Cortical activity underlying overt and covert language generation measured using high-density diffuse optical tomography

Using naturalistic language generation tasks (e.g. overt speech) to capture the neural correlates of speech production is important, as less naturalistic (but often used) tasks such as covert language generation are not a reliable substitute for accurately assessing cortical activation associated with naturalistic speech. fMRI poses challenges to implementing naturalistic language tasks, especially in clinical populations, because it is noisy, physically constraining, and contraindicated in populations with metal implants. High-density diffuse optical tomography (HD-DOT) is particularly well-suited for naturalistic language tasks because it is silent, wearable, portable, and metal-compatible. This study investigates cortical activity underlying naturalistic language generation using HD-DOT. Six adult subjects aged 20-26 years completed two scans on two separate days consisting of three different tasks: Covert word reading (RW), covert verb generation (CV), and overt verb generation (OV). Cortical responses were apparent in expected anatomical areas for all tasks and RW, CV, and OV evoked responses of increasing strength (peak ΔHbO (μMol) = 7.58, 10.3, and 11.0, respectively). Notably, OV recruits additional activation in Broca's area and right-lateralized primary motor cortex as compared to CV. These findings are consistent with those obtained using fMRI^9,10,11 and underscore the need to use naturalistic language tasks when assessing the neural representations of natural speech. These results motivate extension to further investigations of naturalistic language processing of increased complexity, both receptive and productive, such as within-room conversation.