|
Weakly electric fish use their electric organ discharge (EOD) for electrolocation as well as for communication. One class of communication signals are so-called chirps, transient increases in the frequency of their otherwise very stable quasi-sinusoidal EOD. In agonistic contexts, Apteronotus leptorhynchus emit type II chirps with a small frequency excursion and a short duration. When two fish interact, both their EODs superimpose to a beat, an amplitude modulation of the frequency difference between the two EODs. Chirps cause an amplitude and a phase modulation of this beat, which is encoded by the electroreceptor (p-unit) population. As has been shown, the response of p-units is largely enhanced in response to type 2 chirps presented at low beat frequencies. This has been explained by a model of spike-frequency adaptation with a cutoff frequency of 23 Hz, acting as a high pass filter for the transient chirp. However, both fish emit chirps at a similar rate. If the emitting fish has the lower EOD frequency of the two, the chirp changes the signal from a faster oscillation to a slower one and might therefore cause the opposite effect in the cells. Also, until recently small chirps were thought to be relevant only at small beat frequencies up to 30Hz. A recent study has shown, however, that behavioral responses to type II chirps such as attack rates, increase with beat frequency. At these higher beat frequencies the signal is already above the cutoff frequency of the adaptation. How chirps are encoded in this regime, is not known so far. We here fit a model of spike-frequency adaptation to results from single-unit recordings of receptor cells. That way we assess the role of adaptation in the encoding of communication signals at different background oscillation. The EOD frequency of each fish depends on its gender, size and social status. Hence, different beat frequencies reflect different social encounters. Spike-frequency adaptation might play a different role in response to communication signals depending on the social context. |
|