Neural dynamics of speech encoding and maintenance during phonological working memory

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PROJECT SUMMARY Phonological working memory is the capacity to maintain and manipulate representations of speech sounds temporarily in working memory. It is believed to play a critical role in many aspects of language development, such as vocabulary learning, sentence processing, and acquisition of literacy skills, and deficits of phonological working memory are diagnostic of many developmental language disorders. The classic theoretical framework for phonological working memory posits that information is held in a dedicated working memory storage buffer, separate from speech perception cortex; although more recent cognitive models favor a view in which the same cortical substrate responsible for perception of phonological information also supports maintenance of those neural representations during working memory delay. Indeed, recent human neuroimaging studies show phonological working memory load-dependent recruitment of speech-perception cortex during a variety of tasks and in particular, activation in left superior temporal gyrus has been shown to correlate with out-of-scanner performance on clinical assessments of phonological working memory. However, because these studies are not able to temporally distinguish activation during phonological encoding from persistent maintenance of phonological representations after encoding, we do not know which of these processes to attribute to the observed correlations with behavior. Therefore, the theoretical implications of these findings, as well as their clinical interpretations, critically depend on resolving phonological encoding and working memory maintenance processes in time and space. We propose to investigate the neural dynamics of phonological working memory using intracranial electrocorticography, which allows for the simultaneous recording of many discrete neural populations with millimeter- and millisecond-resolution by placing electrodes directly on the cortical surface of the brains of awake, behaving human patients undergoing surgery for medically intractable epilepsy. This technique will allow us to observe directly the relationship between speech encoding and maintenance during phonological working memory. Through three specific aims we will (1) determine the time course of neural activation during phonological working memory across the cortex, (2) determine whether the same perceptual systems that represent phonological information during encoding also actively maintain those representations during working memory, and (3) determine whether perceptual speech systems participate in broader network-level dynamics during working memory maintenance. By achieving these aims, this project will advance our understanding of the role perceptual speech systems play during phonological working memory. Ultimately, such findings can be used to inform the theoretical basis for the role of phonological working memory in language development and has implications for the clinical characterization and remediation of phonological working memory deficits.