Distributional analyses and event-related brain potential were used to show that effects of irrelevant spatial stimulus-response correspondence consist of 2 qualitatively different automatic components that can be distinguished on the basis of their dependencies on relative response speed and on computational requirements of the primary task. One component reflects priming of the spatially corresponding response by an abrupt stimulus onset that does not depend on the nature of the primary task. This unconditional component exhibits a biphasic pattern, with initial facilitation later turning into inhibition, analogous to that found for spatial cuing in visual detection tasks. The 2nd component reflects automatic generalization of task-defined transformations of relevant stimulus information to spatial codes; this conditional component does not depend on relative response speed. Possible connectionist implementations of the conditional mechanism are discussed.

No abstract

We report an experiment that assesses the effect of variations in memory load on brain activations that mediate verbal working memory. The paradigm that forms the basis of this experiment is the "n-back" task in which subjects must decide for each letter in a series whether it matches the one presented n items back in the series. This task is of interest because it recruits processes involved in both the storage and manipulation of information in working memory. Variations in task difficulty were accomplished by varying the value of n. As n increased, subjects showed poorer behavioral performance as well as monotonically increasing magnitudes of brain activation in a large number of sites that together have been identified with verbal working-memory processes. By contrast, there was no reliable increase in activation in sites that are unrelated to working memory. These results validate the use of parametric manipulation of task variables in neuroimaging research, and they converge with the subtraction paradigm used most often in neuroimaging. In addition, the data support a model of working memory that includes both storage and executive processes that recruit a network of brain areas, all of which are involved in task performance.

This article reports 4 experiments that used the psychological refractory period procedure to characterize how people perform multiple tasks concurrently. For each experiment, a primary choice-reaction task was paired with a secondary choice-reaction task that had two levels of response-selection difficulty. Experiments 1 and 2 varied secondary-task response-selection difficulty by manipulating the number of stimulus-response (S-R) pairs. The effect of this factor on secondary-task reaction times (RTs) decreased reliably as the stimulus onset asynchrony (SOA) decreased. Experiments 3 and 4 varied secondary-task response-selection difficulty by manipulating S-R compatibility. Again, the effect of this factor on secondary-task RTs decreased reliably as SOA decreased. These results raise doubts about the existence of an immutable structural response-selection bottleneck and suggest that response selection for 2 concurrent tasks may overlap temporally.

No abstract