The shape of the high frequency flicker sensitivity curve

For a light flickering with a modulation amplitude delta L, there is a single frequency f above which the light appears steady (fused). The relationship between delta L and f has been measured often under a variety of conditions, but its mathematical form is disputed. Three candidate functions have been proposed: (1) log delta L alpha f [Ferry-Porter law], (2) log delta L alpha f1/2 [diffusion model], and (3) log delta L alpha log f [cascaded integrator model]. Although all three functions roughly fit flicker fusion data (after appropriate linear transformation), they differ in curvature. We compared these functions using the general expression (4) log delta L alpha f lambda, where lambda denotes a curvature parameter. Functions (1)-(3) are special cases of (4) with lambda = 1, 0.5, and 0 respectively. When applied to 35 sets of flicker fusion data, the mean values of the best fitting lambda for each data-set was 0.919 (95% confidence interval 1.027 to 0.812). We conclude that the Ferry-Porter law describes most data better than a diffusion model, and that an integrator model deviates consistently in every case.