1. Two possible modes of action of the electric receptors of Gymnarchus niloticus are described. In one, received pulses are ‘smoothed’ and a sensory discharge occurs dependent only on the smoothed level (‘pulse-frequency-modulation’). According to the other mechanism, sensory nerve impulses are produced at the same frequency as the received pulses, but after a delay dependent on their amplitude (‘pulse-phase-modulation’).

2. An experiment has been devised to distinguish between the two mechanisms, and, if the pulse-frequency-modulation mechanism is the operative one, to measure the smoothing time-constant.

3. The results favour the pulse-frequency-modulation mechanism, and indicate a smoothing time-constant of 25 msec. It appears also that adaptation takes place in the electric receptors.

4. The origin of the smoothing action is discussed.

5. The sensitivity of electric receptors is compared with that of other sense organs.

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