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The forcefulness of key press responses was measured in stop-all and selective stopping versions of the stop-signal paradigm. When stop signals were presented too late for participants to succeed in stopping their responses, response force was nonetheless reduced relative to trials in which no stop signal was presented. This effect shows that peripheral motor aspects of primary task responses can still be influenced by inhibition even when the stop signal arrives too late to prevent the response. It thus requires modification of race models in which responses in the presence of stop signals are either stopped completely or produced normally, depending on whether the responding or stopping process finishes first. (Contains 4 footnotes, 1 table and 4 figures.)

In a simple reaction time (RT) experiment, visual stimuli were stereoscopically presented either to one eye (single stimulation) or to both eyes (redundant stimulation), with brightness matched for single and redundant stimulations. Redundant stimulation resulted in two separate percepts when noncorresponding retinal areas were stimulated, whereas it resulted in a single fused percept when corresponding areas were stimulated. With stimulation of noncorresponding areas, mean RT was shorter to redundant than to single stimulation, replicating the redundant signals effect (RSE) commonly found with visual stimuli. With stimulation of corresponding areas, however, no RSE was observed. This suggests that the RSE is driven by the number of percepts rather than by the number of stimulated receptors or sensory organs. These results are consistent with previous findings in the auditory modality and have implications for models of the RSE. (Contains 3 figures, 1 table and 3 footnotes.)

Five psychological refractory period (PRP) experiments were conducted with an especially time-consuming first task (Experiments 1, 3, and 5: mental rotation; Experiments 2 and 4: memory scanning) and with equal emphasis on the first task and on the second (left-right tone judgment). The standard design with varying stimulus onset asynchronies (SOAs) was extended by a condition with blocked SOAs in Experiments 3 and 4. Based on the optimization account (Miller, Ulrich, & Rolke, 2009) it was expected that participants would--at short, but not long SOAs--tend to process the relatively fast central stage of Task 2 before the time-consuming central stage of Task 1 and consequently emit the response to Task 2 before the response to Task 1. Such an optimization tendency was found, more so for the mental rotation task and for the blocked SOA condition. The results indicate that preparation, Task 1 characteristics, and TRT (total reaction time) optimization are--among others--factors influencing central processing order in PRP tasks. (Contains 4 footnotes and 15 figures.)

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 right 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.

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.

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

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

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

In target detection tasks, responses are faster when displays have 2 targets (redundant-targets effect; RTE) and slower when they have no targets (nontargets effect; NTE), relative to displays with a single target. The psychological refractory period paradigm was used to localize these effects. In Experiment 1, participants classified tones as high or low and then classified letters as targets or nontargets after a short or long stimulus onset asynchrony (SOA). The magnitudes of the RTE and NTE did not depend on SOA. In Experiment 2, the order of the tasks was reversed, and at short SOAs the RTE and NTE had similar magnitudes for both tone discrimination and target detection responses. These findings suggest that the RTE and NTE arise during response selection. Interactive effects of tone pitch with the number and type of target features were also observed, and these were tentatively interpreted as synesthetic effects.

Online response preparation was assessed in a visual search task using rapid serial visual presentation. In each trial, a series of letters was presented sequentially, and participants were instructed to make a target-present response if a prespecified target letter was presented or a target-absent response if it was not. Measurements of response preparation using both probe reaction time and the lateralized readiness potential indicated that preparation of the target-absent response increased near the end of the sequence. Most of the increase appeared to be due to direct priming of the target-absent response by nontarget letters, but part was due to the increased conditional probability of this response near the end of the sequence. These results extend previous studies of response preparation by showing online response preparation during a temporally extended reaction time task.

Information-processing models differ about whether stimulus intensity affects the speed of motor processes involved in response activation and execution. Previous studies of intensity are reviewed, but they are not decisive on this point because they have used indirect approaches requiring strong assumptions. Two experiments examined the effects of stimulus intensity on the lateralized readiness potential (LRP), a measure of hand-specific response activation. In Experiment 1, visual stimulus intensity influenced the time from stimulus onset to LRP onset but not the time from LRP onset to the keypress response. In Experiment 2, auditory stimulus intensity did not influence either of these time intervals, although it did influence the time from stimulus onset to the N100 and P300 components of the evoked potential. The results indicate that stimulus intensity does not influence the duration of motor processes in choice reaction time tasks.

Three experiments were conducted to investigate whether response processes can start before memory scanning has finished when both are required in the same task. In Experiment 1 the color of a stimulus letter determined which hand might respond, and the letter's memory set membership determined whether that response should be made or withheld. Electrophysiological data suggested that lateralized response preparation was not initiated until memory scanning finished. Experiment 2 replicated these results with a consistent stimulus-response mapping to make the scanning process easier. Experiment 3 tested for earlier response priming with a probe reaction time paradigm, and the results suggested that color information can be used to activate a response before memory scanning is finished. The results of Experiments 1-3 suggest that interference between memory scanning and response preparation precludes the concurrent operation of these processes.

