Dissociation Of The Short-term Memory For Segment And Tone In Mandarin As Well As The Underlying Mechanism

Verbal short-term memory refers to the short-term memory for verbal stimuli. It can be influenced by several factors, including features of stimuli and personal experience. Syllable is the basic unit of speech, and it is considered to include two phonological units:a segmental unit such as vowel and constant, and a supra-segmental unit such as tone or stress. Whether memory for segmental unit and memory for supra-segmental unit dissociate from each other remains unknown. And if so, its underlying neural mechanism is still unclear. By manipulating the segmental unit and supra-segmental unit in Mandarin, this paper explored these scientific questions in two steps.The first step was to disclose the effect of music training on short-term memory for segmental unit and for supra-segmental unit. As an important kind of human experience, music training can improve the perception of supra-segmental information. We assumed that if there exists dissociation between the process of memory for segmental unit and memory for supra-segmental unit, music training may have different influence on the short-term memory for these two types of units. In our study, native Mandarin speakers with or without formal music training (categorized as musicians and non-musicians) were recruited. By manipulating the segmental and supra-segmental (tonal) features of Mandarin, three experimental conditions were adopted:segmental variety condition, tonal variety condition and dual (segmental&tonal) variety condition. Memory span under these three conditions were measured in both musicians and non-musicians, and also in both auditory and visual modalities. We observed that:musicians outperformed non-musicians on verbal short-term memory span in both auditory and visual modalities, but only in the tonal variety condition; the memory span of segment was larger than that of tone in both groups and in both modalities. These results suggest that music training may improve the short-term memory for tone, but not for segment, indicating there exists dissociation between the short-term memory for segment and for tone.The second step was to explore the underlying neural mechanism of short-term memory for segmental and supra-segmental units. We recruited college students who were native Mandarin speakers and had never received music training before as participants, and collected brain imaging data while they performed the task of short-term memory for segment and tone using fMRI. By analyzing the imaging data of the three stages of short-term memory (stage of encoding, stage of maintenance and stage of retrieval), we examined the difference on brain activation between memory for segment and memory for tone. Our observation showed:at different stages, there were different brain activation patterns for both segmental memory and tonal memory; while at the same stage, segmental memory and tonal memory revealed similar activation pattern. Specifically, brain activation were found mainly in the areas of temporal lobe during the stage of encoding, while the frontal lobe played a more important role during the stage of maintenance, and wider extent of regions were involved in the stage of retrieval, including various regions of frontal, temporal, parietal and occipital lobes, as well as subcortical structures. Both hemispheres were activated, with stronger intensity in the left hemisphere than that in the right hemisphere in the conditions of both segment and tone, suggesting left-side dominance in the process of verbal short-term memory. However, although similar activation pattern was found in different syllable units, there still existed some difference in the specific activated brain regions and the intensity of activation. In summary, these results reveal that there exists dissociation in the neural mechanism related with segmental memory and tonal memory of Chinese Mandarin.Taken together, our study examined the effect of music training on the short-term memory for segment and for tone, and explored the associated neural mechanism. The results demonstrate that there exists certain dissociation between segmental memory and tonal memory in Mandarin, and provide initial perspective for the exploration of its underlying mechanism.