Figure 8. Human and ideal observer speed discrimination performance improves similarly when probed with drifting sinewaves.

(a) Thresholds for discriminating speed with 2 c.p.d. sinewave stimuli (grey) versus natural stimuli (black). Human thresholds (symbols) and ideal thresholds (solid curves) decrease by approximately the same proportion (0.7 versus 0.63). Degrading ideal performance with both stimulus types by the same amount (arrows) produces a good fit of human performance (dashed curves). (b) The human psychometric data with sinewave stimuli (symbols) is predicted by the degraded ideal observer (solid curves). The efficiency parameter for the degraded ideal was obtained from the human data with natural stimuli (values of the efficiency parameter are given in Fig. 7b). The 30% decrease in human threshold with sinewave stimuli is quantitatively predicted with zero free parameters.