11. ELECTRORECEPTION
551
piratory movements ( Dijkgraaf and Kalmijn, 1966). Conceivably, changes
in these responses could signal the presence of objects.
In many afferent fibers the presence of insulators and conductors alters
the number of impulses evoked by the electric organ discharge (Bullock
and Chichibu, 1965; Hagiwara and Morita, 1963; Hagiwara et al., 1965a,b;
Szabo and Fessard, 1965). Except for the stimulation of tonic receptors
noted above, all of these responses are presumably from phasic receptors,
as has been confirmed by direct identification in Gymnotus (Suga, 1967a)
and mormyrids (Roth, 1967; Szabo and Hagiwara, 1967). Although both
medium and large receptors of mormyrids are activated by organ discharge, changes in afferent responses have been fully reported only for
medium receptors. Because of the great sensitivity of large receptors,
they may be maximally activated by the organ and transmit negligible
information about distortions of its field. If so, their primary function may
be detection of weak high frequency signals from other mormyrids, which
is perhaps of significance in communication ( cf. Moller, 1970).
The effects of objects on afferent discharges, presumably from a
phasic receptor, are illustrated for the gymnotid Steatogenys in Fig. 37.
On the left are shown oscilloscope records with histograms of the number
of impulses after each organ discharge when a silver plate was in the
indicated positions. On the right are graphs of mean number of impulses
in two fibers for successive positions of paraffin blocks as well as silver
plates. The monopolar threshold measurements indicate that for the
upper graph, at least, the receptor lies at about the point where the
minimum response is obtained with the silver plate and the maximum
response is obtained with the paraffin block.
A variety of coding mechanisms have been described whereby changes
in afferent impulses evoked by organ discharge may signal distortions of
the organ fields (Bullock and Chichibu, 1965; Hagiwara and Morita,
1963; Hagiwara et al., 196%; Szabo and Hagiawara, 1967). In fish emitting discharges at a low frequency, a fiber may carry several impulses following each discharge; changes in the field may alter the number of
impulses per discharge as in Fig. 37. Latency may or may not be changed
concurrently. Particularly in fish emitting discharges at high frequencies,
the impulses may fail to follow the organ discharge frequency one to one,
and changes in the field may alter the probability of a single impulse’s
occurrence as well as its latency. Latency changes are very pronounced
in discharges from some medium receptors of the mormyrid Gnathonemus. Other medium receptors show less change in latency but greater
changes in number of spikes. Phasic receptors continue to give an altered
response in the presence of a stationary stimulating object, although there
may be some adaptation. The continued responding has sometimes been
551
piratory movements ( Dijkgraaf and Kalmijn, 1966). Conceivably, changes
in these responses could signal the presence of objects.
In many afferent fibers the presence of insulators and conductors alters
the number of impulses evoked by the electric organ discharge (Bullock
and Chichibu, 1965; Hagiwara and Morita, 1963; Hagiwara et al., 1965a,b;
Szabo and Fessard, 1965). Except for the stimulation of tonic receptors
noted above, all of these responses are presumably from phasic receptors,
as has been confirmed by direct identification in Gymnotus (Suga, 1967a)
and mormyrids (Roth, 1967; Szabo and Hagiwara, 1967). Although both
medium and large receptors of mormyrids are activated by organ discharge, changes in afferent responses have been fully reported only for
medium receptors. Because of the great sensitivity of large receptors,
they may be maximally activated by the organ and transmit negligible
information about distortions of its field. If so, their primary function may
be detection of weak high frequency signals from other mormyrids, which
is perhaps of significance in communication ( cf. Moller, 1970).
The effects of objects on afferent discharges, presumably from a
phasic receptor, are illustrated for the gymnotid Steatogenys in Fig. 37.
On the left are shown oscilloscope records with histograms of the number
of impulses after each organ discharge when a silver plate was in the
indicated positions. On the right are graphs of mean number of impulses
in two fibers for successive positions of paraffin blocks as well as silver
plates. The monopolar threshold measurements indicate that for the
upper graph, at least, the receptor lies at about the point where the
minimum response is obtained with the silver plate and the maximum
response is obtained with the paraffin block.
A variety of coding mechanisms have been described whereby changes
in afferent impulses evoked by organ discharge may signal distortions of
the organ fields (Bullock and Chichibu, 1965; Hagiwara and Morita,
1963; Hagiwara et al., 196%; Szabo and Hagiawara, 1967). In fish emitting discharges at a low frequency, a fiber may carry several impulses following each discharge; changes in the field may alter the number of
impulses per discharge as in Fig. 37. Latency may or may not be changed
concurrently. Particularly in fish emitting discharges at high frequencies,
the impulses may fail to follow the organ discharge frequency one to one,
and changes in the field may alter the probability of a single impulse’s
occurrence as well as its latency. Latency changes are very pronounced
in discharges from some medium receptors of the mormyrid Gnathonemus. Other medium receptors show less change in latency but greater
changes in number of spikes. Phasic receptors continue to give an altered
response in the presence of a stationary stimulating object, although there
may be some adaptation. The continued responding has sometimes been