Fusion prevents the redundant signals effect: evidence from stereoscopically presented stimuli

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.     

Effects of redundant visual stimuli on temporal order judgments

Four experiments were conducted in order to compare the effects of stimulus redundancy on temporal order judgments (TOJs) and reaction times (RTs). In Experiments 1 and 2, participants were presented in each trial with a tone and either a single visual stimulus or two redundant visual stimuli. They were asked to judge whether the tone or the visual display was presented first. Judgments of the relative onset times of the visual and the auditory stimuli were virtually unaffected by the presentation of redundant, rather than single, visual stimuli. Experiments 3 and 4 used simple RT tasks with the same stimuli, and responses were much faster to redundant than to single visual stimuli. It appears that the traditional speedup of RT associated with redundant visual stimuli arises after the stimulus detection processes to which TOJs are sensitive.     

Testing the race inequality: A simple correction procedure for fast guesses

In speeded response tasks with redundant signals, parallel processing of the redundant signals is generally tested using the so-called race inequality. The race inequality states that the distribution of fast responses for a redundant stimulus never exceeds the summed distributions of fast responses for the single stimuli. It has been pointed out that fast guesses (e.g. anticipatory responses) interfere with this test, and a correction procedure (‘kill-the-twin’ procedure) has been suggested. In this note we formally derive this procedure and extend it to the case in which redundant stimuli are presented with onset asynchrony. We demonstrate how the kill-the-twin procedure is used in a statistical test of the race model prediction.     

The influence of dichotical fusion on the redundant signals effect,localization performance,and the mismatch negativity

In two experiments, each including a simple reaction time (RT) task, a localization task, and a passive oddball paradigm, the physical similarity between two dichotically presented auditory stimuli was manipulated. In both experiments, a redundant signals effect (RSE), high localization performance, and a reliable mismatch negativity (MMN) was observed for largely differing stimuli, suggesting that these are coded separately in auditory memory. In contrast, no RSE and a localization rate close to chance level (experiment 1) or at chance (experiment 2) were observed for stimuli differing to a lesser degree. Crucially, for such stimuli a small (experiment 1) or no (experiment 2) MMN were observed. These MMN results indicate that such stimuli tend to fuse into a single percept and that this fusion occurs rather early within information processing.     

The auditory redundant signals effect: An influence of number of stimuli or number of percepts?

In two experiments, we examined simple reaction times (RTs) for detection of the onsets and offsets of auditory stimuli. Both experiments assessed the redundant signals effect (RSE), which is traditionally defined as the reduction in RT associated with the presentation of two redundant stimuli, rather than a single stimulus. In Experiment 1, with two identical tones presented via headphones to the left ear, right ear, or both, no RSE was found in responding to tone onsets, but a large RSE was found in responding to their offsets. In Experiment 2, with a pure tone and white noise as the two stimulus alternatives, RSEs were found for responding to both onsets and offsets. The results support the notion that the occurrence of an RSE depends on the number of percepts, rather than the number of stimuli, and on the requirement to respond to stimulus onsets versus offsets. The parallel grains model (Miller & Ulrich, 2003) provides one possible account of this pattern of results.     

On quantifying multisensory interaction effects in reaction time and detection rate

Both mean reaction time (RT) and detection rate (DR) are important measures for assessing the amount of multisensory interaction occurring in crossmodal experiments, but they are often applied separately. Here we demonstrate that measuring multisensory performance using either RT or DR alone misses out on important information. We suggest an integration of RT and DR into a single measure of multisensory performance: the first index (MRE*) is based on an arithmetic combination of RT and DR, the second (MPE) is constructed from parameters derived from fitting a sequential sampling model to RT and DR data simultaneously. Our approach is illustrated by data from two audio–visual experiments. In the first, a redundant targets detection experiment using stimuli of different intensity, both measures yield similar pattern of results supporting the “principle of inverse effectiveness”. The second experiment, introducing stimulus onset asynchrony and differing instructions (focused attention vs. redundant targets task) further supports the usefulness of both indices. Statistical properties of both measures are investigated via bootstrapping procedures.     

