Two classes of models have been proposed to explain how redundant information extracted from separate sources comes to activate a single response. Each provides a fundamentally different account of why responses to redundant signals are typically faster than those to either signal alone (the redundant-signals effect). Independent race models assume that a race occurs between perceptual codes on independent channels and that only the winner activates the response. Coactivation models assume that there is some form of energy or activation-strength summation, with information being pooled across channels prior to decision. An intermediate class of models is introduced and a specific exemplar, the interactive race model, is tested in a series of redundant-target detection experiments. In particular, we examine the effects on performance of two types of contingency that have previously been overlooked as sources of task-relevant information. The results reveal that response times are significantly influenced by both interstimulus and stimulus-response contingencies. The interactive race model provides a natural account of these findings as well as several otherwise puzzling results in the divided-attention literature.

When attention is divided between spatially distinct objects, the time to detect a target decreases when two or more targets are present. This redundancy gain can be accounted for by an interactive race model (Mordkoff & Yantis, 1991) in which separate decisions are made about each object, but environmental contingencies among the objects can influence decision times. In the present study, we examined whether the model also accounts for performance when attention must be divided between stimulus attributes other than spatial location. Subjects made targetpresent responses when displays included a prespecified color, a prespecified letter, or both target features. The data violated the predictions of all separate-activations models, including the interactive race model. Two control experiments ruled out an alternative account based on task complexity. We conclude that coactivation occurs when target attributes from two separable dimensions are simultaneously present, but not when target attributes come from the same dimension. A modular hybrid of race and coactivation models is tentatively proposed.

Sequential modulation is the finding that the sizes of several selective-attention phenomena-namely, the Simon, flanker, and Stroop effects-are larger following congruent trials than following incongruent trials. In order to rule out relatively uninteresting explanations of sequential modulation that are based on a variety of stimulus-and response-repetition confounds, a four-alternative forced choice task must be used, such that all trials with any kind of repetition can be omitted from the analysis. When a fouralternative task is used, the question arises as to whether to have the proportions of congruent and incongruent trials be set by chance (and, therefore, be 25% congruent and 75% incongruent) or to raise the proportion of congruent trials to 50%, so that it matches the proportion of incongruent trials. In this observation, it is argued that raising the proportion of congruent trials to 50% should not be done. For theoretical, practical, and empirical reasons, having half of the trials be congruent in a four-alternative task aimed at providing unambiguous evidence of sequential modulation should be avoided.

Previous research examining response time has supported coactivation under certain conditions. Other research has found more forceful responses to redundant-target than to single-target displays, suggesting coactivation in the motor component. The authors tested for motor coactivation using response time, response force, and other psychophysiological measures. Experiments 1 and 2 showed that response force is determined by the number of stimuli, not the number of targets, when target-distractor discriminations are required. In Experiment 3, 1 stimulus was presented on each trial, and the number of target features was varied. The response time results showed that coactivation occurred somewhere in the information-processing system, but no evidence of motor coactivation was found using any psychophysiological measure. These data disconfirm the motor-coactivation hypothesis for tasks that require visual discriminations.

On what basis does the visual system use recently sampled information to update existing representations of the world? One possibility is that representations are updated through an image-based point-for-point replacement process. An alternative possibility is that representations are updated on the basis of perceptually organized units that reflect objects in the scene rather than locations within the visual field. We report a new effect involving a modulation of visible persistence that seems to support this alternative possibility. In particular, we show that a moving stimulus leaves a visible trace of itself when it undergoes an abrupt and transient change in size but does not do so when the stimulus does not change. Further we show that this effect is substantially reduced when a scene-based reason for the abrupt change in size is provided (i.e., the object is shown to be passing behind an occluding surface that has a very small window in it through which the stimulus shows briefly). We suggest that the visible persistence in the face of change reflects a disruption of the normal updating process which is object-based and disrupted because of the discontinuity of the object. Providing a scene-based reason for the discontinuity allows the object representation to be maintained, and thus does not result in a disruption of the updating process.