W.R. Adey

EFFECTS OF MODULATED VHF FIELDS ON THE CENTRAL NERVOUS SYSTEM

The existence of brief epochs in which electroencephalographic and neuronal activities are strongly correlated has been repeatedly established in different areas of the brain. For example, periodic slow oscillations of neuronal membrane potentials, frequency related to the concomitant local electroencephalogram (EEG), have been recorded in various cortical areas1-’ and in hippocampal and thalamic Therefore, the EEG appears to reflect the attenuated undulations of the membrane potential of a surrounding population of neurons, and rhythmic electroencephalographic patterns could be generated by extracellular summation of simultaneous transient slow electrical events in a population of cells. On the other hand, weak extracellular voltage gradients (1-5 mV/mm) have been shown to significantly affect the excitability, or firing thresholds, of stretch receptor neurons in the crayfish and spinal motor neurons in the As pointed out earlier by Nelson,’ complex structural organization of brain tissues, as seen in the cerebrum, should be highly favorable for multiple electric field interactions, both in the intricate rate of overlapping dendritic trees and between the macromolecules of the extracellular space and the glycoproteins of the outer surface of the cell membrane.’-“ Indeed, extremely weak vhf fields [ I47 MHz, 1 mW/cm2]], amplitude modulated at brain wave frequencies, have been shown to strongly influence spontaneous and conditioned EEG patterns in the cat.“.l” The hypothesis was offered that the weak electrical forces induced in the brain were modifying the excitability of the central neurons and that these changes were reflected in the recorded transient EEG episodes. The extracellular electrical gradients could exert their forces on the multiequilibrium system that exists in the outer zone of the neuronal membrane, where monoand divalent cations compete for binding sites on polyanionic macromolecules and polar ends of intrinsic membrane proteins.L6-20 Local variations in the surrounding electric field could result in slight modifications of the negative binding sites, either by triggering configurational changes of the surface macromolecules or by inducing small displacements of the surface-bound cations. These alterations would, in turn, influence the activity of adjacent ions, which would disturb further the molecular arrangement of the surface macromolecules, thus propagating and multiplying the initial electrical disturbance. Concurrent work in this laboratory*’ indicated that weak pulsed electric currents (2-5 mV/mm, 200 pulses/sec) applied across the cat cortex were able to trig-

Spectral analysis of low frequency components in the electrical activity of the hippocampus during learning.

Abstract Electrographic correlates of learning in the dorsal hippocampus of cats were studied in a training experiment involving a light-dark discrimination task. Separate analyses were made of the period in which the animal discriminates and approaches the goal, of the period following the warning stimulus and of periods of record corresponding with the beginning and the end of each trial. Spectral analysis methods were applied at 1 c/sec resolution for analysis periods as short as 1.6 sec. Comparison of analyses in different stages of learning indicated characteristic and consistent changes in the period of advanced training, above 80% performance level. In this stage of learning sharp peaks of spectral density shift very regularly in steps of 1 c/sec from the pre-stimulus to the approach period. A 6 c/sec frequency was constantly correlated with the first part of the approach period. The consistency of two characteristics, namely the regular shift of frequencies and the sharpness of the peaks in the approach period in this stage of learning, suggests that these electrographic features are correlated with the consolidation processes of the learned behavior in which the hippocampus plays an important role. Distinct differences were found in hippocampal wave patterns accompanying incorrect responses with evidence that “attention sets” immediately preceding the incorrect performance were substantially different from those in correct performances. In the period immediately before reaching the food reward, the EEG often appeared “desynchronized”. A detailed analysis of this phenomenon showed that its main characteristic is an abrupt decrease in amplitude of the waves which was interpreted as a correlate of a very tense state of the animal in front of the food. This attenuation phenomenon succeeds the decision part of the approach behavior and is superimposed on the last part of the approach epoch. The importance of achieving required levels of resolution when working with spectral analyses is discussed in relation to a short analysis period in studies of learning mechanisms.

