Neural circuits and representational hierarchies supporting human auditory-verbal memory

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The human brain frequently faces a difficult problem: it receives complex, structured information from the environment and must maintain this information "in mind" for some time before making an appropriate decision or action. The structured information that humans must maintain is often linguistic, arriving from the environment in the form of spoken speech or visual signs. Psychologists and neuroscientists have long been attempting to understand how the brain "breaks down" linguistic information, and then builds it up again into the internal mental structures that can be easily and conveniently manipulated to facilitate thought, action, and other complex mental activities such as verbal working memory. Decades of research on verbal working memory has generally focused on the brain's utilization of the human speech system as a way to temporarily store linguistic information-and the "phonological code" has generally been assumed to be the dominant format for such short-term storage. However, recent research suggests that the working memory system is highly flexible and opportunistic, making use of numerous kinds of "internal codes", distributed throughout the brain, to allow for the manipulation of different types of information in different contexts. By examining distributed patterns of neural activity in different brain regions and relating such patterns to systematic variations in stimulus features, we can use functional magnetic resonance imaging (fMRI) to probe the representations that are active during verbal working memory. In the current program of research, we examine how verbal working memory has the capacity to represent different aspects of human speech, from relatively "low-level" acoustic-phonetic features, to variations in pitch that contribute to prosody, and semantic features that are important to word meaning but are largely unrelated to phonology. We propose three main experiments, each sharing a basic form in which (1) a set of syllables or words are presented to a participant during fMRI scanning, (2) the participant must silently rehearse the words/syllables in mind over a delay, and, finally, (3) recall the items as best they can in serial order. By varying the inter-relationships (phonological, semantic, prosodic) among the word lists across trials, we can use statistical methods to infer which brain regions support the maintenance of different feature representations. We hypothesize that, even though the phonological code is indeed critical for verbal working memory as prior research has shown, that other components of the hierarchical set of features that comprise speech are also important for maintaining information "online". Taken together, this work will greatly expand our understanding on the multidimensional nature of verbal working memory, and the brain systems and networks that support it.