evoked through a vaginal probe. Cortical responses were measured through cup electrodes affixed to the scalp at Cpz and Fpz. Stimulus parameters were duration 0.1?ms, frequency 4.1?Hz, filters 5–5,000?Hz, at three times sensory threshold.

Results—DNC and perineal nerve SEPs from both the right and left sides were reproducible and easily discerned. The mean P1 latencies were: right DNC 39.4?ms (SD 2.8?ms), left DNC 39.3?ms (SD 3.3?ms), right perineal nerve 37.8?ms (SD 3.4?ms), left perineal nerve 37.6?ms (SD 3.1?ms). We recorded SEP responses from 90 to 92% of subjects for the DNC, and 69% for the perineal nerve.

Conclusions—We are able to evoke somatosensory potentials from the four primary somatic nerves that mediate female genital cutaneous sensation. In healthy subjects, the DNC responses are robust and maintain laterality. The perineal nerve responses are less consistently obtained, but when recorded, are easily discerned. These preliminary data provide a foundation from which to study female genital innervation, particularly as it applies to sexual function.     

New techniques in female pudendal somatosensory evoked potential testing

OBJECTIVE: The primary nerves innervating the female genitalia are the dorsal nerve of the clitoris (DNC) and the perineal nerve, which innervate the clitoris and the external genitalia/distal vagina, respectively. We describe two novel electrodiagnostic techniques for evaluating the integrity of these female genital somatosensory pathways. SUBJECTS AND METHODS: Seventy-seven healthy women (mean age 29.3 years) were enrolled for this study. We performed DNC somatosensory evoked potentials (SEPs), stimulating through self-adhesive disk electrodes on either side of the clitoris. Perineal nerve SEPs were evoked through a vaginal probe. Cortical responses were measured through cup electrodes affixed to the scalp at Cpz and Fpz. Stimulus parameters were duration 0.1 ms, frequency 4.1 Hz, filters 5-5,000 Hz, at three times sensory threshold. RESULTS: DNC and perineal nerve SEPs from both the right and left sides were reproducible and easily discerned. The mean P1 latencies were: right DNC 39.4 ms (SD 2.8 ms), left DNC 39.3 ms (SD 3.3 ms), right perineal nerve 37.8 ms (SD 3.4 ms), left perineal nerve 37.6 ms (SD 3.1 ms). We recorded SEP responses from 90 to 92% of subjects for the DNC, and 69% for the perineal nerve. CONCLUSIONS: We are able to evoke somatosensory potentials from the four primary somatic nerves that mediate female genital cutaneous sensation. In healthy subjects, the DNC responses are robust and maintain laterality. The perineal nerve responses are less consistently obtained, but when recorded, are easily discerned. These preliminary data provide a foundation from which to study female genital innervation, particularly as it applies to sexual function.     

Effects of hypothermia on short latency somatosensory evoked potentials in humans

Short latency somatosensory evoked potentials (SSEPs) elicited by median nerve stimulation were monitored in 14 adult patients undergoing cardiac surgery under cardiopulmonary bypass and induced hypothermia. SSEPs were recorded at 1–2°C steps as the body temperature was lowered from 37°C to 20°C to determine temperature-dependent changes. Hypothermia produced increased latencies of the peaks of N₁₀, P₁₄ and N₁₉ components, the prolongation was more severe for the later components so that N₁₀−P₁₄ and P₁₄−N₁₉ interpeak latencies were also prolonged. The temperature-latency relationship had a linear correlation. The magnitude of latency prolongation (msec) with 1°C decline in temperature was 0.61, 1.15, 1.56 for N₁₀,P₄ and N₁₉ components, respectively, and 0.39 and 0.68 for interpeak latencies N₁₀−P₁₄ and P₁₄−N₁₉, respectively. The rise time and duration of the 3 SSEP components increased progressively with cooling. Cortically generated component, N₁₉ was consistently recordable at a temperature above 26°C, usually disappearing between 20°C and 25°C. On the other hand, more peripherally generated components, N₁₀ and P₁₄, were more resistant to the effect of hypothermia; P₁₄ was always elicitable at 21°C or above, whereas N₁₀ persisted even below 20°C. The amplitude of SSEP components had a poor correlation with temperature; there was a slight tendency for N₁₀ and P₁₄ to increase and for N₁₉ to decrease with declining temperature. Because incidental hypothermia is common in comatose and anesthetized patients, temperature-related changes must be taken into consideration during SSEP monitoring under these circumstances.     

