Figure 1.

Parametric model of transduction in insect photoreceptors. Light fluctuation as a function of time, x(t), passes through an initial stage that reduces its amplitude, and changes its mean level by an exponentially decaying function of time after initial stimulation. Three parameters define this stage: α, the total proportional change in amplitude; μ, the total change in mean level; and η, the time constant for both. The resulting signal, u(t), is convolved with the log-normal photoreceptor filter function of Payne and Howard (1981), with its two parameters, τ and σ. Finally, the output of the log-normal filter, v(t) passes through a static (time independent) nonlinearity formed by a second-order polynomial function with parameters a, b, and c.