The Jamming Avoidance Response inEigenmannia revisited: The structure of a neuronal democracy

Summary

The Jamming Avoidance Response (JAR) is a gradual shift in frequency of the fish's electric organ pacemaker in an attempt to increase a small difference,δf, between the fundamental frequency of electric organ discharges (EODs) of a conspecific and that of the animal's own EODs. The JAR can be elicited in curarized animals by replacing the silenced EOD with a periodic electric stimulus, S1; and by simulating EODs of a conspecific with a periodic stimulus, S2, whose frequency differs byδf from the frequency of S₁. Similar to the natural JAR, the frequency of the pacemaker will rise and fall in response to negative and positiveδfs respectively, provided that S₁; but not S₂, shares critical features with the EOD of the animal. Ambivalent or even opposite responses (Anti-JARs) may result if S₁ lacks critical EOD features (Fig. 1). In search of these features the following results were obtained.

• 1. To elicit JARs, S₁ need not be phaselocked to the pacemaker. The JAR can thus be driven exclusively by electroreceptive afference, without reference to the pacemaker.

• 2. S₁ and S₂ may be pure sinewaves as long as their field geometries differ sufficiently. Higher harmonics, which may be added to a sinewave to mimic the EOD wave shape, are required only if S₁ and S₂ have identical geometries, i.e., if they are presented through the same pair of electrodes. The animal may thus use two different strategies to determine the sign of theδf: one which is based on differences in stimulus field geometries and one which is based on the presence of higher harmonics. Only the former is considered in the following.

• 3. The S₁, but not the S₂, field geometry should approximate the natural EOD field geometry. To the extent that this condition is violated, sufficiently high S₂ intensities may elicit Anti-JARs (Fig. 4).

• 4. Evidence is given that the JAR is controlled, in a cumulative manner, by local interactions of neighboring electroreceptive fields on the animal's body surface which, as a consequence of different S₁ and S₂ field geometries, experience different degrees of contamination of S₁ by S₂. Simultaneous stimulations of remote areas of body surface result in almost linear summation of their associated effects on the pacemaker (Figs. 5, 6). Theoretically, no unitary central EOD representation is required.

• 5. Based on the results in 3. und 4., we propose that correct JARs are elicited to the extent that the majority of electroreceptors is predominantly driven by S_l rather than by S₂, and this condition is fulfilled to the extent that the S₁ field geometry approximates that of the natural EOD.

• 6. Effective S₂ stimuli have a periodicity near that of the EOD (S₁) fundamental frequency, f. This includes all stimuli with a power peak at a frequency of n·f+δf, n=l,2,3,t (Fig. 2), with the optimalδf being 3 to 8 Hz and identical for all n. Such stimuli cause consistent distortions in successive EODs (S₁ pulses), which gradually travel through the EOD (S₁) cycle (Fig. 3). This “motion” leads to periodic fluctuations in the amplitude of the joint signal, EOD (S₁)+S₂, and the phase of its positive zero-crossings with regard to those of the EOD (S₁ (Fig. 7). The modulation of these two variables can be represented by a motion along a closed graph in a two-dimensional state plane (Fig. 8), which is reproducedδf times per s. The direction of motion along this graph reflects the sign of theδf. Evidence is given that this motion is detected by a mechanism comparable to a motion detector in the realm of vision.