Bursting Activity in Weakly Electric Fish - Computational Neuroscience: A Comprehensive Approach - page 217

is specialized in processing fast variations in stimulus amplitude and phase. It is
quite fascinating that electrosensory processing in fish and auditory processing in
barn owls and bats have evolved similar computational algorithms for time coding
(e.g., [21, 66, 67]). The multiple two-dimensional topographical representations of
the sensory surface (electroreceptors in the skin of the fish) within the brain are found
similarly in the visual system where there are multiple topographical representations
of the retina [62]. Additionally, the principal electrosensory neurons in the hindbrain
come as ON- and OFF-types, have center-surround receptive fields, and as in the case
of mammalian thalamic neurons (e.g., [29, 107, 111]), their responses are shaped by
descending feedback.

8.2.1 Behavioral significance of electrosensation

Electroreception comes in two types, passive and active. The passive sense takes ad-
vantage of the electric fields generated by living organisms or, as has been shown in
sharks, the electromagnetic field of the earth (e.g., [63]). Unlike passive electrosen-
sation and most other sensory modalities, active electrosensation relies on signals
originating from the animal itself. The fish generates an electric field through dis-
charge of an electric organ extending along most of the caudal part of its body ( Fig-
ure 8.3 ). The Gymnotiformes are one of two groups of teleosts that independently
evolved active electrosensing [88]. Fish of the two Gymnotiform genera treated here,
Eigenmannia and Apteronotus , produce a quasi-sinusoidal electric organ discharge
(EOD) waveform with frequencies between 200 and 1200 Hz, the exact range being
species-specific.
Objects or animals with impedance different from that of water perturb the electric
field surrounding a fish. Electroreceptors in the skin monitor these distortions and
thus provide information about obstacles, approaching predators, or prey (Figure 8.3;
[2, 89, 90]). Nearby conspecifics also engage in electric communication, for example
in the context of courtship [48, 55, 87]. Thus, the active electrosense allows weakly
electric fish to forage and to communicate under conditions when other senses are
more or less useless as is the case in their natural habitat: They are nocturnal animals
and live in turbid tropical freshwaters, which strongly limits the usefulness of vision.
Similar to echolocation in bats, active electrosensation opens an ecological niche
that is safe from most diurnal predators. Additionally, it opens a new channel for
intraspecific communication.

8.2.2 Neuroanatomy of the electrosensory system

Two sets of primary afferents transmit information on electric field perturbations
from electroreceptors in the skin to the first central processing stage in the hindbrain,
the electrosensory lateral line lobe (ELL). So-called T-receptor afferents fire strictly
phase-locked to each cycle of the EOD, thus carrying information about phase distor-
tions [103]. We will, however, focus on the amplitude-coding pathway that involves
a different set of afferents, P-receptor afferents. These nerve fibers fire action po-
tentials in a probabilistic fashion (thus the ”P”) depending on EOD amplitude (see