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Transcranial magnetic stimulation reveals a causal link between TMS, neural function, and behavior in speech processing paradigms
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Poster A86 in Poster Session A, Tuesday, October 24, 10:15 am - 12:00 pm CEST, Espace Vieux-Port
Lindy Comstock1, Vinicius Rezende Carvalho2, Claudia Lainscsek3, Aria Fallah4, Terrence J. Sejnowski3; 1Semel Institute for Neuroscience and Human Behavior, UCLA, 2Postgraduate Program in Electrical Engineering, UFMG, 3Computational Neurobiology Laboratory, The Salk Institute for Biological Studies, 4Department of Neurosurgery, UCLA
Transcranial magnetic stimulation (TMS) has been widely applied to study neural mechanisms with the underlying assumption that TMS exerts a causal influence on neural circuits involved in sensory perception and motor output, which is then observable as a task-relevant behavioral response. According to this assumption, an excitatory TMS paradigm would augment the activity of neurons involved in a relevant behavioral task. However, questions remain about the exact nature of the neural response elicited by TMS and whether stimulation is in fact directly related to the same neurons involved in volitional motor commands [1-3]. Speech decoding paradigms typically utilize neural signals arising from regions of the motor cortex associated with speech production to serve as inputs for neural speech decoding paradigms [4-6]. Therefore, it is plausible that the stimulation of motor processing areas associated with phoneme articulation could result in an increased neural response in these speech decoding inputs. We sought to determine whether TMS administered to cortical regions governing either lip or tongue movements would (i) produce a significant effect on task performance, and (ii) result in a more accurate prediction of the associated phoneme in a classification analysis. We first reproduced a neurofunctional double disassociation in a phoneme discrimination task [7] and replicated these findings in a separate trial. Next, we found the same double dissociation in a speech decoding classification analysis performed on neural data collected during the behavioral task. The behavioral task was assessed by means of a two-way analysis of variance (ANOVA) with (i) two category levels (bilabial, alveolar) and two target levels (lip, tongue) and (ii) four phoneme levels (b, p, d, t) and two target levels (lip, tongue), followed by a post-hoc Tukey’s HSD Test for multiple comparisons. A novel nonlinear signal processing technique, delay differential analysis (DDA) [8,9], was adopted for the decoding analysis. Significant results were obtained between the TMS and control conditions for the behavioral and decoding tasks, which showed increased performance when TMS targeted a brain region associated with articulation of the stimulus (i.e., /b/ and lip motor cortex) and decreased performance when regions not associated with the stimulus were targeted (i.e., /b/ and tongue motor cortex). A Pearson’s correlation coefficient test was computed to quantify the association between the discrimination accuracy achieved by participants in the behavioral task and the decoding accuracy achieved by the classification analysis. There was a positive correlation between the two variables, r(157) = -.21, p = 0.008. Together, these experiments illustrate that the stimulation of cortical regions involved in phoneme production influence speech perception on the behavioral and neural level. We found improved performance on the behavioral task to be a significant predictor of neural speech decoding accuracy, suggesting a causal link between TMS, neural function, and behavior. While additional refinement of the method is required, our findings support neuromodulation as a prospective means of increasing accuracy in neural speech decoding.
Topic Areas: Speech Perception,