In vitro study of phase resetting and phase locking in a time-comparison circuit in the electric fish, Eigenmannia.

The electric fish Eigenmannia generates on oscillating weak electric field. The amplitude and timing information of this electric field is perceived by electroreceptors distributed on its skin. The pathway of timing information, consisting of spherical cells and giant cells, was studied in an in vitro preparation. The giant cells were identified to be endogenous oscillators and thus have the functional advantage of phase locking more easily to a periodic stimulus with a frequency in the range of the intrinsic frequency. Their spontaneous rhythmic activity was perturbed by delivering excitatory single pulses or periodic pulses via their synaptic inputs. The regular and irregular dynamics produced by periodic stimulation were discussed in the context of a mathematical analysis of the response to single pulses. Ambiguous representations of the timing of the stimulus pulse were observed and could be related to this analysis. Some spontaneously firing cells could be silenced with periodic excitatory stimulation in a narrow frequency and amplitude range. Some irregularly firing cells continued to fire periodically for several seconds after phase locking to a periodic stimulus. This study is the first description of an endogenous oscillator in a system devoted to the precise timing of sensory events.