Cerebral evoked potentials after rectal stimulation

We obtained reproducible cortical evoked potentials (EPs) in response to electrical stimulation of the rectum with 1 Hz frequency. We found 2 distinctly different EPs in response to rectal stimulation. In 5 females, the EP had an early onset latency (mean 26 msec) with multiple positive and negative peaks. In 10 females, the EP had a later onset latency (mean 52 msec) and a trifid configuration, having a very prominent negative peak. The early onset EPs after rectal stimulation appeared very similar to the wave form of the cortical EPs recorded after pudendal nerve stimulation. Finding similar interpeak latencies in the early onset EP after rectal stimulation and the EP after pudendal nerve stimulation suggests that either the same pathway was used or that rectal stimulation also stimulated the pudendal nerve. It appears that we stimulated visceral afferents when we recorded late onset EPs, because the large EP amplitude declined rapidly with faster stimulation rates and also with greater number of averaging, and the sensation threshold was very unstable, all different to somatosensory EPs.     

Functional aspects of evoked alpha and theta responses in humans and cats

A systems theoretical approach was used to compare possible functional roles of theta (4--7 Hz) and alpha (8--15 Hz) response components of brain evoked potentials. These response components were described earlier by Ba\c sar (1980). We recorded EEG and evoked potentials (EPs) from occipital scalp locations in 11 subjects. We used auditory and visual stimuli as inadequate and adequate stimuli, respectively (``cross-modality' measurements). The combined EEG-EP epochs were analysed in frequency domain with fast Fourier transform and adaptive digital filters. Alpha (8--15 Hz) response components turned out to be dependent on whether the stimulus was adequate or not (median amplitude with inadequate vs. adequate stimulation: vs. ). Theta (4--7 Hz) response components were less dependent on stimulus modality (inadequate vs. adequate stimulation: vs. ). In EP recordings the occipital alpha response almost disappeared in the first 250 ms following auditory stimulation. Comparable behaviour was observed in similar experiments with recordings from the cat visual cortex (area 17) and with occipital magnetoencephalographic recordings. Taking into account the above-mentioned previous reports on intracranial recordings in primary sensory areas of the cat brain and preliminary results of magnetoencephalographic measurements, we propose the following hypothesis: alpha responses in a time window of about 250 ms after stimulation might predominantly reflect primary sensory processing whereas the theta responses in the first 250 ms after stimulation might be more involved in supra-modality -- or cross-modality -- associative-cognitive processing. Received: 25 February 1994 / Accepted in revised form: 5 August 1994     

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.     

Method for recording short latency evoked potentials using an EKG artifact elimination procedure

This paper describes the application of an EKG elimination procedure¹, previously reported by the authors, to evoked potentials (EPs) recording using a non-cephalic reference. The method consists of three separate steps: data acquisition, EKG artifact elimination, and EP averaging. EKG artifacts are eliminated from the raw EEG by applying a four step procedure to the simultaneously recorded EEG, EKG and stimulus pulse. The steps are: synchronized partition of the raw EEG, EKG averaging, synchronized repetition, and synchronized subtraction of the EKG estimate from the raw EEG. Average EP values are then obtained by averaging the processed EEG using the stimulus pulse as a trigger. Somatosensory evoked potentials to a hand reference, averaged using the proposed method, were compared with those obtained by two conventional averaging methods, and were shown to be more clearly defined. The advantage of the proposed method for recording short latency EP values with a non-cephalic reference is that it requires fewer sweeps and thus takes less time than other methods. The proposed method may also be applicable to the recording of other EP values.     

