Stimulus-specific processing of auditory deviants and repetitive stimulus sequences in the human brainstem

Detecting unexpected events, also known as deviant stimuli, is essential for survival. Research on the neural mechanisms that underlie deviance detection has gained momentum in the last decades. One major discovery from this research is that deviance detection is not limited to high-order cortical areas but is also present in evolutionary older subcortical structures. However, most of the research studying subcortical deviance detection has so far been limited to animal experiments, with studies in humans mostly focussing on cortical effects. It is therefore still mostly unknown how and to what extend deviance detection manifests in the human brainstem. Here, we aimed to tackle this issue by measuring auditory brainstem responses (ABRs) to investigate the earliest correlates of deviance detection in the human brain. We demonstrate that the human brainstem is sensitive to auditory deviants and that the measured effects depend on the carrier frequency of the stimuli. We found the strongest and fastest deviance detection responses when low-frequency chirps occurred as deviants in a context of high-frequency stimuli. A second experiment revealed that the larger deviant ABR amplitudes can be explained by repetition suppression effects of the more frequent standard stimulus. On the contrary, high-frequency chirps did not elicit deviance detection and caused repetition enhancement instead of suppression. These results show that the human auditory brainstem is sensitive to the stimulus’ probability of occurrence and can use different, stimulus-specific processing mechanisms. Our results reveal a previously unknown complexity of advanced auditory signal processing in the human brainstem.