Predictive Coding beyond simple tone-based oddball designs

As the evidence of predictive processes playing a role in a wide variety of cognitive domains increases, the brain as a predictive machine becomes a central idea in neuroscience. In auditory processing a considerable amount of progress has been made using variations of the Oddball design, but most of the existing work seems restricted to simple stimuli and predictions based on physical features or conditional rules linking successive stimuli. Here we present two experiments that use speech-like stimuli to overcome these limitations and avoid common confounds. Pseudowords were presented in isolation, intermixed with infrequent deviants that contained unexpected phoneme sequences. As hypothesized, the occurrence of unexpected sequences of phonemes reliably elicited an early prediction error signal, compatible with a MMN-like response. These prediction error signals were not modulated by an attentional manipulation induced by different task instructions, suggesting that the predictions are deployed even when the task at hand does not volitionally involve error detection. In contrast, the amount of syllables congruent with a standard pseudoword presented before the point of deviance exerted a strong modulation. Prediction error's amplitude doubled when two congruent syllables were presented instead of one, despite keeping local transitional probabilities constant, this suggest that auditory predictions can be built integrating information beyond the immediate past. In sum, the results presented here attest the predictive capabilities of the human auditory system when facing complex stimuli and abstract rules.