Laminar analysis of hippocampal potentials evoked by peripheral stimulation

From areas SA₁ and SA₂ of the dorsal hippocampus of unanesthetized rabbits immobilized with d-tubocurarine a laminar analysis was made of evoked potentials (EP) in response to stimulation of the sciatic nerves. Inversion of the initial surface-positive phase of the EP was observed at the level of the pyramidal layer. The subsequent surface-negative phase reached a maximum value in the layer of basal dendrites of the pyramidal cells. The initial portion was inverted somewhat above the pyramidal layer and reached its maximum value approximately at the boundary of the pyramidal and radial layers. The change in sign of the remaining portion of this component occurred 0.3–0.4 mm deeper than the pyramidal layer. It is suggested that both components of the EP picked up from the hippocampal surface are due to an excitatory postsynaptic potential (EPSP) at the apical (positive phase), and basal (negative phase) dendrites. The positivity in the region of the pyramidal somata appears to be an extracellular reflection of a composite postsynaptic potential (IPSP) generated in this region.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 4. pp. 434–438, July–August, 1970.     

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.     

Sensory and cognitive evoked potentials in a case of congenital hydrocephalus

We studied auditory and visual evoked potentials in D.W., a patient with congenital stenosis of the cerebral aqueduct. Head CT scans revealed marked hydrocephalus with expanded ventricles filling more than 80% of the cranium and compressing brain tissue to less than 1 cm in thickness. Despite the striking neuroanatomical abnormalities, however, the patient functioned well in daily life and was attending a local community college at the time of testing.Evoked potentials provided evidence of preserved sensory processing at cortical levels. Pattern reversal visual evoked potentials had normal latencies and amplitudes. Brain-stem auditory evoked potentials (BAEPs) showed normal wave V latencies. Na and Pa components of middle-latency AEP had normal amplitudes and latencies at the vertex, although amplitudes at lateral electrodes were larger than at the midline.In contrast to the normal sensory responses, long-latency auditory evoked potentials to standard and target tones showed abnormal P3 components. Standard tones (probability 85%), evoked NN1 components with normal amplitudes (−3.7 μV) and latencies (103 msec), but also elicited large P3 components (17 μV, latency 305 msec) that were never observed following frequent stimuli in control subjects. Target stimuli (probability 15%) elicited P3s in D.W. and controls, but P3 amplitudes were enhanced in D.W. (to more than 40 μV) and the P3 showed an unusual, frontal distribution. The results are consistent with a subcortical sources of the P300. Moreover, they suggest that the substitution of controlled for automatic processes may help high-functioning hydrocephalics compensate for abnormalities in cerebral structure.     

A collector of evoked potentials]

The collector is an adaptive algorithm for pattern recognition. It proposes new in-line fully-automatic technique to learn and recognize effective patterns of input data stream. Evoked potentials (EP) were recorded by ADDA 100 KHz, 4 channels, and described by 200 points per each EP. The collector recognized different studies of conditioned response (CR) by patterns of EPs in amygdalar central nucleus. In dogs with implanted into the limbic structures concentric electrodes an instrumental CR was elaborated to electrical stimulation of the dorsal hippocampus. Generalization or transfer of this CR was tested by means of electrostimulation of amygdalar basal nucleus. The generalization in the first experiment took place approximately in 86% of cases, in the second one in 52% of cases. In the first experiment the amplitudes of initial negativity and of late positive waves were smaller than those in the second one and in the experiments before conditioning.     

Time-frequency component analysis of somatosensory evoked potentials in rats

Background  

Somatosensory evoked potential (SEP) signal usually contains a set of detailed temporal components measured and identified in a time domain, giving meaningful information on physiological mechanisms of the nervous system. The purpose of this study is to measure and identify detailed time-frequency components in normal SEP using time-frequency analysis (TFA) methods and to obtain their distribution pattern in the time-frequency domain.     

