Involvement of superior temporal areas in audiovisual and audiomotor speech integration.

Perception of speech sounds is affected by observing facial motion. Incongruence between speech sounds and watching somebody articulating may influence the perception of auditory syllable, referred to as the McGurk effect. We tested the degree to which silent articulation of a syllable also affects speech perception and searched for its neural correlates. Listeners were instructed to identify the auditory syllables /pa/ and /ta/ while silently articulating congruent/incongruent syllables or observing videos of a speaker's face articulating them. As a baseline, we included an auditory-only condition without competing visual or sensorimotor input. As expected, perception of sounds degraded when incongruent syllables were observed, and also when they were silently articulated, albeit to a lesser extent. This degrading was accompanied by significant amplitude modulations in the beta frequency band in right superior temporal areas. In these areas, the event-related beta activity during congruent conditions was phase-locked to responses evoked during the auditory-only condition. We conclude that proper temporal alignment of different input streams in right superior temporal areas is mandatory for both audiovisual and audiomotor speech integration.

When distance matters: Perceptual bias and behavioral response for approaching sounds in peripersonal and extrapersonal space.

Studies on sound perception show a tendency to overestimate the distance of an approaching sound source, leading to a faster reaction time compared to a receding sound source. Nevertheless, it is unclear whether motor preparation and execution change according to the perceived sound direction and distance, particularly when the sound falls inside the individual's peripersonal space. In this study we developed several auditory stimuli by means of two speakers, generating sounds moving toward the perceiver but stopping at different distances from her/him. Participants were asked to raise their arms as soon as the sound stopped, and their premotor and motor movement components were recorded by means of electromyography (EMG). Error in locating the perceived sound distance was also measured by asking participants to walk to the point in space where they believed the sound had stopped. Results showed that action initiation was anticipated as a function of sound distance: the closer the sound, the earlier the movement onset, when the sound entered the subject's peripersonal space. Less error for distance estimation was present when the sound was inside the peripersonal space with a modulation in the order of a few centimeters. Overall, our results reveal a link between perceptual bias in sound distance evaluation and peripersonal space, suggesting the presence of motor plan specificity.