Abstract:

Auditory and speech signals are undisputedly processed in both left and right hemispheres, but this bilateral allocation is likely unequal. The Asymmetric Sampling in Time (AST) hypothesis proposed a division of labor that has its neuroanatomical basis in the distribution of neuronal ensembles with differing temporal integration constants: left auditory areas house a larger proportion of ensembles with shorter temporal integration windows (tens of milliseconds), suited to process rapidly changing signals; right auditory areas host a larger proportion with longer time constants (∼150–300 ms), ideal for slowly changing signals. Here we evaluate the large body of findings that clarifies this relationship between auditory temporal structure and functional lateralization. In this reappraisal, we unpack whether this relationship is influenced by stimulus type (speech/nonspeech), stimulus temporal extent (long/short), task engagement (high/low), or (imaging) modality (hemodynamic/electrophysiology/behavior). We find that the right hemisphere displays a clear preference for slowly changing signals whereas the left-hemispheric preference for rapidly changing signals is highly dependent on the experimental design. We consider neuroanatomical properties potentially linked to functional lateralization, contextualize the results in an evolutionary perspective, and highlight future directions.

Abstract:

Covert speech involves the internal generation of articulatory movements and their sensory consequences. While overt speech involves a combination of feedforward and feedback signals, feedback signals may be substantially different, or even absent, during covert speech. Despite the differences, we conjectured that sensorimotor interareal communication during covert speech is implemented through the same channels recruited during overt speech. An influential overt speech model proposed that feedforward and feedback signals are segregated to the left and right hemispheres, respectively. Here, we used magnetoencephalography to investigate the lateralization of functional connectivity before and after covert speech production. The data reveal leftward lateralization preceding and rightward lateralization following predicted covert speech onset. This alternating lateralization pattern is observed only in the connection between premotor and auditory regions and in the alpha frequency band. The electrophysiological data, derived entirely from covert speech, add a provocative perspective to adjudicate between overt speech motor control models.

Abstract:

Speech can be defined as the human ability to communicate through a sequence of vocal sounds. Consequently, speech requires an emitter (the speaker) capable of generating the acoustic signal and a receiver (the listener) able to successfully decode the sounds produced by the emitter (i.e., the acoustic signal). Time plays a central role at both ends of this interaction. On the one hand, speech production requires precise and rapid coordination, typically within the order of milliseconds, of the upper vocal tract articulators (i.e., tongue, jaw, lips, and velum), their composite movements, and the activation of the vocal folds. On the other hand, the generated acoustic signal unfolds in time, carrying information at different timescales. This information must be parsed and integrated by the receiver for the correct transmission of meaning. This chapter describes the temporal patterns that characterize the speech signal and reviews research that explores the neural mechanisms underlying the generation of these patterns and the role they play in speech comprehension.