Clinical value of the pudendal somatosensory evoked potential

The pudendal evoked potential was recorded in 126 patients who had presented with various uro-genital complaints. The patients were divided into two groups depending on whether or not there were clinical signs of neurological disease. Group I consisted of 83 patients (66%) who on clinical examination were neurologically normal. In group 11 there were 43 patients (34%) who had physical signs suggesting underlying neurological pathology. The pudendal evoked potential was abnormal in 10 patients from the group with neurological signs (group 11) but only 1 patient from group I (a man who had made an excellent recovery from previous transverse myelitis). The conclusion of this study is that the pudendal evoked potential is of no greater value than the clinical examination in the assessment of patients with uro-genital dysfunction. The recommendation that the potential should be recorded as part of the routine assessment of patients suspected of having a neurogenic disorder of the bladder and sexual function should be reconsidered.     

Somatosensory evoked potentials from the thalamic sensory relay nucleus (VPL) in humans: correlations with short latency somatosensory evoked potentials recorded at the scalp

Somatosensory evoked potentials (SEPs) were recorded in humans from an electrode array which was implanted so that at least two electrodes were placed within the nucleus ventralis posterolateralis (VPL) of the thalamus and/or the medial lemniscus (ML) of the midbrain for therapeutic purposes. Several brief positive deflections (e.g., P11, P13, P14, P15, P16) followed by a slow negative component were recorded from the VPL. The sources of these components were differentiated on the basis of their latency, spatial gradient, and correlation with the sensory experience induced by the stimulation of each recording site. The results indicated that SEPs recorded from the VPL included activity volume-conducted from below the ML (P11), activity in ML fibers running through and terminating within the VPL (P13 and P14), activity in thalamocortical radiations originating in and running througn the VPL (P15, P16 and following positive components) and postsynaptic local activity (the negative component). The sources of the scalp-recorded SEPs were also analyzed on the basis of the timing and spatial gradients of these components. The results suggested that the scalp P11 was a potential volume-conducted from below the ML, the scalp P13 and P14 were potentials reflecting the activity of ML fibers, the small notches on the ascending slope on N16 may potentially reflect the activity of thalamocortical radiations, and N16 may reflect the sum of local postsynaptic activity occurring in broad areas of the brain-stem and thalamus.     

Parkinson's disease and aging: Changes of somatosensory evoked potentials in humans

Somatosensory evoked potentials (SSEP) elicited by electrical stimulation of the median nerve were compared in patients with Parkinson's disease and individuals without clinical manifestations of extrapyramidal insufficiency (46 and 55 persons, respectively). The amplitude of the N31 component was found to diminish in Parkinsonian patients while the latency of the P44 component increased significantly. In addition, these parameters depended on the age of the tested subjects; the direction of age-related changes of the N31 and P44 components coincided with those typical of parkinsonism. Our findings seem to suggest that changes in the somatic afferentation caused by Parkinson's disease and aging are of the same type and depend on disturbances in the nigrostriatal dopaminergic system.Neirofiziologiya/Neurophysiology, Vol. 26, No. 3, pp. 141–145, March–April, 1994.     

Changes in late components of somatosensory evoked potentials in humans during repetitive stimulation

Human evoked potentials to somatosensory stimuli of non-painful and painful intensity recorded from the vertex have been studied. The indices of variability of N150 and P250 components registered in the same subject as well as indices of interrelationship between spontaneous changes of these components decreased when stimulus intensity increased. A supposition is advanced that the role of general source responsible for generations N150 and P250 components diminished when stimulus intensity increased, accordingly participation of autonomic sources became more prominent.     

The development of the somatosensory evoked potential in the unanaesthetized fetal sheep

Somatosensory evoked potentials were recorded in utero from 13 chronically instrumented fetal lambs (97 to 148 days of gestation) following electrical stimulation of the upper lip or upper limb. Several clear and reproducible peaks were observed. Following upper lip stimulation, peaks were seen with mean peak latencies of 9, 13.2, 17.8, 21.3, 33.8 and 206 ms at a gestational age of 125 days. Similar peaks, but of slightly later mean latencies, were seen following limb stimulation. These peaks demonstrated significant gestational age related falls in peak latencies (P less than 0.05). Several of the mid to late latency peaks, notably those occurring at 21.3, 33.8 and 206 ms, demonstrated changes (P less than 0.05) in both latency (longer in low voltage) and amplitude (reduced in low voltage) dependent on electrocorticographic state. Rate of stimulus presentation also had a significant effect on both amplitude and latency of several peaks (P less than 0.05) with this effect lessening with advancing gestational age. Evoked potentials can thus be successfully obtained from chronically instrumented fetal lambs and provide a useful indice for studies of neural maturation.     