Evoked frequency stabilization in the electrical activity of the cat brain

In this study, our previous results on the important relation between EEG and EPs were extended by experiments with chronically implanted and freely moving cats, which had electrodes at the acoustical cortex, inferior colliculus and reticular formation. During the experiments the frequency stabilization upon sound stimulation was shown in the frequency domain by comparison of the pre-stimulus power spectra and post-stimulus amplitude frequency characteristics. Comparative frequency domain analysis of about 75 EEG-EPograms (sample of spontaneous activities just prior to stimulation and single evoked potentials following the stimulation), which were recorded from all the brain nuclei mentioned above and from each of the 11 cats, was performed as follows: 1) Power spectra of the EEG-records prior to stimulus were evaluated. 2) Instantaneous frequency characteristics of single EPs were obtained by the Fourier transform. 3) Distribution of the amplitude maxima of the EP-frequency characteristics and the distribution of the EEG-spectral peaks were compared by plotting two types of histograms containing relevant spectral peaks before and after the stimulation. In a frequency range between 1–1000 Hz, the frequency distribution of the EP records from RF and IC were accumulated in narrow discrete frequency channels, whereas, the distribution of the spectral peaks of the EEG depicted frequency spread in broad channels. The frequency stabilization of the EP records from GEA, in the alpha frequency range, was also observed. This effect was described by a factor which we called as the Frequency Stabilization Factor. The results presented in this study showed that the frequency stabilization of the brain's electrical activity induced by sensory stimulation displayed a fluctuation leading to frequency stabilization factors between 0.95 and 5.00. The frequency stabilization and relevant power enhancement upon stimulation strongly support our contention that evoked potential results from the frequency stabilization of the spontaneous activity, triggered by stimulation.Supported by Grant No. TAG-345 of the Scientific and Technical Research Council of Turkey     

Long latency transient evoked potentials to rapid random stimulation: responses to single and multiple concurrent stimuli

In EP testing, regular (periodic) stimulation at increasing rates produces progressive fusion of responses into steady-state wave forms. When stimuli are presented randomly in time this fusion does not occur. Medium and long latency transient EPs can be recorded to stimulation at interstimulus intervals which are much shorter than the EP wave form latencies. Individual transient EPs can be obtained to multiple independent stimuli presented concurrently when the stimuli are presented randomly to one another. The ability to obtain responses to rapid stimulation and to multiple independent stimuli provides opportunities for increased efficiency and complexity of testing, particularly involving long latency responses.     

Systemic Organization of Perceptive Activity in Children with Different Levels of Mental Development

Visual evoked potentials (EPs) of the left and right hemispheres in response to relevant and irrelevant stimuli in the structures of the left and right hemispheres have been studied in healthy young schoolchildren, learning-disabled (LD) children, and mentally retarded (MR) children. In healthy children, the largest EP variations depending on the stimulus relevancy have been found in associative structures of the left hemisphere. In LD children of the same age, the amplitude and temporal characteristics of left-hemispheric EPs to target and nontarget stimuli are the same. In MR children, EPs to relevant and irrelevant stimuli do not differ from each other in either the left or the right hemisphere. EP latencies are significantly longer in MR children than in healthy children. The results of simultaneous recording of EPs in the left and right hemispheres during isolated stimulation of the right and left visual half-fields indicate that interhemispheric interaction is impaired in children with deviations in mental development. The results of the study are discussed in terms of the psychological characteristics and learning ability of children.     

Brainstem auditory evoked potentials in the rabbit.

Eight white New Zealand rabbits were submitted to auditory stimulation in order to obtain normative BAEP parameters. A monaural alternating 0.1 ms click stimulation at 20 Hz, 90 dB was used. Two series of 1000 responses were averaged (10 ms time-base, 160-3000 Hz band-pass) and highly reproducible peaks were obtained. Peaks P1, P2, P3, P4 were obtained in all ipsilateral recordings, whereas peak P5 was detectable in only 6 animals. In contralateral recordings P1 was absent and the following peaks were similar to those of ipsilateral recordings. Normative values of absolute and interpeak latencies, peak amplitudes and amplitude ratios were obtained. The procedure was repeated 24 hours after basal recordings and measures of test-retest variability were obtained.     

