Electroencephalographic studies in toads

Abstract

During two consecutive years, experiments were done at monthly intervals on the EEG pattern of acute, curarized and non-anesthetized, or freely moving toads ( Bufo arenarum Hensel), bearing chronically implanted electrodes. Animals were kept during several weeks under fixed environmental conditions (22°C temperature, humidity at saturation and lighting 16 h daily), before observations were made. According to the data obtained there exists a marked seasonal variation in the “spontaneous” electrical activity of the brain as well as in the EEG responses to sudden changes in the degree of illumination. In effect, during spring and summer a typical fusiform “alpha-like” rhythm could be detected over the olfactory bulb and forebrain, in over 90% of recordings. Also, the appearance of spindles at this time of the year was an outstanding phenomenon. Simultaneously, a very high sensitivity to changes in lighting was observed, with a marked increase in the frequency and voltage of recorded brain potentials. On the other hand, during winter, in spite of the fact that external conditions remained the same and were carefully controlled, the incidence of “alpha-like” discharges was as low as 6.6%. Also, a complete abolition of spindles and a clear diminution in electrographic evoked responses to changes of lighting was evident.