Speech map in the human ventral sensory-motor cortex

Abstract

The study of spatial maps of the ventral sensory-motor cortex (vSMC) dates back to the earliest cortical stimulation studies. This review surveys a number of recent and historical reports of the features and function of spatial maps within vSMC towards the human behavior of speaking. Representations of the vocal tract, like other body parts, are arranged in a somatotopic fashion within ventral SMC. This region has unique features and connectivity that may give insight into its specialized function in speech production. New methods allow us to probe further into the functional role of this organization by studying the spatial dynamics of vSMC during natural speaking in humans.

Figure 1. Somatotopic organization of vSMC

A) Spatial organization of the lips, jaw, tongue in the ‘homunculus’ as described by classic early stimulation studies. Adapted from Penfield 1959. B) Functional organization of the vSMC derived using electrocorticographic recordings during speech. The overall ordering of representations of the vocal tract is the same as previously described by Penfield, except that two laryngeal areas were identified in the dorsal- and ventral-most aspects of the vSMC. The layout of speech articulators was more fractured and overlapping than previous depictions (Bouchard et al., 2013).

Figure 2. vSMC electrode dynamics

Each axis corresponds to high gamma activity from a given electrode representing selected speech articulators (e.g. lips, dorsal tongue, coronal tongue). These plots help visualize the trajectory of the ‘cortical state’ across time during the production of a speech sound. Speech sounds that each have a different primary articulator (e.g. labial, coronal tongue, and dorsal tongue, in /ba/, /da/, and /ga/, respectively) (A) show divergent trajectories across the timecourse of the production, while speech sounds that have the same primary articulator (e.g. the coronal tongue in /na/, /la/, /ta/)(B) have very similar trajectories.