The neural coding of speech across human languages

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PROJECT SUMMARY The basic mechanisms underlying comprehension of spoken language are unknown. We are only beginning to understand how the human brain extracts the most fundamental linguistic elements (consonants and vowels) from a complex and highly variable acoustic signal. Traditional theories have posited a ‘universal’ phonetic inventory shared by all humans, but this has been challenged by other newer theories that each language has its own unique and specialized code. An investigation of the cortical representation of speech sounds across languages can likely shed light on this fundamental question. Previous research has implicated the superior temporal cortex in the processing of speech sounds. Most of this work has been entirely carried out in English. The recording of neural activity directly from the cortical surface from individuals with different language experience is a promising approach since it can provide both high spatial and temporal resolution. Here, we propose to examine the mechanisms of phonetic encoding, by utilizing neurophysiological recordings obtained during neurosurgical procedures. High-density electrode arrays, advanced signal processing, and direct electrocortical stimulation will be utilized to unravel both local and population encoding of speech sounds in the lateral temporal cortex. We will examine neural encoding of speech in patients who are monolingual and bilingual in Mandarin, Spanish, and English, the most common spoken languages worldwide and feature important contrastive differences of pitch, formant, and temporal envelope. We will test a novel hypothesis that speech processing across languages will reflect a general auditory encoding of relevant phonetic properties, but that processing is modified by language-specific “tuning”. A cross-linguistic approach to the neural encoding of speech will powerfully advance our understanding of how the brain processes sound pattern variability within and across languages. This will provide fundamental insights into the shared mechanisms of auditory processing and experience-dependent plasticity in humans. The results may have significant implications for the development of new diagnostic and rehabilitative strategies for language and neurological disorders (e.g., aphasia, dyslexia, autism). Furthermore, this proposal strives to achieve a broader view of diversity and inclusion in the neuroscience of language.