Abstract
Mormyrid fish rely on reafferent input for active electrolocation. Their electrosensory input consists of phase and amplitude information. These are encoded by differently tuned receptor cells within the Mormyromasts, A- and B-cells, respectively, which are distributed over the animal’s body. These convey their information to two topographically ordered medullary zones in the electrosensory lateral line lobe (ELL). The so-called medial zone receives only amplitude information, while the dorsolateral zone receives amplitude and phase information. Using both sources of information, Mormyrid fish can disambiguate electrical impedances. Where and how this disambiguation takes place is presently unclear. We here investigate phase-sensitivity downstream from the electroreceptors. We provide first evidence of phase-sensitivity in the medial zone of ELL. In this zone I-cells consistently decreased their rate to positive phase-shifts (6 of 20 cells) and increased their rate to negative shifts (11/20), while E-cells of the medial zone (3/9) responded oppositely to I-cells. In the dorsolateral zone the responses of E- and I-cells were opposite to those found in the medial zone. Tracer injections revealed interzonal projections that interconnect the dorsolateral and medial zones in a somatotopic manner. In summary, we show that phase information is processed differently in the dorsolateral and the medial zones. This is the first evidence for a mechanism that enhances the contrast between two parallel sensory channels in Mormyrid fish. This could be beneficial for impedance discrimination that ultimately must rely on a subtractive merging of these two sensory streams.
Highlights
Neuronal maps in sensory physiology have been studied from at least two perspectives, defining a map as a neuronal representation that is based on the topography of the receptor array and/or as a topographic neuronal representation of features that are computed independently from the topography of the receptor array
No evidence for convergence between both sensory streams has been found at the level of the electrosensory lateral line lobe (ELL) in previous works [20,31]
As the medial and the dorsolateral maps of the ELL are interconnected [39] and phase information is overrepresented in the part of the dorsolateral zone (DLZ) map that receives input from the foveal chin appendix [32], this lack of interzonal processing in the ELL is surprising
Summary
Neuronal maps in sensory physiology have been studied from at least two perspectives, defining a map as a neuronal representation that is based on the topography of the receptor array and/or as a topographic neuronal representation of features that are computed independently from the topography of the receptor array.
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