1. 1.

    Microphonic and summating potentials were recorded extracellularly from lateral line organs in the suborbital canal of the perch in response to sinusoidal movements of canal fluid.

  2. 2.

    These potentials were changed in amplitude, shape and phase, relative to the mechanical stimulus, by electrical stimulation of efferent fibres in the lateral line nerve.

  3. 3.

    The receptor potential amplitude/stimulus intensity relationships for the microphonic and summating potentials saturated at high levels of stimulation, and at progressively lower amplitudes with increasing frequencies of mechanical stimulation. Efferent stimulation tended to reduce this rate of saturation.

  4. 4.

    Amplitude versus frequency relationships plotted at different stimulus intensities for the microphonic potential showed that the lateral line organs were most sensitive to frequencies between 35–65 Hz (centre frequency), and at these frequencies efferent stimulation caused the greatest increase in amplitude.

  5. 5.

    Analysis of the second order and third order harmonic components of the microphonic showed that these were reduced by efferent stimulation and that the strongest reduction occurred at the centre frequency.

  6. 6.

    The phase of the receptor potential led that of the mechanical stimulus at very low frequencies by nearly 90°. This changed to zero phase at the centre frequency and to a phase lag at higher frequencies. Efferent stimulation caused no change in phase of the microphonic relative to the control state at the centre frequency, but caused a progressive phase lead and lag as the frequency was decreased and increased respectively about the centre frequency.

  7. 7.

    In the linear response range, the lateral line organs responded as critically damped low frequency resonators to the velocity of the stimulus. Efferent stimulation appeared to alter the damping of this resonance. The possibility is discussed that efferent stimulation can alter the mechanical properties of the lateral line hair cells.

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