An investigation of attention allocation deficits in aphasia.

Nine aphasic and 18 normal adults participated in a dual task with components of phoneme monitoring and semantic judgment. Subjects listened to lists of recorded spoken words and were required to detect semantic and phonetic targets. Two within-subject variables were manipulated: the probability of target occurrence for each dual-task component (.2, .5, and .8) and the explicitness of instruction (explicit vs. implicit). Only in the explicit condition were subjects told about the probability structure and given the attention allocation strategy congruent with that structure. Reaction times were recorded on-line for analysis by an IBM-AT. The results showed that normal subjects' overall latencies in detecting phonetic targets decreased as the target occurrence probability increased in both implicit and explicit instruction conditions. The detection latencies for semantic targets showed a similar probability effect but only in the explicit condition. By contrast, the aphasic subjects did not show the probability effect in any condition, although they performed above the chance level on both semantic and phonetic tasks. The presence of the probability effects for the normal subjects indicated that an efficient attention allocation mechanism was operating so as to optimize the performance level. On the other hand, the absence of probability effects for the aphasic subjects suggests that their attention control system might be inefficient so that task-demand cues (probability and attention allocation instruction) were not utilized properly.