Figure 4: Modeling contributions to rat PII during a light step. Average amplitudes (±SEM) as a function of luminance at different fixed times (90 and 220 ms) after stimulus onset. (a) and (b) show a single hyperbolic function fit to the data (thick grey line). Arrows indicate the possible presence of a secondary mechanism at brighter intensities. A combination of two hyperbolic functions (grey trace and dashed trace) provides a better fit to the data than a single hyperbolic function. The black trace shows the sum of the two hyperbolic functions. The optimized (c) and (d) parameters are plotted as a function of time for the two hyperbolic functions. (c) shows the combined amplitude for the two hyperbolic functions (black trace) compared with the actual waveform for the 2.82 log cd·m−2 (grey trace) stimulus. The arrows in (d) indicate changes in , which are likely to represent a transition between mechanisms.