Abstracts

Rationale: Neuroimaging and cyto-architectonic studies have led to the division of the left inferior frontal gyrus (LIFG) into distinct sub-regions: pars opercularis (POp), pars triangularis (PT), pars orbitalis (POr), and primary motor cortex (M1). However, an understanding of how these regions operate in concert with each other to select and produce the appropriate response is unknown. We performed electro-corticography (ECoG) in 27 left hemisphere dominant patients to evaluate the dynamics of LIFG during visual naming.Methods: Subjects were presented pictures of nouns, verbs or scrambled images and asked to overtly name them during ECoG recording. SDEs overlying right (n=10) or left (n=22) IFG were classified as pars orbitalis (POr), pars triangularis (PT), pars opercularis (POp) or ventral primary motor cortex (M1). All subjects were left hemisphere language dominant. Data were filtered into the gamma (70-110Hz) frequency band using a Hilbert transform and then the power envelope was computed. Amplitude envelope correlations (AEC) were calculated between channels in each region to assess functional connectivity. Directionality of AEC was estimated by using time-lagged correlations. Attractor state dynamics were modeled using k-means clustering of information flow between all LIFG sub-regions. Similar connectivity patterns were clustered in time and visualized in phase space using principal components.Results: The onset of naming led to a dramatic decrease in gamma power in POr below baseline at 250ms after stimulus onset and preceded concurrent activation of PT, POp and M1. Between 300-600ms, negative correlation from POr to PT and from POr to POp was noted during both noun and verb generation. Around 400ms, connectivity increased bi-directionally between POp and M1 until articulation, after which, M1 became negatively correlated with PT, suggestive of a stop signal. Verb naming resulted in a distinct unique positive correlation between POr and M1.Conclusions: During language, parallel, bi-directional processes occurred in serial fashion. POr initializes this state through negative modulation of POp and PT. Later, descending signals from M1 are involved in semantic retrieval and termination of lexical processing. This study conclusively rejects rigid, rostro-caudal hierarchy and provides insight into frontal lobe function in retrieval and selection.