ALBERT

Notes: 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 Sl 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.

Notes: Summary The jamming avoidance response (JAR) is an increase or decrease in an electric fish's frequency of electric organ discharges (EODs), in order to avoid “jamming” of its electrolocation sense by another fish at a slightly lower or higher frequency. This study is concerned with the pulse speciesHypopomus occidentalis andGymnotus carapo, which fire their electric organs in brief pulses separated by intervals of silence. The JAR can be obtained in curarized preparations in which a train of EOD-mimic (S 1) pulses is scanned by a train of jamming pulses (S 2) mimicking a “foreign” fish at a slightly lower frequency (“left-right scan”) or a higher frequency (“right-left scan”); see Fig. 1. Thus the behavior is a function of a single temporal waveform, providing an experimentally convenient preparation in which to study a temporal pattern discrimination problem. Neurophysiological studies of the sensory encoding of JAR-eliciting stimuli show that burst duration coders (BDCs, Fig. 2), but not pulse markers (Fig. 3) or ampullary cells (Fig. 5), provide information related to normal scanning stimuli. BDCs fire a burst of spikes, phase-locked to the largeS 1 pulse; the burst is modulated by theS 2 scanning stimuli. These cells respond differentially to the EOD-mimic signal and the other fish's signals on the basis of amplitude. BDC responses to left-right and right-left scans are approximate mirror images (Fig. 2). This and other observations indicate that BDC activity reflects primarily the most recentS 1+S 2 waveform. The behavioral discrimination of left-right vs. right-left scans is invariant with respect to an inversion of the “foreign” fish's signal, yet this procedure reverses the responses of BDCs (Fig. 7). Consequently the sequence of excitation and inhibition from any one receptor is ambiguous. BDCs can be classified into four categories, based on the range ofS 1/S 2 latencies at which they respond and by their current direction sensitivity (Fig. 8):α-cells have lowest thresholds to negative (inward) current,β-cells to positive (outward) current, and theγ andδ types to both (Fig. 10). The categories are not sharp (i.e., intermediate types exist). The types are scattered along the length of the fish (Fig. 11). One proposal is that the central nervous system compares the relative timing of responses from the different classes of BDCs, in order to discriminate left-right vs. right-left scans. An alternative idea is that the JAR is controlled by the sequence of “gentle” vs. “strong” perturbations of overall electroreceptive feedback.

Notes: Summary Gymnotoid electric fish with pulse-type electric organ discharges (EODs) shorten (lengthen) their EOD intervals as pulses of a slightly slower (faster) train scan their EODs (Figs. 1, 2). They thus minimize the chance of pulse coincidence by transient accelerations (decelerations) of their EOD rate. Studies in curarized preparations demonstrate that this Jamming Avoidance Response (JAR) is controlled by electroreceptive input alone and without reference to an internal electric organ pacemaker-related signal (Fig. 8). A sufficient stimulus input consists of a train of strong, EOD-like stimulus pulses (S1), which mimic the animal's experience of its own EOD, and a train of small pulses (S2) of slightly different repetition rate, which mimic EODs of a neighbor. Correct behavioral responses require S1 pulses of sufficient intensity to recruit pulse-markertype receptors; also spatial and temporal patterns must closely resemble those of the animal's EOD. These features are of little significance for S2 pulses which, while scanning S1 pulses, only provide a small perturbation of electroreceptive feedback from S1 pulses. Inappropriate S1 stimulation impairs and sometimes reverses (Fig. 7) the behavioral discrimination of scan directions. The JAR is explained in terms of excitatory and inhibitory processes (Fig. 3) which are triggered by S2 stimulation, at specific phases within the S1 cycle (Figs. 4–6). The JAR in pulse species strongly resembles the JAR in wave-species (Bullock et al., 1972) and could be considered an evolutionary ancestor of the latter. It is a response to a particular novelty in electroreceptive feedback.

Notes: Summary South American pulse species of weakly electric fish change their frequency of electric organ discharges (EODs) to avoid jamming of their electrolocation sense by other fish at similar frequencies (Fig. 1). If a ‘foreign’ fish is at a lower frequency its pulses scan from negative to positive latencies with respect to the fish's EOD, resulting in a strong acceleration in EOD frequency. Pulses of another fish at a higher frequency scan from positive to negative latencies, evoking deceleration, or only a very small acceleration. Thus the jamming avoidance response (JAR) takes the form of a differential response to opposite scan directions. Curarized specimens ofHypopomus occidentalis andGymnotus carapo were used for behavioral studies of the JAR. A regular train of stimulus pulses mimicked the curare-blocked EOD. A series of foreign pulses just preceding the EOD-mimic pulse (negative latencies) evoked large EOD accelerations, while near-coincident pulses (near-zero latencies) resulted in decelerations, or only small accelerations (Fig. 2). If foreign pulses were placed first at a negative latency, then at a near-zero latency, a large acceleration resulted, while the opposite sequence produced deceleration (Fig. 4). Therefore these ‘double latency clamps’ seem to mimic scanning stimuli, and lend support to a theory for the pulse species' JAR: pulses at negative latencies activate an excitatory (E) process (resulting in acceleration), while those at near-zero latencies activate an inhibitory (I) process (preventing response to the first, and/or causing deceleration); the process activated first dominates the response. A previous neurophysiological study (Baker 1980) of electroreceptor encoding of scanning stimuli suggested two alternative theories of how receptor information could be used to drive the E and I processes. One way would be for αe and δ type receptors (which respond to foreign pulses at negative as well as nearzero latencies) to activate E, while α, β, and/or γ types (which respond only to near-coincident pulses) could activate I. An alternative mechanism would be to pool responses from all receptor types and allow ‘weak’ perturbations of the overall electroreceptive afference to drive E, while ‘strong’ disturbances of all types, which occur only with near-coincident foreign pulses, would activate I. The latter hypothesis was supported by results of ‘double amplitude modulation’ experiments (Fig. 6) in which pulses at fixed latencies are amplitude-modulated; small vs. large modulations resulted in directional responses very similar to those obtained from scans and double latency-clamps.