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In three experiments we measured response time (RT) and peak force (PF) to investigate the grouping of left- and right-hand key press responses in a dual-task paradigm involving two independent go/no-go tasks. Within each task, a go stimulus within one of two modalities (i.e., visual versus auditory) required a response by one hand. In Experiment 1 with simultaneous go stimuli in the two tasks, responses appeared to be grouped in approximately 75-80% of trials, compared with nearly 100% grouping in a single-task condition requiring bimanual responses to the onset of any stimulus in either modality. In Experiment 2 with stimulus onset asynchronies (SOAs) of 0-400 ms between the two go stimuli, response grouping clearly declined as SOA increased, although some grouping was still evident even at the longest SOA. The same pattern was observed in Experiment 3 with the same range of SOAs but unpredictable stimulus order, suggesting that grouping is not strongly dependent on prior knowledge of the likely response order. These results emphasize the pervasiveness of response grouping in bimanual dual-task RT paradigms and provide useful clues as to its nature.

In this dual-task study, we applied both eross-talk logic and locus-of-slack logic to test whether participants can retrieve semantic categories in Task 2 in parallel to Task 1 bottleneck processing. Whereas cross-talk logic can detect parallel memory retrieval only in conditions of categorical overlap between tasks, the locus-of-slack approach is independent of such restrictions. As was expected, using the cross-talk logic, we found clear evidence for parallel retrieval of semantic categories when there was categorical overlap between tasks (Experiment 1). Locus-of-slack-based evidence for parallel semantic retrieval was found, however, both in conditions with (Experiment 1) and in those without (Experiment 2) categorical overlap between tasks. Crucially, however, increasing the demand for resources required to switch from Task 1 to Task 2 eliminated even the locus-of-slack-based evidence for parallel memory retrieval during the psychological refractory period (Experiment 3). Together, our results suggest that parallel retrieval is not bound to conditions of categorical overlap between tasks but, instead, is contingent upon resources needed for switching between tasks (e.g., Oriet, Tombu, & Jolicceur, 2005).

Two experiments tested predictions of the hemispheric coactivation model for redundancy gain (J. O. Miller, 2004). Simple reaction time was measured in divided attention tasks with visual stimuli presented to the left or fight of fixation or redundantly to both sides. Experiment 1 tested the prediction that redundancy gain--the decrease in reaction time when 2 stimuli are presented rather than 1--should increase when the response requires a greater degree of bilateral control by both hemispheres. In different blocks, the response was made with the left hand, the right hand, or both hands simultaneously. In accordance with the model, redundancy gain was larger with bimanual than with unimanual responses. Experiment 2 tested and found support for the prediction that redundancy gain should decrease when both hemispheres are activated by an irrelevant auditory accessory stimulus. Thus, the results support the hemispheric coactivation model. Keywords: reaction time, redundancy gain, accessory stimuli, bimanual responses

In three visual search experiments participants were asked to make a target response if either of two targets was present and to make a nontarget response if neither target was present. Some target-absent displays included only nontarget features that never occurred in the same displays as target features, whereas other target-absent displays included nontarget features that did sometimes occur with target features. Nontarget responses were reliably faster in the former case than in the latter. This 'associated nontargets effect' appears to arise from participants' ability to learn and to use contingencies between the presence of certain nontargets and the absence of any target.

ABSTRACT. When people must respond discriminatively to 1 or 2 stimuli by making 1 or 2 taps of a response key, they initiate the response more rapidly when the correct [...]

In four visual search tasks participants were asked to make a target response if either of two targets was present and to make a nontarget response if neither target was present. Some target-absent displays included only nontarget stimuli of features that never occurred in the same displays as targets, whereas other target-absent displays included nontarget stimuli or features that did sometimes occur with targets. Nontarget responses were reliably faster in the former case than in the latter. This 'associated nontargets effect' indicates that nontargets are not simply classified as nontargets but in addition are discriminated from one another. Current visual search models may underestimate the degree to which nontargets are processed during search.

A diffusion model for simple reaction time (RT) and temporal order judgment (TO J) tasks was developed to account for a commonly observed dissociation between these 2 tasks: Most stimulus manipulations (e.g., intensity) have larger effects in RT tasks than in TOJ tasks. The model assumes that a detection criterion determines the level of sensory evidence needed to conclude that a stimulus has been presented. Analysis of the performance that would be achieved with different possible criterion settings revealed that performance was optimal with a lower criterion setting for the TOJ task than for the RT task. In addition, the model predicts that effects of stimulus manipulations should increase with the size of the detection criterion. Thus, the model suggests that commonly observed dissociations between RT and TOJ tasks may simply be due to performance optimization in the face of conflicting task demands. Keywords: temporal order judgment, simple reaction time, statistical decision making, intensity effects, mathematical model

Three choice reaction time experiments documented a Simon-type congruence effect involving the numbers of stimuli and responses. In Experiment 1 the stimulus consisted of one or two high-or low-pitch tones, and participants were required to respond with one or two taps of a response key depending on stimulus pitch. Responses were faster when the number of tones matched the required number of response taps than when these numbers mismatched. Experiment 2 showed that a weaker version of this effect can also be obtained using visual stimuli, and Experiment 3 showed that the analogous effect can even be obtained, albeit very weakly, with bimodal stimuli. There was also evidence that the number of stimuli affected the rate of emitting the two-tap response. These results indicate that stimulus numerosity is processed automatically to the level at which it can affect the selection and possibly the execution of a varying number of responses.

Bottleneck models of psychological refractory period (PRP) tasks suggest that a Task 1 response should be unaffected by the Task 2 response in the same trial, because selection of the former finishes before selection of the latter begins. Contrary to this conception, the authors found backward response-level crosstalk effects in which Task 2 response force requirements influenced the force-time dynamics of Task 1 responses. Specifically, Task 2 required a hard or soft keypress response. Task 1 responses were harder when the upcoming Task 2 response was to press hard rather than soft, suggesting some activation of Task 2 response parameters before Task 1 processing reached the final ballistic motor output stage. A 3rd experiment using a flankers paradigm showed that this effect did not arise from automatic activation of responses by the stimuli associated with hard and soft responses. This backward response-level crosstalk extends previous findings of backward crosstalk in the PRP paradigm by showing that crosstalk can affect motor output as well as response time and can arise even when the 2 tasks being performed are not semantically related. Keywords: psychological refractory period, bottleneck models, response force, backward crosstalk