Weakly electric sh (WEF) are one of nature's great curiosities. There are two major groups of WEF: mormryids from Africa and gymnotids from South America. Both groups live in muddy rivers where visual perception is impossible. Like elasmo-branchs, they, too, use underwater electric eld for guidance. There is one important difference, however. Instead of only sensing electric elds passi vely in their environments, WEF have electric organs that produce a periodic (ac) electric eld of constant frequency. The electric organ output is called the electric organ discharge (EOD) in the literature. The electric organ is generally located on the sides of the WEF's near the tail.
WEFs of both groups of sh sense the distortion of their self-generated electric elds by nearby animate and inanimate objects with conductivities different from the water in which they are swimming. Gymnotids sense the temporal phase shift of their EODs caused by an external object close to part of their body relative to the EOD phase sensed at a distance from the object. Mormyrids sense reactive (capacitive)-caused distortions of their ac EOD waveform caused by a nearby organic object (von der Emde, 1999). (Inanimate objects such as rocks do not have signi cant capacitance at the audiofrequencies of the EOD, and thus produce no phase shift.)
It should be pointed out that the electric guidance system of WEF has a relatively short range (10s of cm) because of the way an electric eld caused by a voltage dipole is attenuated with distance. It is easy to show that the (scalar) electric potential of a dipole attenuates as R-2, while the electric eld (v ector) magnitude drops off as R-3, where R is the distance from the observation point to the center of the dipole, d is the dipole separation, and R > d (Corson and Lorrain, 1962).
Several hundred knollenorgans are distributed over the body of a mormyrid sh; they are concentrated on the head and on the sides near the tail. Mormyrids also have smaller mormyromast electroreceptors, and a scattering of ampullary electrore-ceptors. Figure 2.5-2 illustrates a typical knollenorgan. One to eight electrosensory cells with diameters of 40 to 60 ^m are clustered in a hemispherical mass inside the organ cavity. Each receptor cell makes electrotonic synaptic contact with the terminal branches of the single afferent nerve ber . There is a loose plug of epithelial cells lling the skin pore o ver the organ. Knollenorgan afferents travel in the lateral line nerve to the electrosensory, anterior lateral line lobe in the sh's brain, thence information is sent to other parts of the CNS. Much is known about the central processing of knollenorgan sensory information, and how it interacts with the brain center that controls the EOD. The interested reader is urged to see Heilingenberg (1991) to pursue this topic in further detail.
Knollenorgans can be organized into two subclasses. The T units behave like tuned band-pass lters ha ving tuning peaks ranging from 100 Hz to 20 kHz, depending on the mormyrid sh species. The T nerves re on the rising edge (positi ve rst derivative) of the stimulus, one spike per EOD cycle positive zero crossing, with very little phase jitter. P units re asynchronously with the EOD; their instantaneous frequency of ring is e vidently proportional to the average peak amplitude of the stimulus (or maybe its rms value). Taken together, the P and T afferent signals from one part of the sh's body code the exact phase of the ac electric eld in the w ater at that point, and also its average peak amplitude. The CNS compares left and right signals from corresponding parts of the body and looks for difference-mode signals, i.e., asymmetry between left and right sides, as a signal that some nearby object is distorting the ac E eld from the EOD.
Rasnow and Bower (1999) have pointed out that a weakly electric sh has the task of mapping the features of an object in real (underwater) space into its perceptual space. Object features surely include physical parameters such as the range and bearing of the object from the sh, and the object' s size, shape, conductivity, and
dielectric constant. Perceptual features are based on the "electric image" features which include the amplitude and phase distribution of the E eld along the sh's body, including left vs. right asymmetries in the sensed ac E eld amplitude and timing. From this sensed information the sh can make simple decisions whether the object is dangerous, food, a potential mate, or merely something like a rock in its immediate environment.
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