Effects of modulated very high frequency fields on specific brain rhythms in cats

Abstract The effects of exposures to low intensity (1 mW/sq.cm or less), very high frequency (VHF) (147 MHz) electrical fields, amplitude-modulated at biological frequencies (1–25 Hz), were studied on untrained and conditioned chronically implanted cats. The fields were applied between two aluminum plates (identical voltages, 180° phase shift) firmly anchored to the floor of an isolation booth, especially designed for use of VHF fields. The animals were restrained in a hammock, the longitudinal axis of the body kept parallel to the field plates. EEG and EOG were recorded through a system of low pass filters on a Model 6 Grass electroencephalograph and an Ampex FR 1100 tape recorder; behavior was continuously observed through a closed circuit TV. A series of animals was operantly trained to produce specific transient brain rhythms following periodic (every 30 sec) presentations of a light flash stimulus. The levels of performance were established (visual and spectral analysis) during conditioning and extinction schedules for a series of cats submitted to VHF fields amplitude-modulated at the dominant frequencies of the selected transient patterns and for a control group, in the absence of fields. The irradiated animals differed markedly from the control group in the rate of performance, accuracy (in terms of frequency bandwidth) of the reinforced patterns and resistance to extinction (minimum of 50 days versus 10 days). The specificity of the frequency of the modulation was tested on another group of untrained animals where spontaneous transient patterns were used to trigger for short epochs (20 sec following every burst) the VHF fields amplitude-modulated at various frequencies. The experimental results indicated clearly that the fields were acting as reinforces (increasing the rate of occurence of the spontaneous rhythms) only when modulated at frequencies close to the biologically dominant frequency of the selected intrinsic EEG rhythmic episodes. Various possible routes of interaction between the external fields and the CNS are discussed, and the hypothesis is offered that the amplitude-modulated VHF fields could influence the excitability of neuronal membranes.

Comprehensive spectral analysis of human EEG generators in posterior cerebral regions

Abstract Continuous spectral analyses of several minutes of normal human EEGs were prepared by computer, and converted to compact graphic forms. This new presentation of voluminous data in contour maps gives an overview of the evolution of patterns in the EEG. Parameters graphed were spectral intensity (“power spectral density”), and coherence, a quantity expressing strength of relationship between brain areas. In one subject, the strongest relations were among longitudinally oriented parieto-occipital linkages, representing the subjects alpha wave. Wave activity in a similar frequency band, recorded across the occipital midline, was completely incoherent with the alpha wave. Thus, two independent generation processes in the same frequency band, orthogonally polarized, are required to account for these records. Low frequency activity in the O1ue5f8O2 leads was related only partially to the heart-beat. In a second subject, there was somewhat greater coherence from side to side than between homolateral centro-parietal and parieto-occipital pairs of leads. The same configuration of two geometrically orthogonal and statistically uncorrelated generators was observed in this subject. Of 28 other analyzed in this way, eighteen showed the same clear perpendicularity, seven were similar but less marked, while three showed no such pattern. The second illustrative subject also showed coherences flanking his alpha-wave peaks, covering a band considerably wider (2–5 c/sec) than those of high alpha intensity (0.5–1.0 c/sec). Eighteen of 25 other subjects showed a similar disparity of width. Mathematical constraints implied by the common finding of two perpendicular alphaband generators were presented. The wide coherent bands (C3-P3/C4-P4) were interpreted as possibly due to shared “side-bands” of the alpha wave; but other features of these wide bands led to the suggestion that much of the local activity not shared between the sides is caused by the shared activity, or at least causally connected with it.

Electroencephalographic correlates of learning in subcortical and cortical structures

Abstract 1. 1. Spectral analysis was applied to EEG records taken from amygdala, subthalamus, midbrain reticular formation and visual cortex simultaneously with records from hippocampus during a discrimination learning experiment in cats. High spectral peaks were located in all these structres at 1, 2 and 3 c/sec, indicating an increase in regularity of these slow wave rhythms in advanced stages of training. 2. 2. Spectral peaks in the theta range (particularly at 5 and 6 c/sec) appeared during approach period in the advanced training in all these structures, but were less pronounced and more variable than in hippocampus. 3. 3. Computations of coherences between pairings of EEG records from different structures revealed high degrees of linear interrelation. High and consistent coherences were found between homotopic points in the hippocampal formation of the two hemispheres. More variable coherences were seen between hippocampus and subthalamus, midbrain reticular formation and visual cortex. These relationships were most consistent in the theta range, in the approach epoch and in the advanced stage of learning. 4. 4. Our data indicate that during the period of consolidation of learning, the hippocampus plays a leading role in complex interacting neural circuits comprising hippocampus, subthalamus, midbrain reticular formation and cerebral cortex.

