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.     

Somatosensory evoked potentials after removal of somatosensory cortex in man

Somatosensory evoked potentials (SEPs) to median nerve, ulnar nerve, thumb, middle finger, and posterior tibial nerve stimulation were recorded in a patient with a discrete resection of part of the postcentral somatosensory cortex as a treatment for focal epilepsy. Comparison of the different stimulation sites confirmed electrophysiologically the restricted locus of the lesion. The results strongly suggest that the early negative component (N20) and subsequent components recorded postcentrally are of cortical origin and depend upon postcentral gyrus cytoarchitectonic areas 3, 2, and 1. Moreover, these postcentral SEPs are distinct from precentrally recorded activity.     

Localization of sensorimotor cortex in neurosurgery by recording of somatosensory evoked potentials

The current method of localizing somatosensory and motor cortex during neurosurgical removal of abnormal tissue is Penfield's method of cortical stimulation. While useful, this method has drawbacks, in particular the need to operate under local anesthesia. Another method of localization, described here, involves intra-operative recording of short-latency somatosensory evoked potentials to stimulation of the contralateral median nerve, from electrodes placed directly on central cortex. Proper localization involves identification of potentials which invert in polarity across the central sulcus, identification of other potentials which are largest in the medial portion of the hand area of somatosensory cortex and do not polarity invert, and determination of the region of maximal potential amplitude. This method of localization works equally well whether the patient is under local or general anesthesia, but it occasionally fails in patients with tumors abutting or invading the hand area of sensorimotor cortex.     

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.     

The frequency dependence of evoked and postsynaptic potentials in the somatosensory cortex of the cat.

Intracellular records were made from neurones of the somatosensory cortex of cats, during stimulation of the ventrobasal complex of the thalamus. The aim of the experiments was to detect correlations between frequency dependence of surface evoked potentials and that of unit discharges. The amplitude of the surface evoked potential showed a strong diminution when the frequency of the thalamic stimulation was raised from 1 cps to 15 cps. In spite of this, frequency dependence in amplitude of unit discharges was never seen. As regards their frequency of occurrence the unit responses (full spikes, dendritic, postsynaptic potentials) behaved differently: a part of them showed increasing, another part gave decreasing occurrence, and the remaining portion did not change it. The authors conclude that temporal dispersion fails to give account for the frequency dependence, therefore further possibilities have to be examined.     

Steady-state analysis of somatosensory evoked potentials

We report the development of a new method for frequency domain analysis of steady-state somatosensory evoked potentials (SEPs) to amplitude-modulated electrical stimulation, which can be recorded in significantly less time than traditional SEPs. Resampling techniques were used to compare the steady-state SEP to traditional SEP recordings, which are based on signal averaging in the time domain of cortical responses to repetitive transient stimulation and take 1–2 min or more to obtain a satisfactory signal/noise ratio. Median nerves of 3 subjects were stimulated continuously with electrical alternating current at several modulation frequencies from 7 to 41 Hz. Amplitude modulation was used to concentrate the power in higher frequencies, away from the modulation frequency, to reduce the amount of stimulus artifact recorded. Data were tested for signal detectability in the frequency domain using the T_circ² statistic. A reliable steady-state response can be recorded from scalp electrodes overlying somatosensory cortex in only a few seconds. In contrast, no signal was statistically discriminable from noise in the transient SEP from as much as 20 s of data. This dramatic time savings accompanying steady-state somatosensory stimulation may prove useful for monitoring in the operating room or intensive care unit.     

Functional organization of the kinesthetic projections of the 1st somatosensory region of the cat cortex

Electrophysiological studies were performed on adult cats under ethaminal anesthesia. Kinesthetic potentials were evoked by passive extension of the ulnar joint and recorded in contralateral primary somatosensory cortical area. Natural (nonelectrical) stimulation of peripheral kinesthetic receptors was performed according to the author's original method. The results obtained show significantly shorter latent period of contralateral kinesthetic potentials in comparison with somatosensory potentials in response to electrical stimulation of the skin. These data demonstrate the possibility of super-rapid conduction of modal-specific volleys to the cortical projection centres in the kinesthetic system of cats.     

Depression of evoked potentials in rat thalamic ventro-basal complex and somatosensory cortex after reticular stimulation

Experiments on unanesthetized rats immobilized with D-tubocurarine showed that electrical stimulation (100/sec) of the central gray matter and the mesencephalic and medullary reticular formation considerably depressed potentials in the somatic thalamic relay nucleus and somatosensory cortex evoked by stimulation of the forelimb or medial lemniscus. The mean threshold values of the current used for electrical stimulation of these structures did not differ significantly and were 70 (20–100), 100 (20–120), and 120 (50–200) µA, respectively. On comparison of the amplitude-temporal characteristics of inhibition of evoked potentials during electrical stimulation of the above-mentioned structures by a current of twice the threshold strength, no significant differences were found. Immediately after the end of electrical stimulation the amplitude of the cortical evolved potential and the post-synaptic components of the thalamic evoked potential was 50–60% (P<0.01) below the control values. The duration of this depression varied from 0.5 to 1 sec. An increase in the intensity of electrical stimulation of brain-stem structures to between three and five times the threshold led to depression of the presynaptic component of the thalamic evoked potential also. Depression of the evoked potential as described above was found with various ratios between the intensities of conditioning and testing stimuli.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 8, No. 5, pp. 467–475, September–October, 1976.     

