Firing Activity of the Weakly-Electric Fish Marcusenius Macrolepidotus (Mormyridae, Teleostei): Logarithmic Repartition of Inter-Pulse Intervals, and Sequential Inequality Testing

Abstract

AbstractThe electric activity of mormyrid fish consists of short electric pulses, all identical, but separated by variable time intervals (inter-pulse interval or IPI). Temporal structures are here extracted from the sequence of IPIs, to show non-randomness in mormyrid electric signal. The electric activity of isolated Marcusenius macrolepidotus is comparable to that of other mormyrids. The pulse waveform is biphasic and symmetrical, it lasts 0.3 to 0.5 ms and its amplitude is about 13 volts. The pulses follow each other with extremely variable IPIs (14-500 ms) with large instantaneous variations. Two new methods of analysis have been developed here. The first is based on the fact that when IPIs are grouped in populations, longer IPI populations have a wider range than shorter ones, a lognormal rather than a normal distribution. The IPIs are therefore classified according not to absolute- but to relative differences: binwidths are in percent, not in milliseconds. The second, non-parametric method (sequential inequality testing), was first used for neurone firing activity. It considers the sign (+ or -) of the difference between successive IPIs, and compares the sequence of these signs with a theoretical random model. When M. macrolepidotus is resting, the sequence of longer and shorter IPIs is not random, as shown by redundancies of patterns of + and - signs. These redundancies and the IPI populations are typical for each individual. When the fish increases its locomotor activity (either spontaneously for a few seconds, or provoked over more than 20 s) the IPIs are shorter. The patterns of + and - are different when the fish is swimming and when resting. Behavioral and electrophysiological data suggest that receiving conspecifics may recognize these rhythm features. The analysis methods developed here may be useful to other stochastic phenomena, such as neural firing activity.