Age-related differences in interhemispheric visuomotor integration measured by the redundant target effect

Presentation of bilateral redundant visual stimuli produces faster reaction times (RT) than presentation of a single unilateral stimulus; an effect known as the redundant target effect (RTE; Miller, 1982), and is a means of testing interhemispheric visuomotor integration (Ouimet, 2009). RTEs that exceed expectations, based on Miller's race model of inequality (RMI), are referred to as "enhanced RTEs" and imply neural coactivation. Paradoxically, enhanced RTEs are observed in cases of corpus callosum disruption. The Hemispheric Coactivation Hypothesis accounts for this paradox by positing that bihemispheric processing occurs to both unilateral and bilateral stimuli in the normal brain, but occurs only with bilateral stimuli in the disconnected brain. Neuroimaging has revealed decreases in the microstructural integrity of the corpus callosum with age (Ota et al., 2006), but research investigating the bilateral RTE in healthy older individuals is lacking. The present study investigated the bilateral RTE in healthy younger and healthy older adults using simple RT and choice RT tasks. Our prediction that older individuals would show significantly larger RTEs than younger individuals was found to be true for both tasks. Tests of the RMI produced little evidence for coactivation. The crossed-uncrossed difference, generally used as a means of testing visuomotor interhemispheric transfer, was also investigated, but no age effects were found. The observation of greater RTE in age is congruent with the Hemispheric Coactivation hypothesis (Miller, 2004) in which callosal disconnection is associated with increased RTE.     

Redundancy phenomena are affected by response requirements

Results are reported for two go/no-go reaction time (RT) experiments, in which the redundant targets advantage was investigated. These experiments were replications of two earlier choice reaction time (CRT) experiments, in which letter stimuli were used. Important differences between the go/no-go RT experiments and the CRT experiments were obtained. Equal and significant redundancy advantages were obtained whether redundant targets were compared with a single target presented with a noise letter or without noise. In the CRT experiments, the advantage was not obtained in the comparison with a single target presented alone. Noise letters did not slow the RTs to single targets with which they were presented as was the case with CRT. Since the differing results of the two procedures depend on the response requirements, explanation of differing CRT data in terms of perceptual or attentional concepts is probably inappropriate. The presence and absence of response competition in the two situations may be the best interpretation. The results tend to support a conclusion of the parallel processing of two letter stimuli separated spatially by as much as 3 degrees.     

Effects of spatial and selective attention on basic multisensory integration

When participants respond to auditory and visual stimuli, responses to audiovisual stimuli are substantially faster than to unimodal stimuli (redundant signals effect, RSE). In such tasks, the RSE is usually higher than probability summation predicts, suggestive of specific integration mechanisms underlying the RSE. We investigated the role of spatial and selective attention on the RSE in audiovisual redundant signals tasks. In Experiment 1, stimuli were presented either centrally (narrow attentional focus) or at 1 of 3 unpredictable locations (wide focus). The RSE was accurately described by a coactivation model assuming linear superposition of modality-specific activation. Effects of spatial attention were explained by a shift of the evidence criterion. In Experiment 2, stimuli were presented at 3 locations; participants had to respond either to all signals regardless of location (simple response task) or to central stimuli only (selective attention task). The RSE was consistent with task-specific coactivation models; accumulation of evidence, however, differed between the 2 tasks.     

Redundant visual signals boost saccade execution

The redundant signal effect (RSE) refers to the fact that human beings react more quickly to a pair of stimuli than to only one stimulus. In previous studies of the RSE in the oculomotor system, bimodal signals have been used as the goal of the saccade. In consistency with studies using manual response times (RTs), saccadic RTs have been shown to be shorter for redundant multimodal stimuli than for single unimodal stimuli. In the present experiments, we extended these findings by demonstrating an RSE in the saccadic system elicited only by unimodal visual stimuli. In addition, we found that shorter saccadic RTs were accompanied by an increased saccadic peak velocity. The present results are of relevance for neurophysiological models of saccade execution, since the boost of saccades was elicited by two visual transients (acting as a “go” signal) that were presented not at the goal of the saccade but at various other locations.     

Multisensory integration of vision and touch in nonspatial feature discrimination tasks

Multisensory integration of nonspatial features between vision and touch was investigated by examining the effects of redundant signals of visual and tactile inputs. In the present experiments, visual letter stimuli and/or tactile letter stimuli were presented, which participants were asked to identify as quickly as possible. The results of Experiment 1 demonstrated faster reaction times for bimodal stimuli than for unimodal stimuli (the redundant signals effect (RSE)). The RSE was due to coactivation of figural representations from the visual and tactile modalities. This coactivation did not occur for a simple stimulus detection task (Experiment 2) or for bimodal stimuli with the same semantic information but different physical stimulus features (Experiment 3). The findings suggest that the integration process might occur at a relatively early stage of object-identification prior to the decision level.     

Redundancy gain with static versus moving hands: a test of the hemispheric coactivation model

A simple reaction time (RT) experiment was conducted to test the hypothesis that redundancy gain arises partly because of hemispheric coactivation. Stimuli were presented to the left or right of fixation, or redundantly to both, and the participants had to make keypress responses as rapidly as possible to stimulus onset. A "static" condition, in which the participants held their hands at rest, was compared with a "dynamic" condition, in which they moved their hands back and forth in an oscillating motion prior to stimulus onset. As predicted from the hypothesis of hemispheric coactivation, redundancy gain, measured as the decrease in mean RT to redundant stimuli as compared with single stimuli, was smaller in the dynamic condition than in the static one.     

