Direct electrical stimulation of the left frontal aslant tract disrupts sentence planning without affecting articulation

ABSTRACT

Sentence production involves mapping from deep structures that specify meaning and thematic roles to surface structures that specify the order and sequencing of production ready elements. We propose that the frontal aslant tract is a key pathway for sequencing complex actions with deep hierarchical structure. In the domain of language, and primarily with respect to the left FAT, we refer to this as the ‘Syntagmatic Constraints On Positional Elements’ (SCOPE) hypothesis. One prediction made by the SCOPE hypothesis is that disruption of the frontal aslant tract should disrupt sentence production at grammatical phrase boundaries, with no disruption of articulatory processes.  We test this prediction in a patient undergoing direct electrical stimulation mapping of the frontal aslant tract during an awake craniotomy to remove a left frontal brain tumor. We found that stimulation of the left FAT prolonged inter-word durations at the start of grammatical phrases, while inter-word durations internal to noun phrases were unaffected, and there was no effect on intra-word articulatory duration. These results provide initial support for the SCOPE hypothesis, and motivate novel directions for future research to explore the functions of this recently discovered component of the language system.

KEYWORDS:

• Frontal aslant tract
• sentence production
• diffusion MRI
• direct electrical stimulation
• aphasia

Acknowledgments

The authors are grateful to Dr. Michael Tanenhaus for his discussion and contributions to the development of the experimental paradigm used during intraoperative mapping and for his insights throughout the planning phase of this study. The authors thank Keith Parkins for software development (StrongView), Sarah Gannon, Steve Erickson, Kelly Wright, and Michael Schmidt for assistance in the operating room, and Raouf Belkhir and for assistance with the development of response time measures. This research was supported by NSF (BCS1349042) and NIH (R01NS089069, R01EY028535) grants to BZM, a core grant to the Center for Visual Science (P30 EY001319), and by support to the Department of Neurosurgery by Norman and Arlene Leenhouts. BLC was supported by an NSF training grant (NSF DGE-1449828).

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Bradford Z. Mahon http://orcid.org/0000-0002-2018-4797

Notes

1. We are grateful to Dr. Anthony Dick for raising this possibility.

Additional information

Funding

This work was supported by National Eye Institute [grant number P30EY001319, R01EY028535]; Division of Graduate Education [grant number DGE-1449828]; National Institute of Neurological Disorders and Stroke [grant number R01NSO89069]; Division of Behavioral and Cognitive Sciences [grant number BCS1349042].