Burst activity in pulse flows of neuronal populations

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BURST ACTIVITY IN PULSE FLOWS OF NEURONAL POPULATIONS OF THE CORTEX UNDER LOW-INTENSITY MICROWAVE

Chizhenkova Rogneda A.

The study of brain electrical activity under microwaves is great practical and theoretical importance. Precisely such investigations can make it possible to uncover the genesis and patterns of reactions of the whole organism. Among the physiological mechanisms of microwave influence on the brain, the direct action on brain prevails [1, 2]. Unfortunately so far, little attention has been paid to fundamental works of activity of the brain under microwave exposure. Our review of the state of investigations of biological effects of electromagnetic fields over the last 30 years has been conducted by means of the database "Medline". Investigations carried out in the cortex and neurons are only 3.5% and 0.2% accordingly [12].

In our previous investigations it was found that 1-min microwave exposure affected little on the mean frequency of cortical neuron activity but produced significant shifts in evoked activity [3, 5]. Our following works showed that microwave modified inner pattern of pulse flows of neurons [5-8, 10, 11]. The purpose of the present study was to investigate spike bursts revealed at different time threshold in background activity of populations of cortical neurons under rather short low-intensity microwave exposure. The spike activity of cortical neuron populations was considered more informative than the activity of single neurons [4].

Preliminary results on this problem partly were presented in our another papers [7, 8].

Experiments were conducted on 16 unanesthetized nonimmobilized rabbits with electrodes preimplanted under barbitural narcosis into the sensorimotor region of the cortex for recording the neuronal spike activity. The animal's head was exposed to microwave field (wavelength 37.5 cm, 0.5-1 mW/cm-2). The exposure time was 1 min.

Three 1 min recordings of neuronal activity, obtained before, during, and immediately after microwave exposure, were subjected to computer analysis. To analyse the burst activity, threshold levels 5, 10, and 20 mc were used. In first case associations of spikes were taken into account provided intervals between spikes were less 5 ms, in second case if they were less 10 ms, and in third case if they were less 20 ms. The characteristics of the burst activity (burst number, mean spike frequency in a burst, mean spike number for a burst, mean burst duration, and burst duration: interburst interval duration ratio) were considered. All parameters chosen were calculated for 20-s time intervals. Results obtained during and after microwave exposure were compared with the initial data by means of non-parametric criteria.

The experimental data were obtained from 102 exposures to microwaves, which yielded 918 20-s portions of neuronal recording. The total number of analyzed indices were 13770.

The characteristics of initial burst activity preceding microwave exposure are presented in Table 1. The observed burst number depended on the threshold levels of them revelation. The burst number was reduced as the threshold was raised. The bursts revealed by 5-ms level possessed the highest spike frequency, the least number of spikes, and least duration. The bursts revealed by 20-ms level had apposite characteristics. The bursts revealed by 10-ms level possessed intermediate features.

Effect of microwave exposure on the burst activity revealed by the used threshold levels. The burst activity revealed by 5-ms level possessed minimum shifts, though statistically mainly significant, during and after irradiation. The burst activity revealed by 10-ms level showed a complex conversion. Distinctions between directions of changes of frequent and temporary indices are observed. Some increase of the former was accompanied by the considerable decrease of latter. The burst activity revealed by 20-ms level demonstrated opposite changes, particularly the decrease of spike frequency in a burst and the increase of the number of spikes in a burst, which increased the total portion of bursts in pulse flows.

Table 1. The characteristics of burst activity before irradiation

(1) burst number per 20 s; (2) mean spike frequency in a burst (Hz); (3) mean spike number in a burst; (4) mean burst duration (ms); (5) burst duration: interburst interval duration ratio.

CONCLUSION

pulse flow neuron burst

The pattern of pulse flows of cortical neurons is the rather complicated phenomenon. Apart from properties of single neurons, action of a system of neuronal circuits are undoubtedly reflected in organisation of spike bursts [9]. In the present study characteristics of burst activity revealed by different threshold levels were described. It was established that 1-min low-intensity microwave irradiation produced rearrangements in pulse flows of cortical neuron populations. Directions of shifts depend on threshold levels of revelation of burst activity. The effect probably consists in reduction of short bursts and in promotion of that of large bursts. Such rearrangements in inner pattern of neuronal pulse flows point to alterations in processes of integration in the cortex under 1-min low-intensity microwave irradiation.

Investigations supported by the Grant of Russian Foundation of Fundamental Investigations No. 00-04-48139.

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