Electric Organ Discharge Frequency Modulations Evoked in Response to Jamming Stimuli in the Weakly Electric Brown Ghost Knifefish (Apteronotus Leptorynchus)
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Gaul, Timothy B.
Gymnotiform electric fish emit a weak electric organ discharge (EOD) for the purpose of electrolocation. Although individual fish emit at unique EOD frequencies, they can actively modulate the frequency of their EODs during social interactions with conspecifics. Gradual frequency shifts occur primarily during the jamming avoidance response (JAR); a behavior in which an animal slowly shifts the frequency of it EOD away from a similar and interfering frequency of a conspecific. An animal will perform the JAR to minimize the detrimental effects of a neighbor’s EOD on its own ability to electrolocate. The JAR has been primarily studied in two genera of electric fish (Eigenmannia and Apteronotus) using a frequency clamp stimulus regime. Frequency clamped stimuli are electronically controlled to maintain a specific frequency difference (DF) between the animal’s EOD frequency and the frequency of the jamming signal - regardless of how the animal responds to the jamming signal. The sign of the DF is also switched at regular intervals to frequencies above (+DF) and below (-DF) the animal’s EOD frequency. This forces the animal to alternate between different behavioral responses to avoid the effects of +DF and -DF jamming stimuli. Eigenmannia responds to the onset of a +DF clamped stimulus by lowering its frequency below its resting level. When the sign of the DF is reversed (i.e. -DF) the animal raises its EOD frequency above its resting level. However, Apteronotus cannot decrease its EOD frequency below the resting level. Instead, it responds to the onset of a +DF signal by performing a small increase in frequency that has been referred to as the non-selective response (NSR). When the sign of the clamped stimulus is switched to a -DF condition the animal performs a JAR that appears to ride on top of the modulations of the NSR. However, the repeated switching of the sign of the DF has made it difficult to delineate between the relative expression of the NSR and JAR under frequency clamp conditions. Moreover, the functional significance of the NSR has remained an enigma, since it has not been described in the context of jamming avoidance. In attempts to clarify these issues, the present study used non-frequency clamped stimuli to examine jamming induced EOD frequency modulations in Apteronotus. Non- clamped stimuli are set at fixed frequencies before being delivered to the animal. This allows the animal to adjust its EOD frequency independently of the stimulus signal. Moreover, the effects of +DF and -DF stimuli can be examined in separate behavioral tests. The results indicate that Apteronotus responds to -DFs between 1 - 6 Hz by shifting its EOD frequency away from that of the jamming signal (i.e. a conventional JAR). However, in contrast to the clamped results, the animal can increase its EOD frequency to within 0.4 ± 0.07 Hz of a +DF signal. This type of matching response (MR) was only performed in response to +DFs within 6 Hz of the animal’s basal EOD frequency, and it was not observed in response to -DF stimuli. In some cases the MR was followed by a JAR which crossed over and shifted away from the frequency of the +DF stimulus signal. The behavioral transition between the MR and JAR appears to be related to the magnitude of the DF and the intensity of the jamming signal. Thus, Apteronotus may use two different behavioral strategies to avoid the detrimental effects of a jamming signal. The first strategy, the MR, may represent the modulations that have been previously described under clamped conditions as the NSR. The MR may temporarily alleviate the detrimental effects of a +DF jamming signal by reducing the frequency difference between the animal’s EOD and that of the jamming signal. The second strategy, the JAR, can be performed either alone in response to -DF jamming stimuli, or in conjunction with the MR under more intense +DF jamming conditions. This response alleviates the detrimental effects of a jamming signal by increasing the frequency difference between the EOD and that of the stimulus signal.
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