Evoked potentials during the elaboration of a conditioned reflex to hippocampal stimulation in rats

In experiments on 9 rats, the study of evoked potentials (EPs) of the CA1 field of the dorsal hippocampus to stimulation of its symmetrical part serving as a signal of drinking conditioned reflex (CR) showed that during reflex elaboration, the amplitude of the main EP components significantly decreased; CR did not appear when the population spike (PS) was absent in the hippocampal response. PS always accompanying CR was not specific only of it, it was also recorded at other behavioural reactions. Changes of fascia dentata EPs in the process of CR elaboration to stimulation of its symmetrical part consisted in decrease of the initial negative wave and increase of the positive one. The obtained data point to a significant reconstruction of excitatory and inhibitory inputs to the hippocampus and fascia dentata under the influence of conditioned activity.     

Method for single-trial recordings of somatosensory evoked potentials

Evoked potentials (EPs) in response to stimuli are recorded from a human scalp contaminated with noise. To improve the signal-to-noise ratio, averaging methods have been widely used for the recorded data. However, when the waveforms of EP for each stimulus are not identical, the average waveform of the EP deteriorates. Variation of the EP waveform to each stimulus itself is important information for the EP. In this paper, a recording method for single somatosensory evoked potential (SEP) waveform is proposed, in which three kinds of band-pass filters were selectively used during three specific time sectors for each interstimulus interval. For the late section of the interval, an EEG waveform prediction method was applied to eliminate contaminated alpha rhythm components. By using the proposed method, we were successful in detecting the single SEP waveform.     

Topography of alpha and theta oscillatory responses upon auditory and visual stimuli in humans

Brain resonance phenomena and induced rhythms in the brain recently gained importance in electroencephalographic, magnetoencephalographic and cellular studies (Ba\c sar and Bullock 1992). It was hypothesized that evoked potentials are superpositions of induced rhythms caused by resonance phenomena in neural populations (Ba\c sar et al. 1992). According to Ba\c sar (1972), such resonance phenomena are reflected in the main peaks of the amplitude frequency characteristics computed from EEG responses. The present study is based on a frequency domain approach for the evaluation of topography- and modality-dependent properties of oscillatory brain responses. EEG and evoked potentials were recorded from vertex, parietal and occipital scalp locations in 24 volunteers. Two combined methods were applied: (1) amplitude frequency characteristics were computed from the transient evoked responses, and (2) frequency components of the transient responses were obtained by adaptive digital filtering. Our main goal was to investigate theta (4--7 Hz) and alpha (8--15 Hz) response components. (1) Amplitude frequency characteristics. Auditory stimuli elicited theta-alpha compound responses in the 4--11 Hz frequency band (e.g. typical peaking frequency around 7 Hz for vertex recordings). Visual stimuli elicited alpha responses (e.g. typical peaking frequency for vertex recordings around 9--12 Hz). Frequency maxima for visual stimuli thus had main peaks at higher frequency values than frequency maxima for auditory stimuli. (2) Digital filtering confirmed these results: for vertex recordings, theta vs. alpha response amplitudes were 9 vs 6 for auditory stimuli and 5 vs 5 for visual stimuli, thus confirming a shift towards higher frequencies, i.e. a more prominent contribution of the alpha range, in the case of visual stimulation. We hypothesize that these properties might reflect site- and modality-specific features of stimulus encoding in the brain in which resonance properties of neuron populations are involved. Furthermore we emphasize the utility of the systems theory approach for a better understanding of brain function by means of EPs. Received: 25 February 1994 / Accepted in revised form: 5 August 1994     

Combined dynamics of EEG and evoked potentials

This study is carried out on single (not averaged) recordings combining the spontaneous activity preceding the stimulus onset and the EP recorded upon acoustical stimulation. These recordings, which we call EEG-EPograms, are measured simultaneously from different subdural structures, such as the auditory cortex, medial geniculate nucleus, inferior colliculus, reticular formation and the hippocampus of the cat brain during the slow wave sleep stage. Using a combined analysis procedure (C.A.P.), the relevant frequency components of spontaneous EEG and EPs, recorded simultaneously from these brain nuclei, are analyzed according to the consistent selectivity bands depicted by the determined amplitude-frequency characteristics for the SWS-stage. In parallel with the results which we obtained for the waking stage, these analyses provide also the following information: (1) there is an important congruency in the time courses of simultaneous response components in common frequency bands, especially in the alpha and beta frequency ranges; (2) there exist significant coupling and synchrony between the evoked amplitude enhancements in the simultaneously recorded single response components; (3) the inter-nuclei coherency in the brain's electrical activity is enormously increased upon stimulation; (4) the evoked response magnitude can be predicted, with reasonable accuracy, from the spontaneous activity preceding the stimulus. All these findings are discussed with reference to those obtained for the waking stage.This study is supported by the Grant TAG-364 of the Scientific and Technical Research Council of Turkey     

