PARALLEL-PROCESSING IN VISUAL SHORT-TERM-MEMORY

Visual short-term memory for the contrast and spatial frequency of sin usoidal gratings was measured in a delayed discrimination task in whic h the 2 stimuli to be compared were separated in time by 1-10 s inters timulus intervals (ISIs). Delayed discrimination thresholds for spatia l frequency and contrast were compared, both when the 2 types of thres holds were measured in separate blocks of trials and when the 2 types of measures were randomly intermixed in an uncertainty paradigm, which required participants to process information about both dimensions on each trial. In both cases, accuracy of memory for spatial frequency w as independent of ISI, but memory for contrast decreased as ISI increa sed. Performance was lower in the uncertainty case, but only by an amo unt predicted by statistical decision theory for independent sources. The results are consistent with a model assuming a set of parallel spe cial-purpose visual discrimination and short-term memory mechanisms.