Dimensional weighting and task switching following frontal lobe damage: Fractionating the task switching deficit.

Deficits in task switching can be found after frontal lobe damage. Here we demonstrate an impairment in task switching specifically linked to when perceptual weights have to be moved between different dimensions of the same stimulus. A patient (DS) with left frontal lobe damage showed normal performance when he responded to the meaning (a word task) or location (a location task) of a word presented to the left or right of fixation when there was no switching between the tasks. However, when the two tasks were switched every 16 trials in a block, DS showed severe difficulty in performing both tasks (Experiment 1). There were then abnormally large switch costs and effects of stimulus-response congruency. The difficulty was not simply due to switching tasks per se: There were no costs of switching when one of the tasks was modified to have different stimulus displays from the other (Experiment 2). The deficit was also not greater when the switch had to be made from a well-practised task to an unpractised task with more arbitrary stimulus-response mappings, indicating no particular problem in disengaging from a learned task or in configuring new stimulus-response links (Experiment 4). We suggest instead that DS was impaired at shifting attentional weights across different dimensions of the same stimulus, a process required with practised and unpractised tasks alike. The results link this process of shifting attention across stimulus dimensions to the left frontal lobe.