324
Gerhard von der Emde
Our experiments finally resolved this puzzle. We used natural EOD stimuli,
which were prerecorded in the presence of several types of capacitive and resistive
objects. The objects caused predictable distortions of the natural signal wave form.
When these stimuli were played back to single mormyromast receptor organs, Aand B-cells responded differently. Responses of A-cells were not much affected
by wave form distortions. B-cells, on the other hand, were extremely sensitive to
even the smallest wave form changes. They responded as if the signal amplitude
had increased strongly, even though it had been kept constant throughout the
whole experiment. In summary, both receptor cell types responded to changes in
signal amplitude but only the B-cells additionally responded to natural changes in
signal wave form (von der Emde and Bleckmann 1997).
These results led us to develop a hypothesis about how the central nervous
system of a mormyrid performs capacitance detection. In the presence of a purely
resistive object only amplitude changes and no wave form distortions occur. When
the brain compares A- and B-receptor responses, e.g:, by subtracting the
information they provide, it will detect no difference between the two. Now
consider the result with a capacitive object. Such an object will change stimulus
amplitude, but in addition it will distort the wave form of the stimulus. The Areceptors still respond only to stimulus amplitude. B-receptors, on the other hand,
are strongly excited by the wave form distortions and therefore respond much
stronger than the A-cells. A subtraction of A- and B-receptor responses by the
brain will reveal these differences, which are proportional to the amount of wave
form distortion and thus proportional to the capacitive value of the object. This
mechanism would allow mormyrids to measure capacitive object properties
quantitatively and therefore could explain our behavioral results. Recently
obtained electrophysiological results suggest that our proposed subtraction
mechanism might indeed exist in the lateral nucleus of the torus semicircularis of
the G. petersii brain (von der Emde and Mohr, in prep.).
3.2 Capacitance Detection in South American Electric Fish
Apart from the fact that electric organs, electroreceptors and the electroreceptive
brain structures of African mormyriforms and South American gymnotiforms
developed separately, they are remarkably similar. This may relate to two main
factors: ( 1) Both groups used preadaptations when developing electric organs
(from muscle tissues), electroreceptors (primitive ampullary receptor organs and
mechanoreceptive neuromasts), and corresponding brain structures (the octavolateralis system). The properties of these existing older structures set the tracks
along which electrogenic and electrosensory structures of both fish groups
developed during evolution. (2) Electric signals work best for orientation and
communication in tropical, freshwater habitats with water of low electrical
conductivity. The environmental conditions under which mormyriforms and
Gerhard von der Emde
Our experiments finally resolved this puzzle. We used natural EOD stimuli,
which were prerecorded in the presence of several types of capacitive and resistive
objects. The objects caused predictable distortions of the natural signal wave form.
When these stimuli were played back to single mormyromast receptor organs, Aand B-cells responded differently. Responses of A-cells were not much affected
by wave form distortions. B-cells, on the other hand, were extremely sensitive to
even the smallest wave form changes. They responded as if the signal amplitude
had increased strongly, even though it had been kept constant throughout the
whole experiment. In summary, both receptor cell types responded to changes in
signal amplitude but only the B-cells additionally responded to natural changes in
signal wave form (von der Emde and Bleckmann 1997).
These results led us to develop a hypothesis about how the central nervous
system of a mormyrid performs capacitance detection. In the presence of a purely
resistive object only amplitude changes and no wave form distortions occur. When
the brain compares A- and B-receptor responses, e.g:, by subtracting the
information they provide, it will detect no difference between the two. Now
consider the result with a capacitive object. Such an object will change stimulus
amplitude, but in addition it will distort the wave form of the stimulus. The Areceptors still respond only to stimulus amplitude. B-receptors, on the other hand,
are strongly excited by the wave form distortions and therefore respond much
stronger than the A-cells. A subtraction of A- and B-receptor responses by the
brain will reveal these differences, which are proportional to the amount of wave
form distortion and thus proportional to the capacitive value of the object. This
mechanism would allow mormyrids to measure capacitive object properties
quantitatively and therefore could explain our behavioral results. Recently
obtained electrophysiological results suggest that our proposed subtraction
mechanism might indeed exist in the lateral nucleus of the torus semicircularis of
the G. petersii brain (von der Emde and Mohr, in prep.).
3.2 Capacitance Detection in South American Electric Fish
Apart from the fact that electric organs, electroreceptors and the electroreceptive
brain structures of African mormyriforms and South American gymnotiforms
developed separately, they are remarkably similar. This may relate to two main
factors: ( 1) Both groups used preadaptations when developing electric organs
(from muscle tissues), electroreceptors (primitive ampullary receptor organs and
mechanoreceptive neuromasts), and corresponding brain structures (the octavolateralis system). The properties of these existing older structures set the tracks
along which electrogenic and electrosensory structures of both fish groups
developed during evolution. (2) Electric signals work best for orientation and
communication in tropical, freshwater habitats with water of low electrical
conductivity. The environmental conditions under which mormyriforms and