Choice and Uncertainty Clog the Bottleneck of Central Processing

1103 In today’s fast-paced computer age, multitasking has become routine. But even the most skillful multitaskers can’t “think” about two tasks at exactly the same time. Psychologists and neuroscientists study parallel processing in the brain by modeling information as traveling sequentially through three processing stages categorized as perceptual, decision (the central black box of the mind), and motor output. A long-standing hypothesis holds that the central decision stage is a passive bottleneck, where two competing tasks are handled on a fi rst-come, fi rst-served basis. In this model, the second task engages only after the fi rst has passed through the bottleneck and does not slow down the reaction time on the fi rst task, even when the second is presented rapidly after the fi rst, also know as short stimulus onset asynchrony (SOA). Growing evidence questions whether the passive bottleneck model fully captures the complexity of taskswitching dynamics. For example, last year in PLoS Biology, researchers Mariano Sigman and Stanislas Dehaene reported that, during dualtasks, response times to the fi rst task lengthened compared with performing the fi rst task alone, independently of SOA. They proposed that since response order was dictated in the experiment, this additional processing time resulted from the participant’s brain dictating—like issuing an executive order—which task to perform fi rst, in keeping with the experimental instructions. In a new study, the researchers employed the same dual-tasks used in their previous study—number comparison and tone discrimination (link to previous synopsis: http:⁄⁄biology. plosjournals.org/perlserv/ ?request=get-document&doi=10.1371/ journal.pbio.0030084). But this time they made task order unpredictable and the subjects decided which task to perform fi rst. With choice, it turns out, task order and reaction times vary in a manner suggesting that uncertainty comes at a cost, and that active processes during the perceptual and central stages are required to decide when to engage and disengage each task. As before, the number and tone tasks involved making simple discriminations as accurately and quickly as possible. Subjects decided whether a number was greater or less than 45. Cognitive challenge resulted from varying the task by notation (Arabic digits or words) and distance (numerical distance between a presented number Choice and Uncertainty Clog the Bottleneck of Central Processing