Motor potentials evoked by paired cortical stimuli

We recorded the motor evoked potentials (MEPs) from the abductor pollicis brevis muscle, after supramaximal electrical transcranial stimulation, and studied the effect of paired transcranial shocks with varying interstimulus time intervals, in 10 normal subjects, 4 patients with median nerve neuropathy and 2 patients with motoneurone disease.In relaxed muscles the amplitude of the MEP evoked by a single shock averaged 30% of the M wave. With intervals from 1 to 2.5 msec 2 shocks evoked one MEP far larger in size than the control MEP (70% of the M wave). With intervals of 10 msec and longer, the 2 shocks evoked 2 independent MEPs; the size of the MEP following the second shock (test) was inversely correlated with the size of the control MEP: the more the control MEP approached the size of the M wave, the smaller the test MEP. Single motor unit records showed that, in the normal subjects and patients with peripheral neuropathy, the same motor unit was activated either by the first or the second shock, whereas in the patients with motoneurone disease it fired twice. In active muscles, the control MEP averaged 70% of the M wave. With intervals of 10 msec and longer the test MEP was markedly suppressed; with 100 msec intervals it fully recovered. In relaxed muscles, by delivering a double shock at a 1.5 msec interval, thus evoking a large MEP, followed by a second double-shock, the test MEP was completely suppressed for a period of 20 msec; it began to recover at 50 msec intervals and fully recovered after 150 msec.These results indicate that: (1) high-threshold spinal motoneurones can profit from temporal summation if double-shocks are delivered at short time intervals; (2) the synchronous excitation of the motoneuronal pool produced by transcranial stimulation is followed by a 20 msec period of absolute inhibition, possibly through a massive activation of the Renshaw system; (3) during voluntary contraction, only a portion of the motoneuronal pool remains refractory, possibly because of the enhanced spinal excitability.     

A posteriori time-varying filtering of averaged evoked potentials

The problem of estimating an unknown transient signal, given an ensemble of waveforms, in which this signal appears as a nonrandom component in the presence of additive noise is considered. This problem is solved by generalizing the method of a posteriori Wiener filtering. In the new method, the ensemble average is filtered by a time-varying system which is based on estimated time-varying power spectra of signal and noise. The nature of this system, and the computational procedures involved, are discussed in detail. A software package for time-varying filtering is briefly described. Application of the method is illustrated by a simulation example, which also provides a comparison to time-invariant a posteriori Wiener filtering.     

The dependence of late evoked cortical potentials on a complex of cognitive factors]

The formation of late evoked cortical responses, N150 and P300, correlates with some factors determining cognitive activity (selective attention, different types of memory, decision making process). Influence of different significance stimuli in a particular situation results in creation of an integral "psychonervous image" reflecting a context of these stimuli, whole complex of cognitive tasks to be solved. It influences significantly on the magnitude of cortical evoked reactions to different stimuli that allows to make a correspondence with the "psychological set" phenomenon and its role in the evaluation of the perceived stimuli.     

Cerebral potentials evoked by rectal distention in humans

The afferent pathways from the rectum can be stimulated and studied after mechanical distention of the rectum. We studied the rectum-brain axis in 24 healthy young adults. The rectum was stimulated with a rectal balloon using inflation volumes of 10 ml and 30 ml air at a stimulation frequency of 0.167 Hz. Additional studies were carried out with 20 ml distention volumes, random stimulation frequency, and stimulation rates of 0.08 Hz and 0.017 Hz. We found two different cortical EPs. An early onset EP was present in 21 of the 24 subjects. P1 latency shortened significantly with increasing distention volumes, but peak amplitudes did not change significantly with larger distention volumes. A late onset EP was present in all subjects with NI latency of 210 ± 15 msec, PI latency of 316 ± 24 msec, and NII latency of 444 ± 34 msec. The different EPs could be due to stimulation of two different visceral afferent pathways which are present in the same individual, due to stimulation of two different fiber populations or due to simultaneous stimulation of afferents in surrounding structures. EP recording after rectal stimulation might be useful in future studies of patients with abnormal rectal sensation.     

Somatosensory evoked potentials by tibial nerve stimulation in the rabbit

Somatosensory evoked potentials by tibial nerve stimulation were obtained in ten New Zealand rabbits. The subcortical or cortical source of the three negative and three positive peaks present in the first 55 ms is discussed viewing the results obtained by different surface electrode locations and by stereotaxic recordings. The authors report interanimal, interhemispheric and test-retest variability of latencies and amplitudes of subcortical and cortical components.     

A psychophysiological analysis of the efficiency of instructing schoolchildren]

Examination of 163 schoolboys in higher forms has revealed that strength of the nervous system and functional state of the CNS (functional level of the system, level of functional possibilities, arousal and reaction stability) do not differ in schoolchildren with various progress in learning. The school teaching efficiency correlated with parameters of strength of the nervous system only in excellent and good pupils, the functional state of the nervous system being of importance for their teaching, especially its such parameters as arousal, level of functional possibilities and reaction stability. In pupils with poor progress those correlations were absent.