Limited-capacity mechanisms of visual discrimination

Discrimination thresholds of spatial frequency and choice reaction times (RT) were measured in three subjects who performed a dual-judgment delayed discrimination task. Two reference gratings were presented side-by-side with a 0-800 msec stimulus onset asynchrony (SOA), which were followed after a 5-sec retention interval by two test gratings. Subjects judged which component changed and which interval had the higher spatial frequency (SF). Thresholds in the dual-judgment task were four to six times higher than thresholds in single-judgment tasks. The SOA had only a moderate effect on discrimination thresholds, whereas the difference between the spatial frequencies of the two components had a highly significant effect. The discrimination thresholds increase with increasing spatial frequency difference for the lower SF component, while they decrease for the higher SF component. An analysis of the distribution of possible error types indicated that all subjects tended to respond more frequently that the higher SF component changed. This tendency led to more errors on trials where the low SF component changed. A posthoc analysis revealed, in two of the three subjects, a significant correlation between delta f/f and RT such that higher delta f/f values were associated with lower RTs and vice versa.