Journal of Neurophysiology, Vol 40, Issue 1 174-187, Copyright © 1977 by APS
Sinusoidal analysis of electroretinogram of squid and octopus
P. H. Hartline and G. D. Lange
1. 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.
