Abstracts

Rationale: The articulatory loop is responsible for short-term storage of auditory information, mediating between perception and vocal production. These processes require functional interactions between perisylvian cortical areas in frontal and temporal lobes. However, the dynamics of neural propagation within this network has not been directly measured. We hypothesized that these dynamics are not simply sequential, but consist of sustained “cross-talk"" among network components.Methods: Event-related causality (ERC) methods were applied to analyze ECoG recordings from ten patients listening to, and reproducing, two consecutive tones by humming. The ERC method is a multichannel extension of the Granger causality concept, which states that an observed time series x(t) causes another series y(t), if knowledge of x(t)'s past significantly improves prediction of y(t). ERC gives an estimate of changes in the intensity and direction of neural activity propagation between recording sites as a function of frequency, in comparison to baseline activity. The spatial-temporal patterns of high-gamma activity (70-110 Hz) propagation were characterized in relation to stimulus onset (stimulus locked trials), and in relation to response onset (response aligned trials).Results: ERC analysis, at the individual level, revealed sustained neural propagation within a network of ECoG recording sites in posterior superior temporal gyrus (pSTG), ventral motor cortex (vMC), and posterior inferior frontal gyrus (pIFG). When ERC results were averaged across patients, they suggested early propagations within pSTG, followed by feedforward propagations from some pSTG sites to vMC and pIFG, and by subsequent feedback propagations (from vMC and pIFG to pSTG). Interestingly, propagations from pIFG to pSTG, and from vMC to pIFG, both increased soon after the onset of the first and second tones, but propagations from vMC to pIFG occurred somewhat later than those from pIFG to pSTG. Immediately before vocal responses, the most prominent propagation occurred within ventral motor cortex, as well as from pIFG to ventral motor cortex.Conclusions: - During a vocal tone repetition task requiring articulatory loop function, we observed sustained reciprocal neural propagation among perisylvian language areas ostensibly responsible for tone perception and vocal production. - These data support a dynamical model of the articulatory loop in which sustained reciprocal network interactions maintain representations for both perception and production in working memory. - Although network interactions are reciprocal, their intensities are directionally asymmetric, generally supporting a sequential model of rapid initiation of motor planning and activation of a forward model, followed ultimately by execution of overt vocalization. - These findings may contribute to a better understanding of the mechanisms of speech impairment following cortical injury. Supported by: NINDS R01-NS040596 (NC)