Multitasking costs in close-head injury patients. A fine-grained analysis.

The issue of whether severe close-head injury (CHI) patients suffer from disproportionate dual-task deficits compared with matched controls was investigated in two experiments. In the first experiment, either one or three masked letters were presented at the center of a monitor, followed by a pure tone at variable stimulus-onset asynchronies (SOAs). In half of the blocks of trials, the task on the letters required a delayed report of the letters at the end of each trial; in the other half of the blocks, the letters had to be ignored. The tone task always required an immediate manual response based on the tone pitch. In the second experiment, either three masked letters or three masked digits were presented with equal probability in each trial, followed by a tone at variable SOAs. The task required the delayed report of the characters only if they were letters, or ignoring the characters if they were digits. In both experiments, CHI patients and matched controls both exhibited an SOA-locked slowing of the reaction time (RT) to the tone: When characters had to be encoded for delayed report, tone RT increased progressively as SOA was decreased. The SOA effect on tone RT was more pronounced for CHI patients than for controls, suggesting that a substantial component of the slower processing time for CHI patients was related to a selective increase at a central stage of processing shared by the two tasks. Implications for models of the CHI effects on human performance are discussed.