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@ -285,7 +285,7 @@ Here we search for such weakly nonlinear responses in electroreceptors of the tw
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We explored a large set of electrophysiological data from primary afferents of the active and passive electrosensory system, P-units and ampullary cells, that were recorded in the brown ghost knifefish \textit{Apteronotus leptorhynchus} to search for weakly nonlinear responses that have been predicted in previous theoretical work\citep{Voronenko2017}. Additional simulations of LIF-based models of P-unit spiking put the experimental findings into context\notejg{very unspecific}. We start with demonstrating the basic concepts using example P-units and respective models and then compare the population of recordings in both cell types.
We explored a large set of electrophysiological data from primary afferents of the active and passive electrosensory system, P-units and ampullary cells \citep{Grewe2017, Hladnik2023}, that were recorded in the brown ghost knifefish \textit{Apteronotus leptorhynchus}. We re-analyzed this dataset to search for weakly nonlinear responses that have been predicted in previous theoretical work \citep{Voronenko2017}. Additional simulations of LIF-based models of P-unit spiking help to interpret the experimental findings in this theoretical framework. We start with demonstrating the basic concepts using example P-units and respective models and then compare the population of recordings in both cell types.
\subsection{Nonlinear responses in P-units stimulated with two frequencies}
Without external stimulation, a P-unit is driven by the fish's own EOD alone (with a specific EOD frequency $f_{\rm EOD}$) and spontaneously fires action potentials at the baseline rate $r$. Accordingly, the power spectrum of the baseline activity has a peak at $r$ (\subfigrefb{fig:motivation}{A}). In the communication context, this animal (the receiver) is exposed to the EODs of one or many foreign fish. Superposition of the receiver's EOD with an EOD of another fish with frequency $f_1$ results in a beat, a periodic amplitude modulation of the receiver's EOD. The frequency of the beat is given by the difference frequency $\Delta f_1 = f_1 - f_{\rm EOD}$ between the two fish. P-units encode this beat in their firing rate \citep{Bastian1981a} and consequently the power spectrum of the response has a peak at the beat frequency (\subfigrefb{fig:motivation}{B}). A second peak at the first harmonic of the beat frequency is indicative of a nonlinear process that here is associated with the clipping of the P-unit's firing rate at zero \citep{Barayeu2023}. Pairing the fish with another fish at a higher beat frequency $\Delta f_2 = f_2 - f_{\rm EOD} > \Delta f_1$ results in a weaker response with a single peak in the response power spectrum, suggesting a linear response (\subfigrefb{fig:motivation}{C}). The weaker response to this beat can be explained by the beat tuning of the cell \citep{Walz2014}. Note, $\Delta f_2$ has been deliberately chosen to match the recorded P-unit's baseline firing rate.