SPATIAL FREQUENCY UNCERTAINTY AND REACTION TIME TO STIMULI-GRATINGS

Reaction time (RT) is often measured in mixed blocks with randomly varying spatial frequency (SF), i.e. under conditions of SF-uncertainty. In the present experiments, the effect of SF-uncertainty was studied with the use of two gratings with different SFs. It was found that SF-uncertainty increased RT and its standard deviation when: a) SFs of the two stimuli were within two different ranges, below 2.6-5.2 c/deg for one of the gratings and above this critical value for the other grating; b) stimulus contrast was low. When SFs of both gratings were within the same range, or the contrast was higher, no effect of SF-uncertainty on RT was observed. The results support our previous suggestion that RT is determined by two mechanisms (transient and sustained) at near threshold contrast, and by one (transient) mechanism at higher contrast.