A. Kamp

Telemetric EEG and video monitoring in epilepsy

We report a clinical evaluation of EEG and video monitoring. In 181 consecutive records, the most common clinical indications were differentiation of epileptic and nonepileptic events (99), seizure recordings for locating a possible focus (23), seizure frequency determination (311, and investigation of possible trigger factors (19). Overall, useful information was obtained in 72% and the clinical question was answered in 67% of records. Prolonged EEG and video monitoring is therefore an important diagnostic aid in patients with proven or suspected epilepsy.

The spectral properties of hippocampal EEG related to behaviour in man.

In an epileptic patient with depth electrodes placed in the hippocampal formation we observed the following: (1) There existed a significant peak in the theta band of the hippocampal EEG of this subject under a number of behavioural circumstances. (2) Four different behaviours were performed at two degrees of intensity. Whenever the values of a certain spectral parameter associated with a low or a high intensity behaviour showed a significant difference, the higher value corresponded with the more intense version of the behaviour. (3) The frequency and rhythmicity of the hippocampal theta component during writing was consistently higher than during sitting or walking. The amplitude was smaller. (4) In a word association task the amplitude, frequency and rhythmicity showed a significant rise during the period of silence immediately following the question and preceding the answer.

Hippocampal EEG and behaviour in dog. II. Hippocampal EEG correlates with elementary motor acts.

A positive correlation has been shown between the speed of forced stepping on a conveyor belt and the amplitude and frequency of the concomitant hippocampal EEG. Significant modulation in the spectral properties of the dogs hippocampal EEG has been found in relation to 3 elementary motor acts: stepping, respiratory movements and a reflex movement in reaction to linear acceleration of the animal. The findings support the idea that within a certain experimental situation the hippocampal EEG reflects the intensity of motor behaviour in a non-specific but predictable way. However, some experimental results indicate that sensory inputs may modulate the hippocampal EEG as well.

HIPPOCAMPAL THETA FREQUENCY SHIFTS AND OPERANT BEHAVIOUR

Abstract 1. 1. A shift of hippocampal dominant theta frequency to 6 c/sec has been demonstrated in the post-reward period in two dogs, which occurs consistently related in time to a well defined behavioural pattern in the course of an operant conditioning paradigm. 2. 2. The frequency shift was detected and quantified by means of sequential and integrated frequency displays and was found to be statistically significant. 3. 3. The occurrence of the hippocampal shift to 6 c/sec became more stable, and the burst of larger amplitude and enhanced regularity, as conditioning proceeded. In respect to the predominance of the component at 6 c/sec a significant difference was found between early and late experiments. 4. 4. A regular pattern of “low voltage desynchronized” hippocampal activity was regularly observed during conditional stimulation, at late stages of conditioning, if followed by correct pressing. When the dog did not perform correctly (inhibition of pressing) there was instead a predominance of theta pattern (peak at 5 c/sec) during stimulation. The difference in contributions of theta rhythm to the total frequency spectra between the two conditions was highly significant. 5. 5. Behavioural correlates of the shift to 6 c/sec and the possible neural mechanisms underlying this phenomenon are discussed.

Contingent negative variation and evoked responses recorded by radio-telemetry in free-ranging subjects

Abstract 1. 1. Using an 8-channel radio-telemetry system (RTE) records were obtained of the EEG, pulse rate, respiration, evoked responses and Contingent Negative Variation (CNV) in four normal subjects and three patients with intracerebral electrodes. The subjects were free to move about within 30 m of the receiving aerial. Two RTE channels were modified to provide time constants of 7 sec. 2. 2. Auditory stimuli, synchronized with the operation of average response computers, were transmitted to the subjects by a separate radio-control link. These were used as conditional and imperative signals to the subjects to perform various tasks: pressing a button to arrest the imperative signals, turning the pages of a book, inserting pegs in a peg-board and coming to a stop on a bicycle. 3. 3. Responses to the signals were averaged on line with two barrier-grid tubes and a 2-channel Enhancetron and the intrinsic rhythms were analysed with a 2-channel frequency analyser. The pulse rate was indicated by a cardiotachometer. 4. 4. The initil responses and CNV during the reception of paired auditory signals were similar to those seen with direct connection, provided that the subjects were engaged in some task related to the signals. The CNV was attenuated during exercise or conversation only while the subject was inattentive to the signals. 5. 5. The amplitude of the intracerebral responses to the conditional and imperative stimuli in the patients with implanted electrodes was reduced when the scalp CNV was attenuated by isolation or distraction. 6. 6. During the performance of fairly complex tasks following the auditory signals, the CNV terminated only at the completion of the task, not at the moment of muscular effort. 7. 7. The radio-control link was also used to instruct an experimenter when to loss a ball to a subject or to feint. In this situation also the CNV developed only when the subject was sure the ball was in the air and terminated when it was caught. 8. 8. These observations suggest that the interactions of evoked responses and CNV seen in laboratory conditions also accompany normal activity and the performance of everyday tasks.