The lateralized readiness potential (LRP) was measured in 2-choice reaction time (RT) tasks with unequally probable stimuli and responses. Responses were faster and more accurate in high- than in low-probability trials. LRPs indicated that participants prepared the high-probability response and suggested that this preparation speeded high-probability responses relative to low-probability responses. When stimulus onset was temporally predictable, the high-probability response was clearly prepared prior to stimulus onset. When stimulus onset was unpredictable, preparation of the high-probability response occurred just after stimulus onset, even on low-probability trials where this preparation was inappropriate. With unpredictable stimulus onsets, RT distributions were adequately represented by a mixture model with prepared versus unprepared states; with predictable onsets, they were not.

Four experiments examined the effects of the number of alternative responses on the lateralized readiness potential (LRP). Two- versus 6-choice tasks differed in both the time from stimulus onset to LRP onset and the time from LRP onset to the keypress response, suggesting that this factor influences the durations of processes both before and after the onset of hand-specific response activation. The effects of response probability were too small to be fully responsible for the effects of the number of alternatives. The compatibility of stimulus-to-finger assignments within a hand influenced primarily the durations of processes after LRP onset, suggesting that hand-specific response activation begins before the response finger has been selected. The results illustrate the utility of the LRP for the localization of factor effects within information-processing tasks.

Previous research indicates that response force increases with stimulus intensity in simple reaction time (SRT) tasks. This result contradicts the common view that the perceptual system activates the motor system via a punctate go signal of fixed size. An elaborated view assumes that the size of the go signal depends on stimulus intensity so that more intense stimuli yield more forceful responses. In order to examine the latter hypothesis, the present experiments manipulated stimulus duration as well as intensity. Response force increased with duration even beyond a critical value of about 60 ms at which stimulus duration no longer affected SRT. In addition, increasing the duration of a stimulus also increased the duration of force output. These findings argue against models with punctate transmission of activation to the motor system. Certain continuous models and variable output models with prolonged go signals provide acceptable accounts of these effects.

Reaction times (RTs) and lateralized readiness potentials (LRPs) were studied to find out whether response preparation begins after mental rotation finishes, as assumed by discrete-stage models. Stimuli were disoriented normal or mirror-image characters, with character name determining which hand would respond. In Experiment 1, the normal/mirror-image information determined whether the response was to be executed (go) or withheld (no-go), and LRPs indicated that responses were weakly prepared before the end of mental rotation. Mental rotation was not required in Experiment 2, and significantly more response preparation was observed. In Experiment 3, probe RT trials embedded in the mental rotation task indicated that hand information is available to the response preparation process during rotation. Apparently, some response preparation occurs before mental rotation finishes, but rotation interferes with response preparation.

Four reaction time experiments examined the mental rotation process using a psychological refractory period paradigm. On each trial, participants made speeded responses to both a tone ([S.sub.1]) and a rotated letter ([S.sub.2]), presented with varying stimulus onset asynchronies (SOAs). If mental rotation of the stimulus letter can proceed while central mechanisms are busy with [S.sub.1], then the effect of orientation should decrease substantially with decreasing SOA. Contrary to these predictions, the effect of orientation was nearly constant across SOAs, suggesting that mental rotation cannot effectively proceed without help from central mechanisms. These results support the conclusion that mental rotation requires access to a single-channel mechanism and must therefore be performed serially with other operations requiring the same mechanism.

Six experiments tested the assumption that the mental rotation process is purely inserted into a mirror-normal discrimination task. In Experiment 1, subjects took significantly longer to respond to upright characters in blocks containing rotated stimuli than in blocks containing only upright stimuli. Experiments 2 and 3 showed that this 'rotational uncertainty' effect was not caused by the need to determine stimulus orientation, and Experiment 4 showed that it was independent of the visual quality of the stimulus. Experiment 5 showed that the effect was greatly reduced when subjects performed a go-no-go task rather than choice reaction time (RT), and Experiment 6 showed that it was independent of the complexity of the response required in the choice task. The results suggest that response selection in a choice RT mirror-normal discrimination task is altered when mental rotation is added, violating the assumption of pure insertion.

Observations of single neurons in the primary motor cortex of 1 monkey provided evidence that preliminary perceptual information reaches the motor system before perceptual analysis is complete. Neurons were recorded during a task in which 1 stimulus was assigned to a wrist flexion response and another was assigned to wrist extension. Two stimuli were assigned to a nogo response; each was visually similar to either the flexion or the extension stimulus. When a nogo stimulus was presented, neurons responded with weaker versions of the discharge patterns exhibited to the visually similar stimulus requiring a movement, suggesting that neurons receive partial perceptual information favoring that movement. Functionally separable neuronal populations were identified, and differences in the activations of these provide evidence about the functional effects of preliminary perceptual output on movement control processes.

In bimodal divided attention tasks, people respond faster to simultaneously presented redundant targets than to single targets. Previous research supports 'coactivation' models, in which redundant targets both activate the response. This study sought to determine whether redundant targets activate the responde independently, with each target producing its own activation, or interactively, with activation produced by redundant targets being a joint function of both their identities. Experiment 1 used auditory targets varying in pitch and visual targets varying in location. Responses to redundant targets were faster when both were high or low than when they were incongruent. Experiment 2 varied joint probability of redundant pairs, and responses were faster to common pairs than rare ones. The results indicate that responses to redundant targets are a joint of both identities, not a concatenation of independent activations.

The influence of frequency of occurrence of a visual stimulus on encoding processes is investigated, to discover what mechanisms allow cognitive processes to modify perceptual processes. Six experiments are described and the results are discussed. (MH)