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Gerhard von der Emde
signals can also be evoked by an object. Objects with complex impedances due to
a capacitive component cause phase shifts of the local signal within the electric
image (Caputi et al. 1998).
3 Detection of Object Properties by Active
Electrolocation
Lissmann and Machin (1958) proved that an electric fish can detect and
discriminate between objects of different electrical properties like a glass rod and
a metal rod. Glass is an electrical insulator and thus, if positioned close to an
electrolocating fish, will lead to a decrease in voltage at electroreceptors in the
fish's skin compared to the situation without the object present. In contrast, a metal
rod will increase the local voltage perceived by the electroreceptors. According to
Lissmann and Machin (1958), fish measure the local amplitude of their selfproduced signals and discriminate between objects on the basis of amplitude
changes. Each object is thought to project a black-and-white image onto the fish's
skin, like an electrical shadow of varying intensity.
When investigating objects found in the natural habitat of African weakly
electric fish, it turned out that inanimate objects, such as stones or dead plant
material, mainly have Ohmic resistances. The impedance of these objects is not
complex, i.e., its electrical resistance is identical for a DC signal and an AC signal
such as an EOD. For an electrolocating fish this means that all frequencies of its
EOD are similarly affected and no phase shifts occur. It follows that a resistive
object changes only the local EOD amplitude but not its wave form. In contrast,
all living matter possesses a complex impedance consisting of considerable
capacitive and a resistive (Ohmic) components (Schwan 1963; Heiligenberg 1973;
von der Emde 1990). Capacitive properties are particularly prominent in living
objects such as plants, other fishes, or insect larvae (which constitute the main
food items of African electric fish) presumably because of the many thin
membranes separated by electrolytes. Capacitive objects affect the locally
perceived EOD in several ways (von der Emde 1990): (I) The peak-to-peak
amplitude of the EOD changes depending on the value of the impedance of the
object. (2) The spectral composition of the EOD is altered because the lower
frequencies within the EOD are attenuated more strongly by a capacitive load than
the higher frequencies. (3) The wave form of the local EOD is distorted due to a
frequency-dependent phase shift of the EOD. In the wave form of a G. petersii
EOD, the relative amplitudes of the two phases change, and a third phase may
occur (Fig. 5 ). Because capacitive properties occur mainly in living objects,
capacitance detection might be particularly useful to electric fish in identifying
food items or other fishes. In order to better understand how weakly electric fish
perceive these capacitive object properties, a line of research was initiated in my
laboratory.
Gerhard von der Emde
signals can also be evoked by an object. Objects with complex impedances due to
a capacitive component cause phase shifts of the local signal within the electric
image (Caputi et al. 1998).
3 Detection of Object Properties by Active
Electrolocation
Lissmann and Machin (1958) proved that an electric fish can detect and
discriminate between objects of different electrical properties like a glass rod and
a metal rod. Glass is an electrical insulator and thus, if positioned close to an
electrolocating fish, will lead to a decrease in voltage at electroreceptors in the
fish's skin compared to the situation without the object present. In contrast, a metal
rod will increase the local voltage perceived by the electroreceptors. According to
Lissmann and Machin (1958), fish measure the local amplitude of their selfproduced signals and discriminate between objects on the basis of amplitude
changes. Each object is thought to project a black-and-white image onto the fish's
skin, like an electrical shadow of varying intensity.
When investigating objects found in the natural habitat of African weakly
electric fish, it turned out that inanimate objects, such as stones or dead plant
material, mainly have Ohmic resistances. The impedance of these objects is not
complex, i.e., its electrical resistance is identical for a DC signal and an AC signal
such as an EOD. For an electrolocating fish this means that all frequencies of its
EOD are similarly affected and no phase shifts occur. It follows that a resistive
object changes only the local EOD amplitude but not its wave form. In contrast,
all living matter possesses a complex impedance consisting of considerable
capacitive and a resistive (Ohmic) components (Schwan 1963; Heiligenberg 1973;
von der Emde 1990). Capacitive properties are particularly prominent in living
objects such as plants, other fishes, or insect larvae (which constitute the main
food items of African electric fish) presumably because of the many thin
membranes separated by electrolytes. Capacitive objects affect the locally
perceived EOD in several ways (von der Emde 1990): (I) The peak-to-peak
amplitude of the EOD changes depending on the value of the impedance of the
object. (2) The spectral composition of the EOD is altered because the lower
frequencies within the EOD are attenuated more strongly by a capacitive load than
the higher frequencies. (3) The wave form of the local EOD is distorted due to a
frequency-dependent phase shift of the EOD. In the wave form of a G. petersii
EOD, the relative amplitudes of the two phases change, and a third phase may
occur (Fig. 5 ). Because capacitive properties occur mainly in living objects,
capacitance detection might be particularly useful to electric fish in identifying
food items or other fishes. In order to better understand how weakly electric fish
perceive these capacitive object properties, a line of research was initiated in my
laboratory.