Comparisons between short-term memory systems for verbal and rhythmic stimuli

Auditory short-term memory is often conceived of as a unitary capacity, with memory for different auditory materials (such as syllables, pitches, rhythms) posited to rely on similar neural mechanisms. One spontaneous behavior observed in short-term memory studies is 'chunking'. For example, individuals often recount digit sequences in groups, or chunks, of 3-4 digits, and chunking is associated with better performance. Chunking may also operate in musical rhythm, with beats acting as potential chunk boundaries for tones in rhythmic sequences. Similar to chunking, beat-based structure in rhythms also improves performance. Thus, it is possible that beat processing relies on the same mechanisms that underlie chunking of verbal material. The current fMRI study examined whether beat perception is indeed a type of chunking, measuring brain responses to chunked and 'unchunked' letter sequences relative to beat-based and non-beat-based rhythmic sequences. Participants completed a sequence discrimination task, and comparisons between stimulus encoding, maintenance, and discrimination were made for both rhythmic and verbal sequences. Overall, rhythm and verbal short-term memory networks overlapped substantially. When contrasting rhythmic and verbal conditions, rhythms activated basal ganglia, supplementary motor area, and anterior insula more than letter strings did, during both encoding and discrimination. Verbal letter strings activated bilateral auditory cortex more than rhythms did during encoding, and parietal cortex, precuneus, and middle frontal gyri more than rhythms did during discrimination. Importantly, there was a significant interaction in the basal ganglia during encoding: activation for beat-based rhythms was greater than for non-beat-based rhythms, but verbal chunked and unchunked conditions did not differ. The interaction indicates that beat perception is not simply a case of chunking, suggesting a dissociation between beat processing and chunking-based grouping mechanisms.