Power spectra of ongoing activity of the snail brain can discriminate odorants

Abstract

To test the hypothesis that different odorants are likely to cause distinctive changes in the ongoing electrical activity of populations of olfactory cells, we investigated field potentials (FP) in the Helix brain and their alterations by odorants as seen by semimicroelectrodes in an isolated preparation of the rostrum with its olfactory organ and whole central nervous system. Five pure chemicals and two natural odorants were applied as stimulants. Signals recorded both from the procerebrum (PC) and the visceral ganglion (VG) were analyzed. In the PC the five pure chemical odorants induce stimulus-specific characteristic responses, mainly in the low frequency range (<15 Hz). Regardless of odor intensity, the frequency of the peak power of sustained induced activity is constant for each chemical: ammonia at 0.2±<0.02 Hz; formic acid at 0.36±0.03 Hz; 2-pentanol at 0.48±0.04 Hz; 2-butanol at 0.67±0.03 Hz; ethanol at 1.31±0.09 Hz (means±95% confidence limits). These peak power frequencies, which we define as ‘odor-specific frequencies’, are confined to the low frequency range of<2.5 Hz. Those of natural odorants are: onion at 0.36±0.14 Hz and apple at 1.1±0.25 Hz. The activities evoked in the PC propagate to VG. The order of behavioural aversion determined by withdrawal reactions of the tentacles, 1% ammonia>formic acid>2-pentanol>2-butanol>ethanol, coincides with ‘the order of molecular affinity’ as well as with the sequence of peak power frequencies. There seems to be a strong correlation among behavioural valence, chemical nature of an odorant, and odor-specific frequency. The finding that, in the Helix olfactory center, odor input is processed as odorant specific low frequency FP activity may represent some general phenomena of olfactory information processing.