Cortical Activity during Postural Audio-Biofeedback: A Study Based on Quantitative EEG

The control of postural sway depends on the dynamic integration of multisensory information in the Central Nervous System. Augmentation of sensory information, such as during auditory biofeedback (ABF) of the trunk acceleration, has been shown to improve postural control. We evaluated, by means of quantitative electroencephalography (EEG), which are the basic processes in the brain involved in the perception and cognition of auditory signals used for ABF. ABF and Fake ABF (FAKE) auditory stimulations were delivered to 10 healthy naive subjects during quiet standing postural tasks, with eyes open and closed. Trunk acceleration and 19-channels EEG were recorded at the same time. Advanced, state-of-the-art EEG analysis and source modeling methods were employed to assess the possibly differential, functional activation and localization of EEG spectral features (power in the α, β, and γ bands) between the FAKE and the ABF conditions, for both the eyes open and the eyes closed tasks. Subjects gained advantage by ABF in reducing their postural sway, as measured by a reduction of the root mean square of trunk acceleration during the ABF compared to the FAKE condition. EEG outcomes support the idea that ABF for postural control heavily modulates (increases) the cortical activation in healthy subjects. Population-wise cortical localization analysis (based on the sLORETA method) performed on the comparison FAKE - ABF revealed: i) a significant decrease (p < 0.05) of α power in the right inferior parietal cortex (Brodmann area 40) for the eyes open task; ii) a significant increase (p < 0.05) of γ power in the left temporo-occipital areas in the eyes open task (Brodmann areas 17,18,19, and 30); iii) a significant increase (p < 0.05) of γ power in left temporo-parietal areas for the eyes closed task (Brodmann areas 21,37, and 41). All in all, the findings reported above suggest a completely different usage of the ABF sound by healthy subjects in the EC Task as compared to the EO Task. It would indeed seem that, in the eyes open task, the spatial information coded in the ABF sound is primarily used by the brain in association with visual information: the significant decrease of α power in the right IPL and the significant increase of γ power in primary and associative visual areas suggest that the ABF sound could i) just modulate the processing of visual information and ii) trigger some specific multi-sensory integration processes in areas associated with multisensory, perceptual integration. It is obvious that, in the eyes closed task, the brain cannot use the visual information. In the absence of visual information, the perception and the elaboration of the spatial content of the ABF sound would originate, at least partially, by an additional processing workload of the associative cortical auditory areas in the left temporal lobe and in the left temporo-parietal junction. This hypothesis is consistent with the fact that a significant increase of γ power due to ABF was found for the eyes closed task, but not for the eyes open task.