The human auditory system rapidly encodes auditory regularities. Evidence comes from the oddball paradigm, in which frequent (standard) sounds are occasionally replaced with a rare (deviant) sound. Deviants relative to standards typically elicit signs of prediction error (e.g., MMN and P3a). It is, however, less clear whether deviants, which also bear predictive information but are encountered less often than standards, might inform auditory prediction. To investigate this, naïve participants listened to sound sequences constructed according to a new, modified version of the oddball paradigm: two kinds of deviants differing in their probability of repetition, yield the sound actually following a deviant either conditionally likely or unlikely. As this sound is either the same deviant (repetition) or a standard (no repetition), it is either unlikely or likely with respect to the global stimulus probability at the same time. In an active deviant detection task, we replicated previous behavioural findings, demonstrating that predictive information carried by deviants (conditional probability) is extracted when behaviourally relevant. Our analyses further reveal that respective effects at the level of response times increase with time. However, in a passive listening setting both MMN and P3a were confined to violations of rules based on global probability, while not being sensitive to conditional probability. Though some sensitivity to conditional probability had been observed in a previous study, these effects were tiny compared to those of global probability. Thus, the auditory system seems to mainly rely on rules that are encountered frequently (standard regularity), at least during passive listening.

Predictive coding is the theory that each level of the nervous system constructs a model of its inputs and then tests new inputs against the model. If the new inputs match the model, then no change in the model is required. If not, then extra brain processing is required to update the model. We tested the precision of such a model in the auditory system by using Shepard tones arranged into a discrete Shepard scale—a series of notes, each comprising sine tones of different amplitudes and one octave apart, and with each tone separated from the next by one semitone, yielding a scale that ascends or descends forever. We unpredictably and occasionally replaced an expected tone in the scale by one that was two thirds of a semitone less, one third of a semitone less, one third of a semitone more, or two thirds of a semitone more. We measured the electrical activity of 20 participants’ brains with 128 scalp electrodes (electroencephalography, EEG) while the tones were delivered to their ears. We found that event-related potentials (ERPs) from 180-220 ms to these unpredictable tones were more negative the farther they were from the predicted note and more negative for tones that were less than the expected note. We conclude that the predictive model for the kind of regularity in a discrete Shepard scale has a sensitivity to tones less than one third of a semitone and is more sensitive to undershoots than to overshoots.

The auditory system has an amazing ability to rapidly encode auditory regularities. Evidence comes from the popular oddball paradigm, in which frequent (standard) sounds are occasionally exchanged for rare deviant sounds, which then elicit signs of prediction error based on their unexpectedness (e.g., MMN, P3a). Here, we examine the widely neglected characteristics of deviants being bearers of predictive information themselves: Naïve participants listened to sound sequence constructed according to a new, modified version of the oddball paradigm including two types of deviants that followed diametrically opposed rules: one deviant sound occurred mostly in pairs (repetition rule), the other deviant sound occurred mostly in isolation (non-repetition rule). Due to this manipulation, the sound following a first deviant (either the same deviant or a standard) was either predictable or unpredictable based on its conditional probability associated with the preceding deviant sound. Our behavioural results from an active deviant-detection task replicate previous findings that deviant repetition rules (based on conditional probability) can be extracted when behaviourally relevant. Our electrophysiological findings obtained in a passive-listening setting indicate that conditional probability also translates into differential processing at the P3a level. However, MMN was confined to global deviants and was not sensitive to conditional probability. This suggests that higher-level processing concerned with stimulus selection and/or evaluation (reflected in P3a) but not lower-level sensory processing (reflected in MMN) considers rarely encountered rules.