Switching or sharing in dual-task line-length discrimination?

In two experiments, we tested whether subjects switched or shared attention between two simultaneously relevant line-length discrimination tasks. Switching models that allowed within- as well as between-trial switching were considered. In the first experiment, stimulus duration was varied randomly from trial to trial. With varied durations, many switching models predict negative contingencies: for a given duration and attentional allocation, accurate responses on one task should be associated with inaccurate responses on the other task. The results, however, showed no negative contingencies, which is consistent with sharing models. In the second experiment, stimulus duration was reduced to 20 msec, yet responses were more than 75% correct overall. This implies that information was obtained about both of the tasks within single trials, contradicting those switching models which predict that information can be obtained from, at most, one task within a period of 20 msec or less. In short, the results of both experiments support sharing models.