Hippocampal EEG and motor activity in the cat: The role of eye movements and body acceleration

In cat the relation between various behaviours and the spectral properties of the hippocampal EEG was investigated. Both EEG and behaviour were quantified and results were evaluated statistically. Significant relationships were found between the properties of the hippocampal EEG and motor acts (walking, sitting, eating, stepping and eye movements). These results were compared with those obtained in dog under similar experimental circumstances. Species differences were found particularly regarding the fact that in the cat a dissociation between frequency and amplitude parameters was obtained for some behaviours; this may explain why appreciable differences in the visual interpretation of EEG records of different species are often reported. A simple model of the modulation of hippocampal EEG by brainstem inputs is presented. Particular attention is paid to species differences regarding the anatomy and physiology of the pathways involved in this modulation. It is concluded that in cat a strong relation exists between the modulation of spectral properties of hippocampal EEG and vestibular inputs and/or eye movements. The effects of body acceleration on hippocampal EEG are put in evidence and related to theories of hippocampal function.

Hippocampal frequency shifts in different behavioural situations

Summary Electrical activity of the dogs hippocampus was recorded in (a) an operant behaviour situation, and (b) a field situation by a radio-telemetering system. The dominant frequency of the theta rhythm shifted consistently from 4–5 c/sec to 6–7 c/sec when a dog (a) withdrew from a pedal after being rewarded or (b) switched from a ‘turning around’ type of activity to an act performed to catch an object thrown some distance away. The frequency shift was quantified by integrated frequency analysis. Such a frequency shift is interpreted in behavioural terms by considering its appearance in different behavioural situations. The correlation between such shifts and switching from one behavioural act to another, within a behavioural mode characterized by movement, is stressed.

The EEG-spectrograph.

Abstract An apparatus is described providing for continuous display of 20 frequencies with a time resolving power of one second approximately. The spectrogram is directly visible on a cathode ray tube and is photographically recorded with another cathode ray tube. Spectrograms of various test signals are shown.

A METHOD FOR AUTO- AND CROSS-RELATION ANALYSIS OF THE EEG.

A comparatively simple method is described which enables the determination of certain relations between different EEG signals. In distinction from auto- and cross-correlation the method is named auto- and cross-relation. By means of auto-relation analysis an indication is obtained of the rhythmicity of an activity or of the average wave form of a recurring isolated phenomenon. By means of cross-relation analysis certain relations between different signals, and their time or phase relations, may be studied. The averaging may be carried out by means of any average response computer. A simple form of pattern recognition is described for increasing the signal to noise ratio by means of a frequency selective method.

Evoked responses to sinusoidally modulated sound in unanaesthetized dogs.

Abstract 1. 1. Responses evoked by sinusoidally amplitude-modulated sound in unanaesthetized dogs have been recorded from inferior colliculus and from auditory cortex structures by means of chronically indwelling stainless steel wire electrodes. 2. 2. Harmonic analysis of the average responses demonstrated that the energy of most of the responses was concentrated in components at fundamental and second harmonic frequencies. 3. 3. The influence of various stimulus parameters was investigated, in particular the effect of variation of modulation frequency between 10 and 80 c/sec. 4. 4. The amplitude/frequency characteristics of the fundamental components obtained from the inferior colliculus were almost flat, with a slight amplitude decrease at frequencies above 60 c/sec. In some of the cortical areas the characteristics showed a maximum of amplitude at modulation frequencies between 15 and 30 c/sec. 5. 5. The phase/frequency characteristics showed a lag of phase with modulation frequency. From this phase lag delay times were calculated. Those at the inferior colliculus were shorter than those at the auditory cortex. At the latter structure the calculated delay times corresponded closely with latencies of the first positive deflection of responses evoked in these same structures by tone bursts. 6. 6. The amplitude modulation depth characteristics showed that the amplitudes of the responses were influenced by modulation depth. At the inferior colliculus an almost linear relation appeared to exist between modulation depth and response amplitudes. In the auditory cortex more complex relations between modulation depth and amplitude occurred. “Saturation” phenomena were observed in two cortical areas. 7. 7. Restricted investigations into the effects of the composition of the carrier signal revealed that change of carrier signal from sine wave at 4000 c/sec to band-limited noise at 3200–5000 c/sec caused change of the response amplitude distribution over the auditory cortex. Decrease of average intensity to 62 dB produced no change.