Spatiotemporal characteristics of evoked potentials in the posterior ventrolateral nucleus of the cat thalamus

Evoked potentials (EPs) in the posterior ventrolateral nucleus (VPL) of the thalamus in response to electrical stimulation of the skin of the contralateral forelimb were studied. It was shown that they are formed by superposition of several electrical dipoles, one of which lies with two poles in VPL while the rest have one pole in VPL and the other in more caudal zones of the somatosensory system. The first phase of the EP in VPL consists of two components with different amplitude—frequency characteristics. The lability of the second component is low and it disappears if the frequency of stimulation rises above 20 Hz. The focus of maximal activity during contralateral stimulation of the skin of the forelimb occupies the same topographical position in VPL during development of the positive and negative phases of the EP. The steepness of rise of the ascending phase and the amplitude of the positive phase of the EP are greatest at the focus of maximal activity; in other regions the lines of equal steepness need not necessarily coincide with the equipotential lines; differences are especially noticeable in the sagittal plane.Institute of the Brain, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 4, No. 4, pp. 439–446, July–August, 1972.     

Non-painful and painful stimulation of human skin and muscle: analysis of cerebral evoked potentials

The present study compared the cerebral processing of non-painful and painful cutaneous CO₂ laser stimulation and intramuscular electrical stimulation in 11 normal subjects. The overall wave form morphology of the long-latency evoked potentials (EPs) at the central vertex (Cz) was identical and surface topographic mappings of the 21-channel recordings showed similar distributions, suggesting involvement of common neural generators. However, the EPs caused by intramuscular stimulation differed from cutaneous stimulation in several distinct ways. First, the latency of the major positive and negative components were significantly shorter with intramuscular stimulation (N 128–145 ms; P 274–298 ms) compared to cutaneous stimulation (N 235–286 ms; P 371–383 ms) (P<0.001). Second, the peak-to-peak amplitude and root-mean-square values of intramuscular EPs recorded at Cz showed a ceiling effect in the painful range, whereas the laser EPs continued to increase in this range. Third, painful intramuscular, but not non-painful, stimulation caused a frontal activity which not was observed with cutaneous laser stimulation at any intensity. Conduction velocity measurements indicated activation of nociceptive A-delta afferents with cutaneous laser stimulation (10.2±0.2 m/s) and activation of a mixed nerve fiber population with intramuscular electrical stimulation (65.8±25.8 m/s). Differences between laser and intramuscular EPs may be due to different types and origins of activated afferent fibers. Laser EPs can be used specifically to assess cutaneous A-delta fiber function, whereas intramuscular EPs reflect the cerebral processing of a mixed afferent input from muscle tissue.     

Evoked potentials in the reticular formation of the rabbit midbrain during formation of a dominant focus in it

During creation of a dominant focus in the midbrain reticular formation (RF) by its multiple stimulation with a high-frequency current (stimulation frequency 200 Hz, pulse duration 0.1-0.5 ms, voltage 1-3 V, duration 5 s) a statistically significant increase of the amplitude of the evoked potential (EP) in RF to light flashes was revealed in comparison with background data. Significant increase of EP amplitude was also observed in RF in response to the same stimuli applied in successive experiments without RF stimulation, which pointed to the existence of a latent dominant focus in the CNS.     

