The auditory brainstem responds more consistently to regular sound sequences than irregular sound sequences, according to a recent study published this September in Neuroscience. The study adds to our understanding of how the brain processes regular sound patterns.
Entrainment is the synchronization of organisms to an external rhythm, and it has been suggested that entrainment to regular patterns in sound may play a major role in important cognitive functions (e.g. the perception of beat in music).
Neurons in the auditory cortex (where sounds are processed) synchronize their responses to regular time patterns in sound input. It has been proposed that these neurons may underlie a pattern-forming system that couple to external sound rhythms, which enables entrainment to different rates of the auditory input.
Along with the cortex, another region known to play a part in the processing of sounds is the brainstem – the lower part of the brain which joins to the spinal cord. Auditory brainstem responses show some of the beat-related effects found in the cortex. Interestingly, there are larger responses in the brainstem for sound that falls on a beat compared to off a beat.
It is known that an omission response occurs in the auditory cortex when sounds are infrequently omitted from a regular sequence of identical sounds. However, it is unknown whether the omission of sounds evokes a brainstem response.
The study, by Alexandre Lehmann (University of Montreal and McGill University), Diana Jimena Arias (University of Quebec at Montreal) and Marc Schönwiesner of (University of Montreal), traced entrainment-related processes along the auditory pathway. They simultaneously recorded the EEG (electrical activity in the brain) responses of 15 normal-hearing participants in their brainstem and cortex whilst regular and irregular sequences of a sound were played. There were then random omissions during the sequences.
The results revealed that the auditory cortex responded strongly to omissions whereas the auditory brainstem did not. However, auditory brainstem responses in the regular sound sequence were more consistent across trials than in the irregular sequence. They also found stronger adaptation in the cortex and brainstem of responses to stimuli that preceded omissions than those that followed omissions.
The researchers concluded, “These results indicate that the auditory brainstem faithfully encodes short-term acoustic properties of a stimulus and is sensitive to sequence regularity, but does not entrain to isochronous sequences sufficiently to generate overt omission responses, even for sequences that evoke such responses in the cortex.”
The research adds to our understanding of how the brain processes regular sound patterns, with the researchers highlighting that this “is an important aspect of human cognitive abilities like rhythm, music and speech perception.”