Frequency transfer of the ventriloquism aftereffect

The ventriloquism aftereffect (VAE) describes the persistent shift of perceived sound location after having been adapted to a ventriloquism condition, in which the sound was repeatedly paired with a displaced visual stimulus. In the latter case, participants consistently mislocalize the sound in the direction of the visual stimulus (ventriloquism effect, VE). Previous studies provide conflicting reports regarding the strength of the VAE, ranging from 0 to nearly 100%. Moreover, there is controversy about its generalization to different sounds than the one inducing the VE, ranging from no transfer at all, to full transfer across different sound spectra. Here, we imposed the VE for three different sounds: a low-frequency and a high-frequency narrow-band noise, and a broadband Gaussian white noise (GWN). In the adaptation phase, listeners generated fast goal-directed head movements to localize the sound, presented across a 70 deg range in the horizontal plane, while ignoring a visual distracter that was consistently displaced 10 deg to the right of the sound. In the post-adaptation phase, participants localized narrow-band sounds with center frequencies from 0.5 to 8 kHz, as well as GWN, without the visual distracter. Our results show that the VAE amounted to approximately 40% of the VE and generalized well across the entire frequency domain. We also found that the strength of the VAE correlated with the pre-adaptation sound-localization performance. We compare our results with previous reports and discuss different hypotheses regarding optimal audio-visual cue integration.

Restricted recovery of external remapping of tactile stimuli after restoring vision in a congenitally blind man

People with surgically removed congenital dense bilateral cataracts offer a natural model of visual deprivation and reafferentation in humans to investigate sensitive periods of multisensory development, for example regarding the recruitment of external or anatomical frames of reference for spatial representation. Here we present a single case (HS; male; 33 years; right-handed), born with congenital dense bilateral cataracts. His lenses were removed at the age of two years, but he received optical aids only at age six. At time of testing, his visual acuity was 30% in the best eye. We performed two tasks, a tactile temporal order judgment task (TOJ) in which two tactile stimuli were presented successively to the index fingers located in the two hemifields, adopting a crossed and uncrossed hand posture. The participant judged as precisely as possible which side was stimulated first. Moreover, we used a crossmodal-congruency task in which a tactile stimulus and an irrelevant visual distracter were presented simultaneously but independently to one of four positions. The participant judged the location (index or thumb) of the tactile stimulus with hands crossed or uncrossed. Speed was emphasized. In contrast to sighted controls, HS did not show a decrement of TOJ performance with hands crossed. Moreover, while the congruency gain was equivalent to sighted controls with uncrossed hands, this effect was significantly reduced with hands crossed. Thus, an external remapping of tactile stimuli still develops after a long phase of visual deprivation. However, remapping seems to be less efficient and to only take place in the context of visual stimuli.

The Continuity Illusion Does Not Depend on Attentional State: fMRI Evidence from Illusory Vowels

We investigate whether the neural correlates of the continuity illusion, as measured using fMRI, are modulated by attention. As we have shown previously, when two formants of a synthetic vowel are presented in an alternating pattern, the vowel can be identified if the gaps in each formant are filled with bursts of plausible masking noise, causing the illusory percept of a continuous vowel (“Illusion” condition). When the formant-to-noise ratio is increased so that noise no longer plausibly masks the formants, the formants are heard as interrupted (“Illusion Break” condition) and vowels are not identifiable. A region of the left middle temporal gyrus (MTG) is sensitive both to intact synthetic vowels (two formants present simultaneously) and to Illusion stimuli, compared to Illusion Break stimuli. Here, we compared these conditions in the presence and absence of attention. We examined fMRI signal for different sound types under three attentional conditions: full attention to the vowels; attention to a visual distracter; or attention to an auditory distracter. Crucially, although a robust main effect of attentional state was observed in many regions, the effect of attention did not differ systematically for the illusory vowels compared to either intact vowels or to the Illusion Break stimuli in the left STG/MTG vowel-sensitive region. This result suggests that illusory continuity of vowels is an obligatory perceptual process, and operates independently of attentional state. An additional finding was that the sensitivity of primary auditory cortex to the number of sound onsets in the stimulus was modulated by attention.

Effects of a Visual Distracter Task on the Gait of Elderly versus Young Persons

Seniors show deficits of dual-task walking when the second task has high visual-processing requirements. Here, we evaluate whether similar deficits emerge when the second task is discrete rather than continuous, as is often the case in everyday life. Subjects walked in a hallway, while foot proprioception was either perturbed by vibration or unperturbed. At unpredictable intervals, they were prompted to turn their head and perform a mental-rotation task. We found that locomotion of young subjects was not affected by this distracter task with or without vibration. In contrast, seniors moved their legs after the distraction at a slower pace through smaller angles and with a higher spatiotemporal variability; the magnitude of these changes was vibration independent. We conclude that the visual distracter task degraded the gait of elderly subjects but completely spared young ones, that this effect is not due to degraded proprioception, and that it rather might reflect the known decline of executive functions in the elderly.

Vibrotactile Feedback for Brain-Computer Interface Operation

To be correctly mastered, brain-computer interfaces (BCIs) need an uninterrupted flow of feedback to the user. This feedback is usually delivered through the visual channel. Our aim was to explore the benefits of vibrotactile feedback during users' training and control of EEG-based BCI applications. A protocol for delivering vibrotactile feedback, including specific hardware and software arrangements, was specified. In three studies with 33 subjects (including 3 with spinal cord injury), we compared vibrotactile and visual feedback, addressing: (I) the feasibility of subjects' training to master their EEG rhythms using tactile feedback; (II) the compatibility of this form of feedback in presence of a visual distracter; (III) the performance in presence of a complex visual task on the same (visual) or different (tactile) sensory channel. The stimulation protocol we developed supports a general usage of the tactors; preliminary experimentations. All studies indicated that the vibrotactile channel can function as a valuable feedback modality with reliability comparable to the classical visual feedback. Advantages of using a vibrotactile feedback emerged when the visual channel was highly loaded by a complex task. In all experiments, vibrotactile feedback felt, after some training, more natural for both controls and SCI users.