Predictive coding
Why do sounds we make ourselves seem quieter to the brain?
Open access · cc by · source: Europe PMC
Before a self-triggered sound, auditory cortex showed stronger alpha rhythms, and people with a bigger alpha boost showed a bigger dampening of the brain's response to the sound.
Study at a glance
- Design
- Human experiment — Within-subject MEG experiment comparing self-triggered tones (finger lift) with computer-presented tones at a regular or jittered rhythm, analysed as evoked fields and source-level time-frequency activity in right auditory cortex.
- N
- N=14 · 14 right-handed healthy volunteers; roughly 100 trials per condition before artefact rejection.
- Population
- Healthy young adults (mean age 22.6) in Glasgow
- Outcome
- M100 evoked field amplitude; pre- and post-stimulus alpha, beta and gamma power and phase locking in auditory cortex, and their correlations
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Key findings
Self-generated tones produced a smaller M100 response, replicating sensory attenuation. Starting about 400 ms before the tone, alpha power (around 10 Hz) was higher in the active condition, and larger alpha increases went with stronger attenuation (rho = -0.74). After the tone, the active condition showed reduced broadband power including gamma and reduced alpha/beta phase locking, while the theta component that carries most of the evoked response did not differ. Higher pre-stimulus alpha predicted lower post-stimulus gamma, both across people and trial by trial, and early gamma appeared to precede alpha/beta phase resetting.
Methodology
Fourteen volunteers heard the same brief tone under different conditions while MEG recorded their brain activity: in the active condition the tone played immediately when they lifted a finger, and in passive conditions the computer played it either at a steady three-second rhythm or at unpredictable intervals. The authors reconstructed activity in right auditory cortex and compared evoked responses, oscillatory power and phase locking between self-generated and regular passive tones, then correlated these measures across people and, in the unpredictable condition, across single trials.
Limitations
The sample was small, so the across-participant correlations are imprecise, as their wide confidence intervals show. Analysis was restricted to one voxel in right auditory cortex to avoid contamination from the left motor cortex, so it says little about the left hemisphere or wider networks. The findings are correlational: alpha, gamma and phase changes are interpreted as prediction precision, prediction error and feedback, but these roles were not manipulated directly, and the authors admit the brief post-stimulus effects may not be true oscillations. The active condition also involves movement and attention differences that are not fully separated from prediction itself.
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