Neural Mechanisms Underlying the Effects of Novel Sounds on Task Performance in Children With and Without ADHD.

Distractibility is one of the key features of attention deficit hyperactivity disorder (ADHD) and has been associated with alterations in the neural orienting and alerting networks. Task-irrelevant stimuli are thus expected to have detrimental effects on the performance of patients with ADHD. However, task-irrelevant presentation of novel sounds seems to have the opposite effect and improve subsequent attentional performance particularly in patients with ADHD. Here, we aimed to understand the neural modulations of the attention networks underlying these improvements. Fifty boys (25 with ADHD) participated in a functional magnetic resonance imaging (fMRI) study in which unique (novel) or repeatedly presented (familiar) sounds were placed before a visual flanker task in 2/3 of the trials. We found that presenting any sound improved task performance in all participants, but the underlying neural mechanisms differed for the type of sound. Familiar sounds led to a stronger increase in activity in the left posterior insula in patients with ADHD compared to typically developing peers. Novel sounds led to activations of the fronto-temporoparietal ventral attention network, likewise in ADHD and TD. These changes in signaling by novelty in the right inferior frontal gyrus were directly related to improved response speed showing that neural orienting network activity following novel sounds facilitated subsequent attentional performance. This mechanism of behavioral enhancement by short distractions could potentially be useful for cognitive trainings or homework situations.

Neural basis of the ventriloquist illusion.

The ventriloquist creates the illusion that his or her voice emerges from the visibly moving mouth of the puppet [1]. This well-known illusion exemplifies a basic principle of how auditory and visual information is integrated in the brain to form a unified multimodal percept. When auditory and visual stimuli occur simultaneously at different locations, the more spatially precise visual information dominates the perceived location of the multimodal event. Previous studies have examined neural interactions between spatially disparate auditory and visual stimuli [2-5], but none has found evidence for a visual influence on the auditory cortex that could be directly linked to the illusion of a shifted auditory percept. Here we utilized event-related brain potentials combined with event-related functional magnetic resonance imaging to demonstrate on a trial-by-trial basis that a precisely timed biasing of the left-right balance of auditory cortex activity by the discrepant visual input underlies the ventriloquist illusion. This cortical biasing may reflect a fundamental mechanism for integrating the auditory and visual components of environmental events, which ensures that the sounds are adaptively localized to the more reliable position provided by the visual input.

On perceived synchrony-neural dynamics of audiovisual illusions and suppressions.

Whenever temporally incongruent audiovisual sequences are presented, the perceived flash rate follows the physical flutter rate. Increasing the auditory flutter rate increases the perceived flicker rate (visual illusions). Likewise, decreasing the flutter rate decreases the perceived flicker rate (visual suppressions). Here, we investigated the electrophysiological correlates of this perceptual phenomenon. Two sequences of visual flashes and auditory beeps were presented either synchronously (both visual flashes (F) and auditory beeps (B) at 3 or 5 Hz, respectively) or asynchronously at different rates (3F5B or 5F3B). Event-related potentials were acquired, while subjects reported the perceived number of flashes (response options: 3, 4, and 5). During asynchronous trials, subjects' flash counts were significantly higher when the flutter rate exceeded the flicker rate (i.e. visual illusions occurred); and lower flutter rate was below the flicker rate (i.e. visual suppressions occurred). Differential brain responses for reported illusions and suppressions (incorrect flash counts) vs. no-illusions/suppressions (correct flash counts) were found over parieto-occipital sites, followed by slow modulations over frontal and occipital areas. Importantly, the modulation over occipital electrodes starting around 500 ms had an inverse polarity for illusions vs. suppressions. These results provide evidence that both sound-induced visual illusions and suppressions are mediated by an interplay of distributed brain regions, in the attempt to fuse asynchronous audiovisual stimuli into a synchronous percept.

Closer in time when farther in space--spatial factors in audiovisual temporal integration.

We investigated the effect of visual eccentricity and spatial alignment on judgments of audiovisual synchrony. Sequences of flashes at 4, 6, and 8 Hz were presented centrally, or at horizontal eccentricities of 6 degrees or 18 degrees. Concurrent sequences of clicks were presented at the same rate as the flashes, or at higher or lower rates. Subjects judged whether the flash rate was the same as (synchronous with), faster than, or slower than the click rate. With the 4- and 6-Hz flash rates, subjects' judgments of audiovisual synchrony increased with increasing eccentricity, but only when the click rate was more rapid than the flash rate. This effect remained even when the size of the peripheral visual stimuli was adjusted to compensate for cortical magnification, and was not significantly influenced by the spatial proximity of the auditory and visual signals. However, it was absent when the auditory and visual stimuli were presented serially rather than concurrently. With the 8-Hz flash rate, synchrony judgments were prevalent irrespective of eccentricity. When two serially presented flash rates were compared, visual-visual matching judgments increased with eccentricity at flash rates of 6 Hz and higher, but decreased at flash rates below 6 Hz. Finally, when two concurrent flash rates were compared, visual-visual synchrony judgments increased with eccentricity at all flash-rate combinations. Together, these results suggest that while perceptual uncertainty can play a role in synchrony judgments at rates of 6 Hz and higher, below 6 Hz eccentricity produces a widening of the window of apparent audiovisual temporal synchrony which perceptual uncertainty cannot explain.

Task-Irrelevant Novel Sounds Improve Attentional Performance in Children With and Without ADHD.

Task-irrelevant salient stimuli involuntarily capture attention and can lead to distraction from an ongoing task, especially in children with ADHD. However, there has been tentative evidence that the presentation of novel sounds can have beneficial effects on cognitive performance. In the present study, we aimed to investigate the influence of novel sounds compared to no sound and a repeatedly presented standard sound on attentional performance in children and adolescents with and without ADHD. We therefore had 32 patients with ADHD and 32 typically developing children and adolescents (8 to 13 years) execute a flanker task in which each trial was preceded either by a repeatedly presented standard sound (33%), an unrepeated novel sound (33%) or no auditory stimulation (33%). Task-irrelevant novel sounds facilitated attentional performance similarly in children with and without ADHD, as indicated by reduced omission error rates, reaction times, and reaction time variability without compromising performance accuracy. By contrast, standard sounds, while also reducing omission error rates and reaction times, led to increased commission error rates. Therefore, the beneficial effect of novel sounds exceeds cueing of the target display by potentially increased alerting and/or enhanced behavioral control.