Spontaneous modulations of the electric organ discharge in the weakly electric fish, Apteronotus leptorhynchus: a biophysical and behavioral analysis

Brown ghosts, Apteronotus leptorhynchus, are weakly electric gymnotiform fi sh whose wave-like electric organ discharges are distinguished by their eno rmous degree of regularity. Despite this constancy, two major types of tran sient electric organ discharge modulations occur: gradual frequency rises, which are characterized by a relatively fast increase in electric organ dis charge frequency and a slow return to baseline frequency; and chirps, brief and complex frequency and amplitude modulations. Although in spontaneously generated gradual frequency rises both duration and amount of the frequenc y increase are highly variable, no distinct subtypes appear to exist. This contrasts with spontaneously generated chirps which could be divided into f our "natural" subtypes based on duration, amount of frequency increase and amplitude reduction, and time-course of the frequency change, Under non-evo ked conditions, gradual frequency rises and chirps occur rather rarely. Ext ernal stimulation with an electrical sine wave mimicking the electric field of a neighboring fish leads to a dramatic increase in the rate of chirping not only during the 30 s of stimulation, but also in the period immediatel y following the stimulation. The rate of occurrence of gradual frequency ri ses is, however, unaffected by such a stimulation regime.