Encoding phase spectrum for evaluating 'electric qualia'

The most broadly expressed and studied aspect of sensory transduction is receptor tuning to the power spectral density of the incoming signals. Temporal cues expressed in the phase spectrum are relevant in African and American pulse-emitting electric fish showing electroreceptors sensing the signals carried by the self- and conspecific-generated electric organ discharges. This article concerns the role of electroreceptor phase sensitivity in American pulse Gymnotiformes. These fish show electroreceptors sharply tuned to narrow frequency bands. This led to the common thought that most electrosensory information is contained in the amplitude spectra of the signals. However, behavioral and modeling studies suggest that in their pulses, Gymnotiformes electroreceptors also encode cues embodied in the phase spectrum of natural stimuli. Here, we show that the two main types of tuberous primary afferents of Gymnotus omarorum differentially respond to cues embodied in the amplitude and phase spectra of self-generated electrosensory signals. One afferent type, pulse markers, is mainly driven by the amplitude spectrum, while the other, burst coders, is predominantly sensitive to the phase spectrum. This dual encoding strategy allows the fish to create a sensory manifold where patterns of 'electric color' generated by object impedance and other potential sources of 'colored' images (such as large nearby objects and other electric fish) can be represented.