Relevance of the callosal transfer in defining the peripheral reactivity of somesthetic cortical neurones

Experiments were performed in 13 chloralose-anaesthetized, curarized cat preparations (monitoring of rectal temperature, heart rate, expired pCO2 and EEG), in order to ascertain whether, and to what extent, the reactivity to ipsilateral skin shocks of the neurones of the anterior ectosylvian and anterior suprasylvian gyri (AEG and ASG, respectively) is dependent on the callosal output of the somatosensory areas of the contralateral hemisphere. Indeed, we knew from previous experiments that a high proportion of AEG and ASG neurones having bilateral peripheral receptive fields (PRFs) can be excited by direct stimulation of the contralateral homonymous areas, and that the callosal fibres originating in the latter carry somesthetic impulses related to contralateral PRFs. A preliminary analysis was carried out on the amplitude and latency relationships between the evoked potentials (EP) recorded simultaneously from the two hemispheres and from the corpus callosum (CC) following stimulation of the forepaw of one side. The results obtained showed good correlations between the onset and development of the EPs picked up from the hemisphere ipsilateral to the stimulated skin, on the one hand, and onset and development of the EPs recorded from the contralateral hemisphere and the corpus callosum, on the other. At a further stage of the experiments, the EPs elicited upon ipsilateral and contralateral skin shocks in the AEG-ASG area have been recorded and averaged before, during and after the reversible inactivation of callosal somesthetic transmission. This was achieved by applying polarizing currents (0.2-1 mA) to the rostral portion of the CC, adequacy and reversibility of this method having been tested by observing, respectively, suppression and prompt restoration of transcallosal EPs and of the asynaptic spiking produced by cortical cells when antidromically invaded from contralateral homotopic cortex. It was seen that during CC blockade the EPs elicited in the AEG-ASG areas did not show any change either in amplitude or time-course if brought about by contralateral peripheral stimulation, whereas those evoked by ipsilateral skin shocks exhibited significant reduction, which was related to the strength of CC polarization and to the reduction of transcallosal EPs. In control experiments similar effects were observed after ablation of somatosensory areas of the hemisphere which send off somesthetic callosal impulses, whereas strychninization of these areas caused effects opposite in sign, i.e., enhancement of the ipsilateral but not of the contralateral EPs in the areas of the untreated hemisphere. By testing the effects of CC polarization on single AEG-ASG neurones, it was observed that the responses of units linked only with contralateral PRFs (Group I; 7 units tested) were unaffected by callosal polarization. The discharges of neurones provided with wide and bilateral PRFs (Group II; 27 units tested) were not affected if elicited by contralateral PRF shocks but were deeply impaired (in 11 neurones out the 27) when provoked by ipsilateral PRF stimulation. The effect consisted chiefly of the disappearance of the first high peak of the PSTHs, and when recording intracellularly graded events, it was mirrored by a large decrease of the postsynaptic excitatory potentials elicited in Group II neurones by ipsilateral PRF shocks. A late scattered histographic component was identified in the PSTHs of such cells, which did not appear to be significantly altered during CC blockade. These effects were observed on the ipsilateral responses of 11 out of the 27 Group II neurones so tested whereas the ipsilateral PSTHs of the remaining 16 Group II neurones either did not undergo significant changes during the callosal blockade or escaped evaluation because of high spontaneous shifts of neural responsiveness. The results are discussed mainly with a view to the possible functional role of the specific somesthetic callosal fibres in defining ipsilateral reactivity for the wide-field cells of the AEG-ASG area.     

Scalp-recorded short and middle latency peroneal somatosensory evoked potentials in normals: comparison with peroneal and median nerve SEPs in patients with unilateral hemispheric lesions

Peroneal somatosensory evoked potentials (SEPs) were performed on 23 normal subjects and 9 selected patients with unilateral hemispheric lesions involving somatosensory pathways.Recording obtained from right and left peroneal nerve (PN) stimulations were compared in all subjects, using open and restricted frequency bandpass filters. Restricted filter (100–3000 Hz) and linked ear reference (A1–A2) enhanced the detection of short latency potentials (P1, P2, N1 with mean peak latency of 17.72, 21.07, 24.09) recorded from scalp electrodes over primary sensory cortex regions. Patients with lesions in the parietal cortex and adjacent subcortical areas demonstrated low amplitude and poorly formed short latency peroneal potentials, and absence of components beyond P3 peak with mean latency of 28.06 msec. In these patients, recordings to right and left median nerve (MN) stimulation showed absence or distorted components subsequent to N1 (N18) potential.These observations suggest that components subsequent to P3 potential in response to PN stimulation, and subsequent to N18 potential in response to MN stimulation, are generated in the parietal cortical regions.     

