W. Storm van Leeuwen

The cortical source of the alpha rhythm

Abstract The question of the intracortical sources of the alpha activity of the EEG has been investigated in dogs, using intracortical electrodes placed along a direction perpendicular to the cortical surface. Alpha rhythms have been quantified using spectral analysis, namely the phase relationships between alpha activities recorded between several electrodes has been computed. It was demonstrated that opposition of phase exists between alpha rhythms recorded at different depths within the visual cortex. The phase reversal (180°) was found at about 1100 μm below the cortical surface. The neurophysiological basis of such alpha activity is discussed.

Combined use of subdural and intracerebral electrodes in preoperative evaluation of epilepsy.

For intracranial recording of partial seizures considered to originate from one of the temporal or frontal lobes, the team in the Utrecht Academic Hospital has used subdural, multicontact, flexible electrodes since 1972. These are introduced through bilateral, frontocentral trephine holes and are manipulated under fluoroscopy to cover most of the cerebral convexity. It became evident that in many patients, additional placements to record from intracerebral structures were indispensable. Therefore, using the same trephine holes, an additional 2 to 4 depth electrodes were stereotactically implanted in the mesial temporal and/or frontal structures, as appropriate. An extensive intra- and extracerebral spatial representation of the epileptogenic zone was thus obtained. We report here the methods for manufacturing and applying these electrodes and our clinical experience with 28 patients. The results obtained so far stress the value of combining subdural and depth electroencephalographic monitoring in the presurgical selection of patients suffering from medically refractory complex partial seizures. By miniaturizing the electrodes, extensive areas of the brain can be investigated without craniotomy or multiple burr holes.

Lack of bilateral coherence of mu rhythm.

An investigation has been carried out in 25 normal subjects on quantitative differences of mu rhythm and alpha rhythm parameters. The parameters were frequency, average power over 102 sec and bilateral coherence. It was found that: (1) in the majority of cases the frequencies of the mu rhythms were higher than those of the alpha rhythms; (2) in the majority of subjects the average power was smaller for the mu rhythms for the alpha rhythms; (3) in none of the subjects was bilateral coherence of mu rhythms observed while in the majority the alpha rhythms were obviously bilaterally coherent. It is proposed that the latter difference represents a fundamental difference in the origin of alpha and mu rhythms.

Automatic detection and localization of epileptic foci

A new method of automatic EEG analysis (ASD or automatic non-stationarity detection) derived from a parametric EEG model (autoregressive filter model) and based on inverse filtering, has been applied to scalp and subdural EEGs of epileptic patients. By this method it was possible to detect the occurrence of transient non-stationarities such as paroxysmal patterns of activity (spikes, spike-and-waves) characteristic of inter-ictal EEGs. The ASD has been implemented in a general purpose digital computer. The method allows: (a) statistical evaluation of paroxysmal patterns; (b) multi-channel analysis, where the interrelationships of different derivations are quantified and displayed by means of a clinically useful spatial map. The application of the ASD method to scalp and subdural EEGs, simultaneously recorded, has revealed that a number of transient non-stationarities detected in the scalp by the program yet not by visual inspection, coincide with clear paroxysmal patterns in subdural derivations. In this way conventional definitions of characteristic paroxysmal patterns at the scalp are put in question. Identical conclusions regarding the localization of an epileptogenic zone in 4 patients were obtained either using the conventional methods of recording electro-clinical seizures in EEGs of long duration or applying the ASD method to short EEG epochs (100 sec long, at most).

Dynamic characteristics of visual evoked potentials in the dog. II. Beta frequency selectivity in evoked potentials and background activity.

Abstract 1. 1. Sinusoidally modulated light (SML) evoked potentials recorded from the posterior marginal gyrus were described as generated by a beta frequency selective system. 2. 2. The beta resonance is related to the dominant mode of spontaneous activity recorded from the same brain area and characterized by means of power spectra. 3. 3. The resonance behaviour of cortical SML evoked potentials was dependent on the state of the animal and disappeared during light Nembutal anaesthesia. 4. 4. A linear model including resonance was devised to account for the cortical frequency selective system. The model was tested experimentally by obtaining the systems impulse response by means of cross-correlating the input and output of the system excited with noise modulated light. 5. 5. The implications of beta selectivity in terms of neural pathways and networks are discussed.

Essential differences between alpha rhythms and barbiturate spindles: Spectra and thalamo-cortical coherences

Abstract 1. 1. Spontaneous alpha rhythms and barbiturate spindles have been recorded in dogs from the same electrode sites (thalamus and cortex). 2. 2. Alpha rhythms were found to differ from barbiturate spindles as regards their spectra and topographic distribution. 3. 3. Thalamo-cortical coherences obtained during alpha rhythm differed significantly from equivalent coherences during barbiturate spindles for many thalamic and cortical sites. The latter were larger than the former. 4. 4. It is concluded that thalamic and cortical sites act more independently of one another in the state in which alpha rhythms occur than under light barbiturate anaesthesia.

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.

Dynamic characteristics of visual evoked potentials in the dog. I. Cortical and subcortical potentials evoked by sine wave modulated light

Abstract 1. 1. Potentials evoked by sinusoidally modulated light (SML) were recorded from the lateral geniculate nucleus and occipital cortex of unanaesthetized dogs. The evoked potentials were analysed by discrete Fourier analysis. 2. 2. The harmonic components of SML evoked potentials were described by frequency response functions. The range of linear and non-linear behaviour was determined. 3. 3. A linear description was only possible regarding the SML evoked potentials recorded from the posterior marginal gyrus. 4. 4. Three types of non-linearities were characterized: saturation, non-linear oscillations responsible for the generation of subharmonics and essential non-linearities. 5. 5. The essential non-linearities correspond to rectification occurring in “on” and “off” neuronal populations; they were the dominant features of SML evoked potentials in the lateral geniculate nucleus and the calcarine region of occipital cortex, but not in the posterior marginal gyrus. 6. 6. The phase functions were shown to be determined in part by a delay time. 7. 7. The relations between SML evoked potential parameters and data obtained at the unit level are discussed.

Normative spectral data on alpha rhythm in male adults

Spectral analysis of the EEGS of 110 normal male subjects was carried out in order to obtain objective normative data on the alpha rhythm. The parameters of alpha rhythm extractd from the spectra and coherency functions were: the frequency and bandwidth, maximum power density, interhemispheric symmetry, reactivity, coherency and their interrelationships.

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.