An isolated eye or eye plus optic lobe preparation (in oxygenated chilled seawater) from Loligo opalescens, Octopus bimaculata, and O. bimaculoides was used to study the electroretinogram (ERG) for small signal intensity-modulated stationary spots of light. 2. If light intensity was modulated sinusoidally (modulation depth 0-50%) the ERG response is sinusoidal with less than 2% of the power present in the next five harmonics compared to the fundamental. Bode plots, amplitude and phase shift plotted against frequency, were constructed from these sinusoidal input-output experiments. 3. Linearity and time invariance were tested: a) an increase in amplitude of sinusoidal modulation by a constant factor caused an increase in response amplitude by the same factor but caused no change in shape of the Bode plot gain or phase curves; b) the transfer function represented by the Bode plot could be used to predict waveshape of the response to a brief flash (Green's or impulse-response function); c) the Fourier transformed square-wave response could be used to obtain a Bode plot which coincided with that obtained by sinusoidal input-output experiments. 4. The Bode plot can be fit by the transfer function of 5-12 (depending on conditions and on the preparations) series cascaded low-pass filters whose corner frequences are distributed between 0.2 and 40 Hz. Alternatively, 3-7 filters plus a delay of 25-130 ms fits the Bode plots equally well. The series filter model is compatible with a simply physical model consisting of cascaded chemical reactions whose forward rate constants are reciprocals of the filter time constants, whose reverse rate constants are negligible, and in which the concentration of an intermediate product controls membrane current. 5. As mean intensity is increased, the gain decreases. This effect is more pronounced at low frequencies than at high frequencies. Thus, the system is nonlinear for large intensity changes. The process of adaptation involves not only a change in gain, but a change in shape of the Bode plot, i.e., change in filter corner frequencies. In terms of the reaction chain model, this means that some rate constants change as the state of adaptation is changed.
