Serial modules in parallel: The psychological refractory period and perfect time-sharing

The authors describe ACT-R/perceptual-motor (ACT-R/PM), an integrated theor y of cognition, perception, and action that consists of the ACT-R productio n system and a set of perceptual-motor modules. Each module (including cogn ition) is essentially serial, but modules run in parallel with one another. ACT-R/PM can model simple dual tasks such as the psychological refractory period (PRP), including subtle results previously explained with executive process interactive control (EPIC, D. E. Meyer & D. E. Kieras, 1997a). The central difference between the theories is that EPIC's productions can fire in parallel, whereas in ACT-R/PM, they are serial. Results from three PRP- like experiments with more demanding cognitive requirements indicate that c ognitive processing for the 2 tasks need not overlap. ACT-R's activation-ba sed retrieval processes are critical in accounting for the timing of these tasks and for explaining the dual-task performance decrement.