Odor input generates approximately 1.5 Hz and approximately 3 Hz spectral peaks in the helix pedal ganglion

In 1999 we reported that odorants evoke in the Helix pedal ganglion (PG) activity patterns which are largely odorant-specific and related to the nature of odor and its behavioral output. Notably, some activities (for example, approximately 1.5 and approximately 3 Hz), nonspecific to odorants, were consistently evoked in PG. The present contribution goes farther in a deeper survey of the intrinsic and odorant-evoked activities of PG with special weight on the nonspecific fluctuations. We address the following questions. (i) What are the features of the activities? (ii) Are they comparable to the activities found in the motor systems of the other invertebrates? (iii) To what functions can they be related? Three main frequency components represented by power peaks at <1 Hz, 1-2 Hz and 2-8 Hz seem to feature the response activities of PG. (a) The aversive odorants induce odorant-specific patterns represented by peak power frequencies at <1 Hz. (b) The oscillation at approximately 1 Hz, which exists intrinsically in the Helix PG, can be specifically enhanced by appetitive odors. Activities induced in the procerebrum (PC), the visceral ganglion (VG) and PG by appetitive odorants, such as ethanol and apple, peak at 1.3-2 Hz, whereas those induced by aversive ones, such as formic acid and onion at <1 Hz. (c) The 2-8 Hz components always accompany the odorant-evoked activities of the PG either as the second or third strongest component, or in the form of conspicuous, long-lasting approximately 3 Hz oscillations. (d) The nonspecific odor-evoked 1-2 Hz and approximately 3 Hz activities, and the intrinsic approximately 1 Hz activity of the PG seem to be interrelated by a degree of mutual exclusion. We may therefore consider these activities as elementary, slow components that are involved in the processing of signals in this ganglion. It can be inferred from the findings in other invertebrates that the 1-3 Hz spontaneous discharge is strongly connected with motor activity that involves the feedback mechanism of the procerebro-cerebro-buccal or -procerebro-cerebro-pedal circuit. Our approach differs from most others reported so far in the following aspects: (i) use of gross steel electrodes for recording population activities; (ii) lengthy stimulation (10 min); (iii) long observation during and after stimulation; (iv) power spectral presentation of temporal evolution of activity patterns; (v) estimation of peak power frequency by Frequency-Amplitude Plot (FAP) (obtained from signals averaged in the frequency domain; a method based on systems theory).