The Spatial and Temporal Signatures of Word Production Components: A Critical Update

In the first decade of neurocognitive word production research the predominant approach was brain mapping, i.e., investigating the regional cerebral brain activation patterns correlated with word production tasks, such as picture naming and word generation. Indefrey and Levelt (2004) conducted a comprehensive meta-analysis of word production studies that used this approach and combined the resulting spatial information on neural correlates of component processes of word production with information on the time course of word production provided by behavioral and electromagnetic studies. In recent years, neurocognitive word production research has seen a major change toward a hypothesis-testing approach. This approach is characterized by the design of experimental variables modulating single component processes of word production and testing for predicted effects on spatial or temporal neurocognitive signatures of these components. This change was accompanied by the development of a broader spectrum of measurement and analysis techniques. The article reviews the findings of recent studies using the new approach. The time course assumptions of Indefrey and Levelt (2004) have largely been confirmed requiring only minor adaptations. Adaptations of the brain structure/function relationships proposed by Indefrey and Levelt (2004) include the precise role of subregions of the left inferior frontal gyrus as well as a probable, yet to date unclear role of the inferior parietal cortex in word production

Oromotor Kinematics of Speech In Children and the Effect of an External Rhythmic Auditory Stimulus

The purpose of this study was to determine the effect of an external auditory rhythmic stimulus on the kinematics of the oromotor musculature during speech production in children and adults. To this effect, the research questions were: 1) Do children entrain labiomandibular movements to an external auditory stimulus? 2) Does the ability to entrain labiomandibular movements to an external auditory stimulus change with age? 3) Does an external auditory stimulus change the coordination and stability of the upper lip, lower lip, and jaw when producing speech sounds? The oromotor kinematics of two groups of children, age eight to ten (n = 6) and eleven to fourteen (n = 6), were compared to the oromotor kinematics of adults (n = 12) while producing bilabial syllables with and without an external auditory stimulus. The kinematic correlates of speech production were recorded using video-based 4-dimensional motion capture technology and included measures of upper lip, lower lip and jaw displacement and their respective derivatives. The Spatiotemporal Index (a single number indication of motor stability and pattern formation) and Synchronization Error (a numerical indication of phase deviations) were calculated for each participant within each condition. There were no statistically significant differences between age groups for the Spatiotemporal Index or for Synchronization Error. Results indicated that there were statistically significant differences in the Spatiotemporal Index for condition; with Post-hoc tests indicating that the difference was between the first condition (no rhythm) and the second condition (self-paced rhythm). Results indicated that both child groups were able to synchronize to an external auditory stimulus. Furthermore, the older child group was able to establish oromotor synchrony with near-adult abilities

Optimizing Experimental Design for Comparing Models of Brain Function

This article presents the first attempt to formalize the optimization of experimental design with the aim of comparing models of brain function based on neuroimaging data. We demonstrate our approach in the context of Dynamic Causal Modelling (DCM), which relates experimental manipulations to observed network dynamics (via hidden neuronal states) and provides an inference framework for selecting among candidate models. Here, we show how to optimize the sensitivity of model selection by choosing among experimental designs according to their respective model selection accuracy. Using Bayesian decision theory, we (i) derive the Laplace-Chernoff risk for model selection, (ii) disclose its relationship with classical design optimality criteria and (iii) assess its sensitivity to basic modelling assumptions. We then evaluate the approach when identifying brain networks using DCM. Monte-Carlo simulations and empirical analyses of fMRI data from a simple bimanual motor task in humans serve to demonstrate the relationship between network identification and the optimal experimental design. For example, we show that deciding whether there is a feedback connection requires shorter epoch durations, relative to asking whether there is experimentally induced change in a connection that is known to be present. Finally, we discuss limitations and potential extensions of this work

Altered Modulation of Silent Period in Tongue Motor Cortex of Persistent Developmental Stuttering in Relation to Stuttering Severity

Motor balance in developmental stuttering (DS) was investigated with Transcranial Magnetic Stimulation (TMS), with the aim to define novel neural markers of persistent DS in adulthood. Eleven DS adult males were evaluated with TMS on tongue primary motor cortex, compared to 15 matched fluent speakers, in a "state" condition (i.e. stutterers vs. fluent speakers, no overt stuttering). Motor and silent period thresholds (SPT), recruitment curves, and silent period durations were acquired by recording tongue motor evoked potentials. Tongue silent period duration was increased in DS, especially in the left hemisphere (P<0.05; Hedge's g or Cohen's dunbiased = 1.054, i.e. large effect size), suggesting a "state" condition of higher intracortical inhibition in left motor cortex networks. Differences in motor thresholds (different excitatory/inhibitory ratios in DS) were evident, as well as significant differences in SPT. In fluent speakers, the left hemisphere may be marginally more excitable than the right one in motor thresholds at lower muscular activation, while active motor thresholds and SPT were higher in the left hemisphere of DS with respect to the right one, resulting also in a positive correlation with stuttering severity. Pre-TMS electromyography data gave overlapping evidence. Findings suggest the existence of a complex intracortical balance in DS tongue primary motor cortex, with a particular interplay between excitatory and inhibitory mechanisms, also in neural substrates related to silent periods. Findings are discussed with respect to functional and structural impairments in stuttering, and are also proposed as novel neural markers of a stuttering "state" in persistent DS, helping to define more focused treatments (e.g. neuro-modulation)