The active electric sense of pulse gymnotiformes

Abstract

Pulse Gymnotiformes generate discrete pulsatile electric discharges to polarize nearby objects, which in turn projects electric images onto a cutaneous electroreceptor mosaic. The active electrosensory channel of these fish is characterized by complex electric organs, quasi-periodically activated via an electromotor coordination system driven by a pacemaker in the medulla. Receptor density, number of subtypes, and central projection are maximal in the perioral region, forming an electrosensory fovea that receives signals from a “foveal receptive field” where objects are polarized by a coherent and coaligned electric field. Foveal electroreceptor subtypes are well-tuned to the main frequencies of the power spectral density, and have distinct, extreme sensitivities to the phase spectra of the self-generated signals, which grants the possibility for evaluation of impedance-related object qualia (“electric color”). Only a single receptor type is found scattered across sides of the body, which receives signals from a “peripheral field” where objects are polarized by site-dependent (in both time course and direction) electric fields. While pulse and wave emitting Gymnotiformes have homologous anatomical similarities, they show important differences associated with object polarization strategies, electroreceptor responsiveness to the self-generated carrier, and central signal processing. In contrast, pulse Gymnotiformes share their pulsatile polarization strategy and some peripheral mechanisms for discriminating impedance-related qualia with the phylogenetically distant Mormyroidea. Convergent evolution has given rise to computational analogies in the two taxa, despite significant structural differences in the anatomical and functional organization of their central nervous systems. In a strikingly similar manner, in both taxa each electric organ discharge generates a neural electric image, which is evaluated against a moving average of the preceding images, reducing the cost of image processing and facilitating the detection of novel events. This chapter focuses on the active electrosensory channel of pulse Gymnotiforms, stressing the features that define their identity as a functional group.