Simple reaction time and statistical facilitation: a parallel grains model

A race-like model is developed to account for various phenomena arising in simple reaction time (RT) tasks. Within the model, each stimulus is represented by a number of grains of information or activation processed in parallel. The stimulus is detected when a criterion number of activated grains reaches a decision center. Using the concept of statistical facilitation, the model accounts for many classical effects on mean simple RT, including those of stimulus area, stimulus intensity, stimulus duration, criterion manipulations, redundant stimuli, and the dissociation between intensity effects on simple RTs and temporal order judgments. The model is also consistent with distributional properties of simple RTs.     

Locus of the redundant-signals effect in bimodal divided attention: a neurophysiological analysis.

We reanalyzed the data from the study of Lamarre, Busby, and Spidalieri (1983). In that study, the activity of single neurons in area 4 of the motor cortex was recorded during a bimodal detection task in which a monkey (Macaca mulatta) had to respond as quickly as possible to a visual or an auditory signal or to both (redundant trials). Manual responses on redundant trials were speeded by the presence of both signals, as is typically found. The times between signal onsets and the first changes in neuronal activity were also speeded by redundant signals, but there was no difference between redundant-signals and single-signal trials in the time between the change in neuronal activity and movement onset. These results suggest that late motor processes are not speeded by redundant signals in bimodal detection tasks.     

On the relationship between the redundant signals effect and temporal order judgments: parametric data and a new model

Paradigms used to study the time course of the redundant signals effect (RSE; J. O. Miller, 1986) and temporal order judgments (TOJs) share many important similarities and address related questions concerning the time course of sensory processing. The author of this article proposes and tests a new aggregate diffusion-based model to quantitatively explain both the RSE and TOJs and the relationship between them. Parametric data (13 stimulus onset asynchronies) from an experiment with pairs of visual stimuli (626-nm LEDs) confirm that, relative to central signals (3 degrees), peripheral signals (35 degrees) yield slower reaction times, more strongly modulated RSE time-course functions, and flatter TOJ psychometric functions. All of these qualitative features are well captured, even in quantitative detail, by the aggregate diffusion model.     

Locus of the redundant-signals effect in bimodal divided attention: A neurophysiological analysis

We reanalyzed the data from the study of Lamarre, Busby, and Spidalieri (1983). In that study, the activity of single neurons in area 4 of the motor cortex was recorded during a bimodal detection task in which a monkey (Macaca mulatta)had to respond as quickly as possible to a visual or an auditory signal or to both (redundant trials). Manual responses on redundant trials were speeded by the presence of both signals, as is typically found. The times between signal onsets and the first changes in neuronal activity were also speeded by redundant signals, but there was no difference between redundant-signals and single-signal trials in the time between the change in neuronal activity and movement onset. These results suggest that late motor processes are not speeded by redundant signals in bimodal detection tasks.     

The anchor effects on the judgment of loudness using reaction time as an index of loudness

The present study was designed to investigate anchor effects on loudness judgments, using reaction time (RT) as an index of loudness. In Experiment 1, anchor effects were reexamined using verbal categories. Two kinds of anchor stimuli, 60- and 90-dB SPL 1,000-Hz pure tones, and four kinds of series stimuli, 60-, 70-, 80-, and 90-dB tones, were used. In this experiment, clear anchor effects were found just the same as in our previous experiment. Experiment 2 was conducted using RT as an index of loudness with stimulus conditions similar to those in Experiment 1. The same anchor effects could be seen in this experiment too. As RT is quite free from the limitations inevitably accompanying the verbal responses, it may be concluded that the anchor effects reflect the shift in perception.     

Response time distributions in multidimensional perceptual categorization

Three speeded categorization experiments were conducted using separable dimension stimuli. The form of the category boundary was manipulated across experiments, and the distance from category exemplars to the category boundary was manipulated within each experiment. Observers completed several sessions in each experiment, yielding 300–400 repetitions of each stimulus. The large sample sizes permitted accurate estimates of the response time (RT) distributions and RT hazard functions. Analyses of these data indicated: (1) RT was faster for stimuli farther from the category boundary, and this stochastic dominance held at the level of the RT distributions; (2) RT was invariant for all stimuli the same distance from the category boundary; (3) when task difficulty was high, errors were slower than correct responses, whereas this difference disappeared when difficulty was low; (4) small, consistent response biases appeared to have a large effect on the relation between correct and error RT; (5) the shape of the RT hazard function was qualitatively affected by distance to the category boundary. These data establish a rich set of empirical constraints for testing developing models of categorization RT.