Does a more predictable background make surprises bigger in the brain?
The same rare tone produced a larger mismatch-like brain response when the surrounding sound sequence was more stable and predictable.
Source
Standard Tone Stability as a Manipulation of Precision in the Oddball Paradigm: Modulation of Prediction Error Responses to Fixed-Probability Deviants
Study at a glance
- Design
- Human experiment — Within-subject passive auditory EEG with four tone sequences in which the standard tone's probability varied (10/11, 7/11, 4/11, 1/11) while the deviant tone stayed at 1/11.
- N
- N=20 · 20 analysed healthy adults aged 21-55 (25 recorded; 2 lost to recording problems, 3 to EEG artefacts).
- Population
- Healthy adult volunteers without neurological, psychiatric or hearing problems, recorded in Spain
- Outcome
- ERP amplitude to the fixed-probability deviant tone (and, secondarily, to the standard and the deviant-minus-standard difference wave)
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What they did
Twenty adults read while passively hearing four blocks of rapid pure tones. In every block a particular 'deviant' tone appeared equally rarely, but the 'standard' tone around it ranged from very common (a classic oddball sequence) to no more common than other tones (a random sequence), with filler tones making up the rest. Because the deviant's own probability never changed, differences in its brain response could be attributed to how confident the brain could be about the regularity, not to how rare the deviant was.
What they found
The deviant tone's EEG response became steadily more negative over frontocentral scalp between 122 and 202 ms as the standard became more probable, peaking around 170 ms, the time range of the mismatch negativity (MMN). The deviant response was significantly more negative in the oddball block than in the random and low-confidence blocks. Responses to the standard tone itself showed no significant change, while the classic deviant-minus-standard difference wave showed a graded MMN and an earlier P50 effect.
The limits
What it doesn't show
The sample is small and ages spanned 21 to 55, and only 16 electrodes were used, so source localisation was not possible. The standard and deviant were physically different tones, so the classic difference wave mixes precision with frequency differences and with adaptation to the more repeated standard; the authors therefore rest conclusions on the deviant response alone. Participants were reading and not attending to the tones, so the results may not extend to attended listening, and the study manipulated only pitch variability, leaving other kinds of uncertainty untested.
Key terms
- Mismatch negativity (MMN)
- A negative brain response around 100-200 ms after an unexpected sound that breaks an established pattern.
- Oddball paradigm
- A sequence where a frequent 'standard' stimulus is occasionally replaced by a rare 'deviant'.
- Prediction error
- The mismatch between what the brain predicted and what it actually received.
- Precision weighting
- The predictive-coding idea that prediction errors count for more when the brain is confident in its predictions and less in noisy contexts.
- Refractoriness / adaptation
- Reduced neural responses to a stimulus that has been repeated often, an alternative explanation for some MMN effects.
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Quiz yourself
What made this oddball design novel?
Common questions
Why keep the deviant tone's probability the same in every block?
If the deviant's response only reflected its rarity, it would not change. Holding it fixed means any change must come from the surrounding context, i.e., how predictable the sequence was.
Why not rely on the usual deviant-minus-standard MMN?
The standard's response is affected by how often it repeats (adaptation), which varied across blocks, so the difference wave cannot cleanly separate precision from repetition effects.
What does this mean for predictive coding?
It supports the claim that the brain scales its surprise signals by how confident it is in its model: the same unexpected event is treated as more surprising in a stable world.
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