Comparison of ongoing compound field potentials in the brains of invertebrates and vertebrates

(1) Ongoing compound field potential fluctuations of higher brain centers (the micro-EEG of some authors) are considered as a biological phenomenon, a sign of the activity in the organized assemblage of cells. Such activity has been compared in several taxa with quite different brain structure to look for possible evolution in the form of the field potentials and for possible explanations of differences and similarities. (2) Recordings were made with semimicroelectrodes in the neuropile of the cerebral ganglion of the mollusc, Aplysia, with comparative observations on Helix, and the arthropods Limulus, Melanoplus, and Cambarus, and in or on the cerebral cortex and optic tectum of rays, cats and rabbits, with comparative observations on sharks, bony fish, turtles and geckos in unstimulated resting or generalized arousal states. Manipulations of state did not alter the main findings. (3) Power spectra in the cerebral ganglia of various higher invertebrates are similar; activity is fast and spikey (with the exception of Octopus). Integrated energy above 50 Hz exceeds that from 2-50 Hz and falls slowly with frequency; in Aplysia the power spectrum falls less than 10 dB between 10 and 300 Hz. In vertebrates from fish to mammals activity is similar in being mainly slow (less than 40 Hz); it commonly falls greater than 20 dB between 10 and 50 Hz. (4) Amplitude is low in invertebrates and lower vertebrates. RMS voltage in Aplysia (3-300 Hz, reference electrode remote) is typically less than 10 microV; in the ray optic tectum less than 25 microV (2-50 Hz); in the dorsal cortex of the gecko less than 30 microV, in the cat cortex greater than 85 microV. In the vertebrates amplitude does not change greatly with small shifts in electrode position, as it does in invertebrates. (5) Coherence decline with distance, measured tangentially at different electrode separations in the millimeter range, is used as an estimator of synchrony. Averaged coherence between loci 1 mm apart is negligible in Aplysia in any band from 3 to 100 Hz; in the ray tectum it is low, 0.25-0.5 between 3 and 16 Hz. In the turtle dorsal pallium it is higher, at 2 mm, 0.6-0.75 in this band. In the rabbit cortex coherence is even higher, typically greater than 0.7 at 1 mm, and greater than 0.3 at 4 mm in this band. (6) Band-pass filtered electrograms, ca. one octave wide, in all species show constant waxing and waning in each band; amplitude is not maintained even for a second. (ABSTRACT TRUNCATED AT 400 WORDS)