Auditory perception during walking is not a passive process, but one that changes as the body moves through the environment. Using auditory steady-state responses (ASSR), recent research shows that walking alters how the brain responds to sound. These findings highlight how sensory processing adapts dynamically during real-world movement.
Experiment 1 & 2: different conditions to analyze auditory perception during walking
Experiment 1: Participants walked in the shape of an 8 while listening to auditory stimulus. During the experiment, they also had two more conditions: standing and stepping. Researchers marked a spot on the walking pathway where participants needed to stand or step in place.
To compare auditory perception during walking vs. other conditions, authors looked at ASSR. To elicit ASSR, participants are exposed to a continuous stream of sinusoidal tones, with amplitude modulation.
The tone that was modulated was either on 39 Hz or 41 Hz. 39 Hz tone is played in the left ear, while 41 Hz tone in the right ear. Authors chose these frequencies based on previous studies, where ASSR reached its max response ~40 Hz in humans.
Experiment 2: Similar to experiment one, it had walking and standing conditions. Also, the ASSR is elicited in the same way – with 39 Hz tone on the left, and 41 Hz tone on the right ear.
However, researchers included perturbations this time. Perturbations tested how walking can change the response to unexpected sounds, and if movement enhances sensitivity to these disruptions.
There were two different types of auditory disruptions. First, the diotic perturbations: central burst tones (both in the left and right ear). And the dichotic perturbations, where the burst is either on the left or right.
Auditory perception during walking: experimental design. (a) Walking paths for experiment 1 and 2. The direction in which the participants had to walk was always marked on the floor. The direction is not constant i.e. they changed in both experiments. The green dot on the pathway is the place where “standing” condition happened. As well as stepping in place in experiment 1. (b) Auditory stimulus for both experiments. The sinusoidal stimulus is present in both experiment 1 and 2. In the bottom, we see the perturbation tones for experiment 2.Recording and analyzing EEG data
Researchers used mbt’s portable EEG system for EEG collection, to examine auditory perception during walking and stationary conditions.
Using Independent Component Analysis (ICA), authors decomposed EEG data and selected components of interest. For ASSR, they looked at components with the clearest response at 39 or 41 Hz. Once they identified them, these components are then used to reconstruct the data back into the sensor space.
A similar procedure is done for finding the alpha components. The criteria for the selected components boil down to:
- Spectral: strong power in the alpha band (6 – 14 Hz)
- Spatial: plausible brain (not noise) scalp topography
- Physiological: not eye, muscle, or movement artifact
- Reliability: present consistently across conditions
(a) Example of an ICA component with high ASSR. (b) The final topography after selecting ASSR components. Topography is presented for all participants, on the left is during experiment 1 and experiment 2 is on the right. (c) ICA with prominent alpha activity in the frequency spectrum. (d) Final topography after alpha component selection, for all participants.For ASSR, the fronto-central electrodes had the strongest ASSR power during processing. So, these are selected for further analysis. For final alpha component power, we can see from the image above that it’s focused in the occipital area (POz, O1 and O2).
ASSR lateralization index
Authors wanted to determine whether walking and turning direction shift how sound is processed. So, they extracted the ASSR lateralization index:
(39Hz ASSR-power – 41Hz ASSR power) / (39Hz ASSR-power + 41Hz ASSR power)
A larger index indicates stronger processing of the input from the left ear. And a smaller index would mean a relatively stronger processing of the right-ear input.
How authors extracted the walking path parameters
Participants had three motion sensors attached in total: two to their ankles and one on their back. These collected velocity and acceleration data. Then they are used to identify important walking path points.
(e) Original and filtered gyroscope data across time. (f) Authors used the filtered gyro data to extract blue and red periods – turning left and turning right. The peak of the sine function is the middle point of each turn. This is taken as the 0-time point which is used for analyzing ASSR lateralization.Walking resulted in higher ASSR power and decreased alpha
(a) Experiment 1: ASSR power during step, walk and stand conditions. We can see two clear peaks on 39 Hz (left ear) and 41 Hz (right). (b) Experiment 1: Alpha power during step, walk and stand (pink, black and green respectively). (c) Experiment 1: correlation results between alpha power difference (stand – walk) and ASSR power difference (stand – walk). (d) EXPERIMENT 2: ASSR power during walk and stand (black and green). (e) Experiment 2: Alpha power during walk and stand (black and green). (f) Experiment 2: correlation results between alpha power difference (stand – walk) and ASSR power difference (stand – walk).ASSR power was significantly higher during walking compared to standing or stepping in place. Alpha power however is decreased in the walking condition compared to standing or stepping. Negative correlations between the two are also significant, in both experiment 1 and 2.
The authors interpret the increase in ASSR during walking as evidence that auditory processing is strengthened during movement. It’s likely to support awareness of the surrounding environment. They also note the importance of the ASSR difference between walking and stepping in place. Similar motor actions will not result in the same sensory processing. It’s the combination of motor output with valid spatial aim that produces the strongest response.
They describe the decrease in alpha power during walking as a reduction in inhibitory brain activity. It indicates a shift toward greater sensory readiness.
Turning caused the ASSR lateralization index to shift dynamically
(a,b) ASSR index modulation during the walking path in both experiment 1 and 2. (c) Schematic walking path in both experiments. Orange dot in all plots represents time point 0.What we can see from the ASSR index and the difference between hemispheres:
- Early in a turn, the hemisphere toward the turn direction.
- Later in the turn, dominance shifted to the opposite hemisphere.
This shows that the brain dynamically reallocates auditory processing to different hemispheres, depending on movement direction. Likely to enhance environmental awareness while navigating.
Effects of auditory perturbations in Experiment 2
ASSR power during (a) left, (b) right and (c) central perturbations. Red is for 41-Hz burst and we can see the result for standing (full) and walking (dashed) conditions. Same thing for 39-Hz burst, only in black. At the bottom of each plot, we see the statistical comparison.We can see that the burst in the left ear – 39 Hz showed stronger response during walking than standing. The same can be observed for the right ear, 41 Hz perturbation, however not as statistically significant. Central perturbations showed to difference between walking and standing.
The authors interpret these findings to mean that walking enhances the processing of sound perturbations perceived in the periphery. This suggests that during locomotion, the auditory system may prioritize peripheral input. This is to detect changes or events potentially better in the surrounding environment.
Because the central perturbations didn’t t show stronger effects during walking, the enhancement appears to be specific to spatially localized (peripheral) auditory input.
ERPs complement the ASSR findings in auditory perception during walking
ERPs were measured as a response to these auditory perturbations. Researchers found that ERPs were larger during walking than standing. Especially to perturbations presented to one ear, instead of central. Walking not only boosts continuous auditory processing (ASSR) but also makes the brain more responsive to discrete, potentially important auditory events.
Check out the original study by Xinyu Chen linked here, for more detail and more significant insights on auditory perception during walking.
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