The value of ulnar somatosensory evoked potentials (SEPs) in cervical myelopathy

In 57 patients with clinical signs and surgical documentation of compressive myelopathy, ulnar nerve somatosensory evoked potentials (SEPs) were more sensitive (with 74% abnormal) than either median or tibial nerve SEPs. The most frequent abnormalities were reduced or absent neck evoked responses and prolonged central conduction time. All subjects who had an SEP abnormality were identified by combined tibial and ulnar SEPs. Median nerve SEP added no additional information. Normal ulnar and tibial nerve SEPs were also able to exclude major cord damage in patients with cervical radiculopathy but little evidence of myelopathy.     

Origin of frontal N15 component of somatosensory evoked potential in man

Origin of the frontal somatosensory evoked potential (SEP) by median nerve stimulation was investigated in normal volunteers and in patients with localized cerebrovascular diseases, and the following results were obtained.

• 1.(1) In normal subjects, SEPs recorded at F3 (or F4) contralateral to the stimulating median nerve were composed of P12, N15, P18.5 and N26. Similar components were recognized in SEP recorded at Fz.
• 2.(2) In patients in whom putaminal or thalamic hemorrhages had destroyed the posterior limbs of the internal capsules, frontal N15 and parietal N18 (N20) disappeared. These components were also absent in patients with cortical (parietal) infarctions. Among these patients, the thalamus was not affected in cases with putaminal hemorrhages and cortical infarctions.

These facts indicate that the generator of the frontal N15 does not exist in the thalamus but that it originates from the neural structure central to the internal capsule, which suggests a similarity to the generator of the parietal N18.Because N15 was recorded in the midline of the frontal region with shorter latency than parietal N18, the frontal N15 might represent a response to the sensory input of the frontal lobe via the non-specific sensory system.     

Median and tibial nerve somatosensory evoked potentials: middle-latency components from the vicinity of the secondary somatosensory cortex in humans

The topography of the middle-latency N110 after radial nerve stimulation suggested a generator in SII. To support this hypothesis, we have tried to identify a homologous component in the tibial nerve SEP (somatosensory evoked potential). Evoked potentials following tibial nerve stimulation (motor+sensory threshold) were recorded with 29 electrodes (bandpass 0.5–500 Hz, sampling rate 1000 Hz). For comparison, the median nerve was stimulated at the wrist. Components were identified as peaks in the global field power (GFP). Map series were generated around GFP peaks and amplitudes were measured from electrodes near map maxima. With median nerve stimulation, we recorded a negativity with a maximum in temporal electrode positions and 106±12 ms peak latency (mean±SD), comparable to the N110 following radial nerve stimulation. After tibial nerve stimulation the latency of a component with the same topography was 131±11 ms (N130). Both N110 and N130 were present ipsi- as well as contralaterally. Amplitudes were significantly higher on the contralateral than the ipsilateral scalp for both median (3.1±2.4 μV vs. 1.7±1.6 μV) and tibial nerve (1.9±1.2 μV vs. 0.6+1 μV). The topography of the N130 can be explained by a generator in the vicinity of SII. The latency difference between median and tibial nerve stimulation is related to the longer conduction distance (cf. N20 and P40). The smaller ipsilateral N130 is consistent with the bilateral body representation in SII.     

Detection of neurological injury using time-frequency analysis of the somatosensory evoked potential

Somatosensory evoked potentials (SEPs) can be monitored during critical surgery to help detect or possibly prevent post-operative injury to the brain. This paper presents the application of time-frequency analysis to detect both temporal and spectral changes in the SEP waveform that may occur due to injury. Time-frequency distributions, which provide a measure of signal energy at both a specific time and frequency, were computed for averaged SEPs acquired from anesthetized cats during various stages of hypoxic injury and then recovery. Wigner distributions of SEP waveforms were found to contain a peak of signal energy at a specific time and frequency, a peak that is altered during injury. Four characteristics of the distribution peak that demonstrate changes due to injury were computed: peak time, peak frequency, peak power, and peak sharpness. Peak time was found to increase while peak frequency, peak power, and peak sharpness were found to decrease during injury. Furthermore, the total signal power in a time-frequency space around the normal peak location was monitored by developing a time-frequency window filter (TFWF) method. For all cases, onset of hypoxia was detected an average of 2.75 min earlier by the TFWF method than by the conventional amplitude measurement method. Time-frequency analysis of EP signals may therefore be useful as a monitoring tool for early detection of brain injury.