Slow waves during sleep in crayfish: a time-frequency analysis.

NREM phases of sleep in vertebrates are characterized by slow waves. Crayfish also sleeps while lying on one side on the surface of the water. At this time the numerous spikes on an almost flat base line generated by the brain when alert are replaced by slow waves of 15-20 Hz. In this work, we conducted experiments to determine the temporal relationship between the lying on one side position and the brain slow waves. We videotaped chronically implanted animals to detect their body position and simultaneously recorded their brain electrical activity. To analyze brain electrical activity, we developed a wavelet based method and correlated the results with body position. Among results are: (a) during sleep signals in the frequency range 30-45 Hz show a large decrease in power; (b) sleep slow waves are generated 1-2 min after the animal lies on one side and are maintained throughout the whole period in such position. We conclude that the strong correlation between brain slow waves and lying on one side position further indicates periods of true sleep in these animals.