THE SENSORIUM AND THE MODULATION OF CEREBRAL STATES: TONIC ENVIRONMENTAL INFLUENCES ON LIMBIC AND RELATED SYSTEMS

In simple organisms, little or no plasticity of behavior is possible. This rigidity is inherent in the restricted neuronal connections typefying simple multicellular organisms. In Hydra, for example, neuroepithelial cells located at intervals on the body surface make direct connections with underlying muscle cells. This simple reflex arc of sensor-effector relationship has been replaced in higher organisms by a series of interneurons, interposed between the sensory and effector cells. It is in a great proliferation of these intercalated neurons that central nervous systems of higher organisms have evolved and, with their appearance, a corresponding plasticity of behavior, Clearly, input-output relationships in central nervous functions are the result of processing and transformation of sensory influxes in all modalities. Significantly, many sensory stimuli are transient, or they may recur at intervals, cyclically or randomly. Yet their effects may far outlast their initial, brief impact on the organism. Indeed, biological survival may be linked to persisting behavioral patterns that are first elicited by a brief stimulus that may recur only at long intervals, but with sufficient frequency to sustain or entrain a biologically important behavioral pattern. Thus, studies of EEG sleep patterns in horses at first suggested an absence of rapid-eye movement (REM) or dream sleep. Subsequent investigation showed that REM sleep only appeared after prolonged adaptation over many weeks in the test environment. If the animal were then roused from sleep by a sudden, brief strange noise, a further long period of adaptation, lasting up to a week, was necessary before REM sleep again a~pea red . ~ The requirement for rapid escape from potential danger has elicited what may be interpreted as a tonic response to specific auditory stimuli. In this example, there is a lasting efficacy of a transient initial stimulus. Quite importantly, the complex behavioral response that it elicits obviously extends substantially beyond the first processing of the information, and the stereotyped responses that might be expected to it, such as the orienting reaction with turning of head and eyes towards the stimulus, or the transient autonomic reactions of altered heart rate and blood pressure.s1 It is also clear that the

The efflux of45Ca2+ and [3H]γ-aminobutyric acid from cat cerebral cortex

Abstract The efflux of both45Ca2+ and [3H]GABA from suprasylvian cortex of cat has been studiedin vivo. After pre-incubating the cortex with radioactivity for 90 min, superfusion with non-radioactive medium was carried out using 0.8 ml vol. changed at 10 min intervals. Increases in the calcium concentration of the medium resulted in greater efflux of both45Ca2+ and [3H]GABA, and the effect on [3H]GABA efflux was potentiated by AOAA. The effect of a1mM increment in Ca2+ concentration was only slightly less than that of a20mM increment. Adding Mg2+ to the medium did not produce increases comparable to added Ca2+, whereas electrical stimulation of the cortex had no effect on the efflux of either45Ca2+ or [3H]GABA. Thiosemicarbazide, an epileptogenic agent, resulted in a slightly irregular efflux of45Ca2+ with peaks visible at times of seizure activity. The efflux of3H2O and14C-(car☐yl)-inulin could not be correlated with any of the above treatments. The efflux of [3H]GABA from the cortex is considered to originate from synaptic terminals and that of45Ca2+ may be the result of reactions at the membrane triggering the release or turnover of calcium.