Short-latency somatosensory evoked potentials in brain-dead patients

Ten adult brain-dead patients were evaluated for the presence of clearly defined median nerve short-latency somatosensory evoked potentials (SSEPs). All met clinical criteria recommended by the President's Commission report (1981), had positive apnea tests, and had electrocerebral silent EEGs. P13-P14 and N20 were absent in all scalp-scalp channels, although 3 patients showed P13-P14 in scalp-non-cephalic channels. Of 6 patients showing N13, 3 lacked P13-P14. Our data suggest a characteristic destruction of N20 and rostral P13-P14 generators, with variable rostral-caudal loss of lower generators, SSEPs can provide valuable information about brain-stem activity in the evaluation of suspected brain-dead patients.     

Temperature-dependent hysteresis in somatosensory and auditory evoked potentials

Fourteen adult patients undergoing open heart surgery under induced hypothermia had median nerve, short-latency somatosensory evoked potentials (SSEPs) recorded during cooling (from 36°C to 19°C) and subsequent rewarming. Similar data on another group of patients who had brain-stem auditory evoked potentials (BAEPs) were also analyzed. Hypothermia produced increased latencies of the major SSEP and BAEP components and the latencies returned to normal with subsequent warming. The temperature-latency relationship during the cooling phase was significantly different from that during the warming phase. For SSEP components the temperature-latency relationship was linear during cooling and curvilinear during warming, whereas for BAEP it was curvilinear both during cooling and warming. Furthermore, the regression curves were different during the two phases of temperature manipulation, particularly for temperatures below 30°C both for SSEP and BAEP components. At the onset of warming there was an initial exaggerated warming response on the evoked potential (EP) latencies and amplitude of the EP components. The temperature-latency regression curves were uniformly less steep during the warming phase compared to those during cooling. These findings suggest the existence of hysteresis in the relationship between temperature and EP latencies. The latencies at a given temperature below 30°C depend on whether that temperature is reached during cooling or during warming.     

痛觉诱发电位的研究进展

痛觉诱发电位的研究在过去的几十年内取得了重要进展 ,出现了许多用于被试的诱发明确疼痛感的刺激技术 ,并与诱发电位方法学联合应用 ,已经成为脑映像学研究中重要的组成部分。本文从刺激技术、痛觉诱发电位成分分析和偶极子源分析等方面出发 ,讨论了痛觉诱发电位的研究进展     

The time window for generation of dendritic spikes by coincidence of action potentials and EPSPs is layer specific in somatosensory cortex

The precise timing of events in the brain has consequences for intracellular processes, synaptic plasticity, integration and network behaviour. Pyramidal neurons, the most widespread excitatory neuron of the neocortex have multiple spike initiation zones, which interact via dendritic and somatic spikes actively propagating in all directions within the dendritic tree. For these neurons, therefore, both the location and timing of synaptic inputs are critical. The time window for which the backpropagating action potential can influence dendritic spike generation has been extensively studied in layer 5 neocortical pyramidal neurons of rat somatosensory cortex. Here, we re-examine this coincidence detection window for pyramidal cell types across the rat somatosensory cortex in layers 2/3, 5 and 6. We find that the time-window for optimal interaction is widest and shifted in layer 5 pyramidal neurons relative to cells in layers 6 and 2/3. Inputs arriving at the same time and locations will therefore differentially affect spike-timing dependent processes in the different classes of pyramidal neurons.     

Somatotopy of human hand somatosensory cortex revealed by dipole source analysis of early somatosensory evoked potentials and 3D-NMR tomography

Somatosensory evoked potentials (SEPs) to median nerve and finger stimulation were analyzed by means of spatio-temporal dipole modelling combined with 3D-NMR tomography in 8 normal subjects. The early SEPs were modelled by 3 equivalent dipoles located in the region of the brain-stem (B) and in the region of the contralateral somatosensory cortex (T and R). Dipole B explained peaks P14 and N18 at the scalp. Dipole T was tangentially oriented and explained the N20-P20, dipole R was radially oriented and modelled the P22. The tangential dipole sources T were located within a distance of 6 mm on the average and all were less than 9 mm from the posterior bank of the central sulcus. In 6 subjects the tangential sources related to finger stimulation arranged along the central sulcus according to the known somatotopy. The radial sources did not show a consistent somatotopic alignment across subjects. We conclude that the combination of dipole source analysis and 3D-NMR tomography is a useful tool for functional localization within the human hand somatosensory cortex.     

Effect of local cooling of the cortex on evoked potentials in the reticular formation in response to somatosensory stimulation

Potentials evoked in nuclei of the reticular formation by electrodermal stimulation of the limbs were investigated in acute experiments on unanesthetized, immobilized rats during cooling of the somatosensory cortex in the area of representation of one forelimb. Evoked potentials in the reticular formation were found to depend on the degree of cold inhibition of the cortical primary response to the same stimulation. The peak time of the main negative wave increased from 40–50 to 60–80 msec with a simultaneous decrease in its amplitude or its total disappearance in the case of deep cooling of the cortex. Cooling of the cortex had a similar although weaker effect on the earlier wave of the evoked potential with a peak time of 14 msec, recorded in the ventral reticular nucleus. In parallel recordings of potentials evoked by stimulation of other limbs they remained unchanged at these same points of the reticular formation or were reduced in amplitude while preserving the same temporal parameters. Cooling of the cortex thus selectively delays the development and reduces the amplitude of the response to stimulation of the limb in whose area of representation transformation of the afferent signal into a corticofugal volley is blocked. Consequently the normal development of both late and early components of the potential evoked in the reticular formation by somatic stimulation requires an additional volley, descending from the cortex, and formed as a result of transformation of the same afferent signal in the corresponding point of the somatosensory cortex.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 13, No. 1, pp. 32–38, January–February, 1981.     

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.