Electric Fields and Electroreception
321
First, we wanted to find out whether weakly electric fish can perceive
capacitive object properties. We used a behavioral paradigm in which fish were
trained to discriminate between a capacitive and a resistive object. We utilized socalled dipole-objects for these experiments that were made out of plastic tubing
into which carbon electrodes were inserted (von der Emde 1990). The advantage
of these objects is that their electrical properties can be changed quickly from
outside the aquarium and they can thus be bestowed with any desired capacitive or
resistive value. With their help we determined which range of capacitive values an
electric fish can detect. It turned out that the range of detectable capacitances is
species-specific: it depends on the frequency composition of the electric signal
which the fish emits (see also Meyer 1982). Small fishes, which emit very short
pulses that contain high frequencies (e.g., Pollimyrus adspersus), detect small but
not large capacitances. Larger fishes that emit EODs of longer duration (e.g.,
Mormyrus rume) are superior in detecting larger capacitance. Fish of intermediate
size, like G. petersii, which emit a signal of 400 J.lS duration, can detect an
intermediate range of capacitances. In all cases, the detectable range of capacitive
values included the values typical of many natural capacitive (living) objects.
Natural capacitances range from a few nF, e.g., in small worms, to several tens of
nF in larger living objects, such as leaves of water plants or other fishes. The
smaller the object, .the smaller its capacitance. This may explain why small fish,
that can eat only small prey, produce short pulses that allow them to detect small
capacitances.
3.1 Capacitance Detection in African Mormyrid Fish
After having determined the detectable range of capacitance of different fish
species, it remained to be shown that mormyrids can perceive capacitive object
properties separate from resistive properties. The problem is that both capacitive
and resistive objects change the peak-to-peak amplitude of the local signal (see
above). If the fish just measure the EOD amplitude, as suggested by Lissmann and
Machin (1958), they should not be able to discriminate between a capacitive and a
resistive object with the same impedance value.
Additional training experiments clarified this point. None of our trained fish
ever had problems discriminating between a fixed capacitive object and a resistive
object, whose value was changed from one testing session to the next (Fig. 4A).
Even if the impedances of the two objects were numerically identical (which lead
to an identical EOD amplitude change as perceived by the electroreceptors), each
fish discriminated between them as quickly and accurately as it discriminated
between objects of completely different impedance values. Mormyrids obviously
perceive capacitive objects as objects having a quality different from resistive
objects. Psychophysical multidimensional scaling experiments further showed that
G. petersii can measure capacitive and resistive properties of natural objects
321
First, we wanted to find out whether weakly electric fish can perceive
capacitive object properties. We used a behavioral paradigm in which fish were
trained to discriminate between a capacitive and a resistive object. We utilized socalled dipole-objects for these experiments that were made out of plastic tubing
into which carbon electrodes were inserted (von der Emde 1990). The advantage
of these objects is that their electrical properties can be changed quickly from
outside the aquarium and they can thus be bestowed with any desired capacitive or
resistive value. With their help we determined which range of capacitive values an
electric fish can detect. It turned out that the range of detectable capacitances is
species-specific: it depends on the frequency composition of the electric signal
which the fish emits (see also Meyer 1982). Small fishes, which emit very short
pulses that contain high frequencies (e.g., Pollimyrus adspersus), detect small but
not large capacitances. Larger fishes that emit EODs of longer duration (e.g.,
Mormyrus rume) are superior in detecting larger capacitance. Fish of intermediate
size, like G. petersii, which emit a signal of 400 J.lS duration, can detect an
intermediate range of capacitances. In all cases, the detectable range of capacitive
values included the values typical of many natural capacitive (living) objects.
Natural capacitances range from a few nF, e.g., in small worms, to several tens of
nF in larger living objects, such as leaves of water plants or other fishes. The
smaller the object, .the smaller its capacitance. This may explain why small fish,
that can eat only small prey, produce short pulses that allow them to detect small
capacitances.
3.1 Capacitance Detection in African Mormyrid Fish
After having determined the detectable range of capacitance of different fish
species, it remained to be shown that mormyrids can perceive capacitive object
properties separate from resistive properties. The problem is that both capacitive
and resistive objects change the peak-to-peak amplitude of the local signal (see
above). If the fish just measure the EOD amplitude, as suggested by Lissmann and
Machin (1958), they should not be able to discriminate between a capacitive and a
resistive object with the same impedance value.
Additional training experiments clarified this point. None of our trained fish
ever had problems discriminating between a fixed capacitive object and a resistive
object, whose value was changed from one testing session to the next (Fig. 4A).
Even if the impedances of the two objects were numerically identical (which lead
to an identical EOD amplitude change as perceived by the electroreceptors), each
fish discriminated between them as quickly and accurately as it discriminated
between objects of completely different impedance values. Mormyrids obviously
perceive capacitive objects as objects having a quality different from resistive
objects. Psychophysical multidimensional scaling experiments further showed that
G. petersii can measure capacitive and resistive properties of natural objects