Factors affecting the release of [14C]taurine from cat brain: The electrical effects of taurine on normal and seizure prone cortex

Summary The efflux of [14C]taurine from cat cerebral cortiex has been studied in vivo using a superfusion technique. The ca2+-dependence of [14C]taurine indicates that large increments in Ca2+ concentration (20.0mM, 5.0mM) are more effective in increasing [14C]taurine efflux than small ones (1.0mM). Weak electrical stimulation or the addition of either25mM taurine or25mM GABA to the superfusion medium produced increases in [14C]taurine efflux, whereas25mM glutamate was without effect. The application of25mM taurine to the cortex also produced an increase in the amplitude of all EEG frequencies. No such EEG effect was observed for25mM GABA. During seizure activity induced by superfusion of the cortex with a low Ca2+ medium, [14C]taurine efflux rose and fell regularly, peaks in efflux often being correlated with seizures. The addition of taurine to the superfusion medium during seizure activity prevented further seizures and stopped the waves in [14C]taurine efflux. The observations are consistent with a direct effect of taurine on cortical excitability.

Responses of cat pallidal neurons to cortical and subcortical stimuli

Abstract Discharge patterns of pallidal neurons of the cat were studied during single-shock and sustained high-frequency stimulation of sensorimotor cortex (SMC), caudate nucleus, nonspecific thalamic nuclei (CM), subthalamus, midbrain reticular formation (MRF) and amygdaloid complex. Three types of spontaneous discharge pattern were found in pallidal neurons: high discharge rate (HDR), low discharge rate (LDR) and repetitive burst patterns were observed. The LDR and repetitively discharging units generally responded to a limited number of the stimulus sites, occasionally to one site exclusively, whereas HDR units generally showed a high degree of convergence. The response patterns of units responding to single-shock stimulation of MRF, CM, and subthalamus were similar. A marked facilitation lasting 20–50 msec was a consistent response which in some units was followed by a prolonged facilitation or inhibition. Sustained high-frequency stimulation always resulted in a net facilitatory effect. Single-shock stimulation of caudate nucleus and SMC generally produced a powerful inhibition. In some units facilitation preceded this inhibition, and in a small number of units, only facilitation was observed. Sustained high-frequency stimulation produced either inhibitory or facilitatory effects. Single-shock stimulation applied to amygdala caused an initial facilitation, followed in some units by inhibition or facilitation. High-frequency stimulation produced either facilitatory or inhibitory effects. The variation in response patterns to stimulation of each stimulus site was dependent upon the individual neuron and not upon the stimulus site. Analyses of the responses to a test stimulus following a conditioning stimulus suggest that the excitability changes of the neuron following single-shock stimulation are well represented by the poststimulus time histogram.

Biophysical and metabolic bases of cooling effects on cortical membrane potentials in the cat

Neuronal and silent cell membrane potentials were recorded in cat cortex during focal cooling by 5–6 C, either transiently for 1–2 min or for 10–15 min. Effects were compared between locally anesthetized, immobilized preparations and those under pentobarbital general anesthesia. The findings suggest actions at common membrane sites in effects of cooling and general anesthesia. In immobilized animals, transient cooling depolarized neurons by 1–2 mv/C. Silent cells responded similarly but with a slower onset. Intraneuronal wave activity and neuronal firing patterns showed related changes during the cooling cycle. Increased cell firing during transient cooling was not confined to depolarizing shifts but also occurred during hyperpolarizing shifts. Raising topical Ca2+ levels to 20 mM blocked depolarization by transient cooling, but 7.5 mM solutions were without effect. Repeated transient cooling or continuous cooling for 10–15 min produced almost complete depolarization of neurons and silent cells for periods extending considerably beyond recovery of brain temperature. Moreover, these cumulative effects beyond the cooling period were potentiated by the weaker 7.5 mM Ca2+ solutions. Pentobarbital anesthesia blocked the effects of sustained cooling on neuronal and silent cell membrane potentials and on unit firing activity. Also, topical 7.5-mM Ca2+ solutions failed to potentiate effects of cooling during barbiturate anesthesia. The effects of transient cooling are discussed in terms of membrane leakage currents that may be blocked by Ca2+. Cumulative effects with prolonged cooling suggest interference with membrane metabolism, and the possible role of cooperative binding and release of cations from membrane surface glycoproteins in these responses is considered.