Costs of a predictable switch between simple cognitive tasks following severe closed-head injury.

The authors used a predictable, externally cued task-switching paradigm to investigate executive control in a severe closed-head injury (CHI) population. Eighteen individuals with severe CHI and 18 controls switched between classifying whether a digit was odd or even and whether a letter was a consonant or vowel on every 4th trial. The target stimuli appeared in a circle divided into 8 equivalent parts. Presentation of the stimuli rotated clockwise. Participants performed the switching task at both a short (200 ms) and a long (1,000 ms) preparatory interval. Although the participants with CHI exhibited slower response times and greater switch costs, similar to controls, additional preparatory time reduced the switch costs, and the switch costs were limited to the 1st trial in the run. These findings indicate that participants with severe CHI were able to take advantage of time to prepare for the task switch, and the executive control processes involved in the switch costs were completed before the 1st trial of the run ended.