Bi-directional audiovisual influences on temporal modulation discrimination

Audiovisual interaction with rate-varying signals

A task-irrelevant, amplitude-modulating sound influences perception of a size-modulating visual stimulus. To probe the limits of this audiovisual interaction we vary the second temporal derivative of object size and of sound amplitude. In the study's first phase subjects see a visual stimulus size-modulating with f ″ ( x ) > 0, 0, or <0, and judge each one's rate as increasing, constant, or decreasing. Visual stimuli are accompanied by a steady, non-modulated auditory stimulus. The novel combination of multiple stimuli and multi-alternative responses allows subjects' similarity space to be estimated from the stimulus-response confusion matrix. In the study's second phase, rate-varying visual stimuli are presented in concert with auditory stimuli whose second derivative also varied. Subjects identified each visual stimuli as one of the three types, while trying to ignore the accompanying sound. Unlike some previous results with f ″ ( x ) fixed at 0, performance benefits relatively little when visual and auditory stimuli share the same directional change in modulation. However, performance does drop when visual and auditory stimului differ in their directions of rate change. Our task's computational demands may make it particularly vulnerable to the effects of a dynamic task-irrelevant stimulus.

Costs and benefits of audiovisual interactions

A strong temporal correlation promotes integration of concurrent sensory signals, either within a single sensory modality, or from different modalities. Although the benefits of such integration are well known, far less attention has been given to possible costs incurred when concurrent sensory signals are uncorrelated. In two experiments, subjects categorized the rate at which a visual object modulated in size, while they also tried to ignore a concurrent task-irrelevant broadband sound. Overall, the experiments showed that (i) losses in accuracy from mismatched auditory and visual rates were larger than gains from matched rates and (ii) mismatched auditory and visual rates slowed responses more than they were sped up when rates matched. Experiment One showed that audiovisual interaction varied with the difference between the visual modulation rate and the modulation rate of a concurrent auditory stimulus. Experiment Two showed that audiovisual interaction depended upon the strength of the task-irrelevant auditory modulation. Although our stimuli involved abstract, low-dimensional stimuli, not speech, the effects we observed parallel key findings on interference in multi-speaker settings.

Decision-Making and Multisensory Combination Under Time Stress

Using a video game platform, we examined how vision-based decision making was affected by a concurrent, potentially conflicting auditory stimulus. Electroencephalographic responses showed that by 150 milliseconds of stimulus onset, the brain had detected the conflict between visual and auditory stimuli. Systematically reducing the intertrial interval (ITI), which subjects described as stressful, undermined decision making. Subjects' arterial pulse variance decreased along with ITI, signaling increased parasympathetic influence on the heart. When successive trials required a shift in processing mode, short ITIs significantly boosted one trial's influence on the next, suggesting that stress reduces cognitive flexibility. Finally, our study demonstrates the heart's and the brain's important influence on decision making.

Feeling the Beat (and Seeing It, Too): Vibrotactile, Visual, and Bimodal Rate Discrimination

Beats are among the basic units of perceptual experience. Produced by regular, intermittent stimulation, beats are most commonly associated with audition, but the experience of a beat can result from stimulation in other modalities as well. We studied the robustness of visual, vibrotactile, and bimodal signals as sources of beat perception. Subjects attempted to discriminate between pulse trains delivered at 3 Hz or at 6 Hz. To investigate signal robustness, we intentionally degraded signals on two-thirds of the trials using temporal-domain noise. On these trials, inter-pulse intervals (IPIs) were stochastic, perturbed independently from the nominal IPI by random samples from zero-mean Gaussian distributions with different variances. These perturbations produced directional changes in the IPIs, which either increased or decreased the likelihood of confusing the two pulse rates. In addition to affording an assay of signal robustness, this paradigm made it possible to gauge how subjects' judgments were influenced by successive IPIs. Logistic regression revealed a strong primacy effect: subjects' decisions were disproportionately influenced by a trial's initial IPIs. Response times and parameter estimates from drift-diffusion modeling showed that information accumulates more rapidly with bimodal stimulation than with either unimodal stimulus alone. Analysis of error rates within each condition suggested consistently optimal decision making, even with increased IPI variability. Finally, beat information delivered by vibrotactile signals proved just as robust as information conveyed by visual signals, confirming vibrotactile stimulation's potential as a communication channel.

Temporal causal inference with stochastic audiovisual sequences

Integration of sensory information across multiple senses is most likely to occur when signals are spatiotemporally coupled. Yet, recent research on audiovisual rate discrimination indicates that random sequences of light flashes and auditory clicks are integrated optimally regardless of temporal correlation. This may be due to 1) temporal averaging rendering temporal cues less effective; 2) difficulty extracting causal-inference cues from rapidly presented stimuli; or 3) task demands prompting integration without concern for the spatiotemporal relationship between the signals. We conducted a rate-discrimination task (Exp 1), using slower, more random sequences than previous studies, and a separate causal-judgement task (Exp 2). Unisensory and multisensory rate-discrimination thresholds were measured in Exp 1 to assess the effects of temporal correlation and spatial congruence on integration. The performance of most subjects was indistinguishable from optimal for spatiotemporally coupled stimuli, and generally sub-optimal in other conditions, suggesting observers used a multisensory mechanism that is sensitive to both temporal and spatial causal-inference cues. In Exp 2, subjects reported whether temporally uncorrelated (but spatially co-located) sequences were perceived as sharing a common source. A unified percept was affected by click-flash pattern similarity and the maximum temporal offset between individual clicks and flashes, but not on the proportion of synchronous click-flash pairs. A simulation analysis revealed that the stimulus-generation algorithms of previous studies is likely responsible for the observed integration of temporally independent sequences. By combining results from Exps 1 and 2, we found better rate-discrimination performance for sequences that are more likely to be integrated than those that are not. Our results support the principle that multisensory stimuli are optimally integrated when spatiotemporally coupled, and provide insight into the temporal features used for coupling in causal inference.