BLUE WHALE, BALAENOPTERA MUSCULUS, VOCALIZATIONS FROM THE WATERS OFF CENTRAL CALIFORNIA

Low-frequency calls produced by blue whales, Balaenoptera musculus , were recorded in the northeastern Pacific Ocean off central California. Two blue whales were sighted during a vessel-based marine mammal survey, and when sonobuoys were subsequently deployed, blue whale calls were recorded. A third recording was obtained during the survey from a blue whale that was not seen. Recordings with 15, 25, and 55 min of calls were obtained from these individuals. The three recordings all contain two-part, low-frequency calls with slight interindividual variation. The calls consist of an amplitude modulated (AM) signal with a mean center frequency of 16.5 Hz, followed by a downsweep whose mean center frequency sweeps from 18.2 Hz to 16.6 Hz. The recordings are compared with blue whale recordings from the Pacific and Atlantic Oceans. The geographic variability suggests that blue whale calls may be used as an acoustic indicator of stock identity.     

Important relation between EEG and brain evoked potentials

In this study, we analyze the important relation between the spontaneous and evoked activities of the substructures of the cat brain, such as the reticular formation, hippocampus, inferior colliculus, medial geniculate nucleus and acoustical cortex, with an ensemble of systems theory methods consisting of the following steps: (1) single auditory and/or visual evoked potentials (EPs) and the spontaneous activities (EEG) just preceding the stimuli are recorded from the brain center under study; (2) selectively averaged evoked potentials (SAEPs) are obtained from the recorded EPs; (3) amplitude frequency characteristics are computed from the AAEPs by means of Fourier transform; (4) the single EEG-EP sweeps are theoretically pass-band filtered with adequate band limits determined according to the selectivities revealed by the amplitude characteristics; (5) the EEG and EP components obtained in this way are compared with regard to the amplification in the population response upon the application of the stimulus. The results of this analysis support quantitatively our prediction of various types of resonance phenomena in a number of nuclei in the cat brain and in a large scale of frequencies from 1 Hz to 1000 Hz and show that the amplification factor related to resonance phenomena has probabilistic nature. Therefore, the analogy which we have recently drawn between the behaviors of a neural population and a random-phase probabilistic harmonic oscillator is extended by assigning also the amplitude and the frequency of the oscillations as random variables. A working hypothesis for the dynamics of neuronal populations is elaborated accordingly.Presented in Part at the Third European Meeting on Cybernetics and Systems Research 1976 in Vienna, April 20–23, 1976Supported by Grant No. TAG-345 of the Scientific and Technical Research Council of Turkey     

Functional maturation of the cat visual cortex during prenatal ontogenesis]

Development of the perceiving function of the visual cortex was studied on the foeti of cats in the second stage of antenatal development with intact placental blood circulation. It has been found that functional maturing of the cortical end of the visual analyser starts at the beginning of the second half of antenatal life. At this period EPs to stimulation of the optic nerve are recorded throughout the dorsal cortex of the contralateral hemisphere. At first they appear as slow three-phase (positive-negative-positive) oscillations of a small amplitude. As the foetus develops, the EP amplitude increases, and the EP configuration in the striate zone of the cortex becomes complex. Two weaks before birth, a short-latency negative wave appears against the background of the primary positive oscillation. In the last week of antenatal development of the foeti and in the first few days of the kittens life, EPs are represented in the specific zone of the visual cortex (g. lateralis) as two negative oscillations, and in the so-called associative zone (the middle part of the suprasylvian gyrus) by one long-latency high-amplitude negative oscillation which corresponds by latency to the second negative EP component in the striate cortex.