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Neural Processing of Speech Using Intracranial Electroencephalography: Sound Representations in the Auditory Cortex  

Liberty S. Hamilton

When people listen to speech and other natural sounds, their brains must take in a noisy acoustic signal and transform it into a robust mapping that eventually helps them communicate and understand the world around them. People hear what was said, who said it, and how they said it, and each of these aspects is encoded in brain activity across different auditory regions. Intracranial recordings in patients with epilepsy, also called electrocorticography or stereoelectroencephalography, have provided a unique window into understanding these processes at a high spatiotemporal resolution. These intracranial recordings are typically performed during clinical treatment for drug-resistant epilepsy or to monitor brain function during neurosurgery. The access to direct recordings of activity in the human brain is a benefit of this method, but it comes with important caveats. Research using intracranial recordings has uncovered how the brain represents acoustic information, including frequency, spectrotemporal modulations, and pitch, and how that information progresses to more complex representations, including phonological information, relative pitch, and prosody. In addition, intracranial recordings have been used to uncover the role of attention and context on top-down modification of perceptual information in the brain. Finally, research has shown both overlapping and distinct brain responses for speech and other natural sounds such as music.

Article

Neural Oscillations in Audiovisual Language and Communication  

Linda Drijvers and Sara Mazzini

How do neural oscillations support human audiovisual language and communication? Considering the rhythmic nature of audiovisual language, in which stimuli from different sensory modalities unfold over time, neural oscillations represent an ideal candidate to investigate how audiovisual language is processed in the brain. Modulations of oscillatory phase and power are thought to support audiovisual language and communication in multiple ways. Neural oscillations synchronize by tracking external rhythmic stimuli or by re-setting their phase to presentation of relevant stimuli, resulting in perceptual benefits. In particular, synchronized neural oscillations have been shown to subserve the processing and the integration of auditory speech, visual speech, and hand gestures. Furthermore, synchronized oscillatory modulations have been studied and reported between brains during social interaction, suggesting that their contribution to audiovisual communication goes beyond the processing of single stimuli and applies to natural, face-to-face communication. There are still some outstanding questions that need to be answered to reach a better understanding of the neural processes supporting audiovisual language and communication. In particular, it is not entirely clear yet how the multitude of signals encountered during audiovisual communication are combined into a coherent percept and how this is affected during real-world dyadic interactions. In order to address these outstanding questions, it is fundamental to consider language as a multimodal phenomenon, involving the processing of multiple stimuli unfolding at different rhythms over time, and to study language in its natural context: social interaction. Other outstanding questions could be addressed by implementing novel techniques (such as rapid invisible frequency tagging, dual-electroencephalography, or multi-brain stimulation) and analysis methods (e.g., using temporal response functions) to better understand the relationship between oscillatory dynamics and efficient audiovisual communication.

Article

Crossmodal Plasticity, Sensory Experience, and Cognition  

Valeria Vinogradova and Velia Cardin

Crossmodal plasticity occurs when sensory regions of the brain adapt to process sensory inputs from different modalities. This is seen in cases of congenital and early deafness and blindness, where, in the absence of their typical inputs, auditory and visual cortices respond to other sensory information. Crossmodal plasticity in deaf and blind individuals impacts several cognitive processes, including working memory, attention, switching, numerical cognition, and language. Crossmodal plasticity in cognitive domains demonstrates that brain function and cognition are shaped by the interplay between structural connectivity, computational capacities, and early sensory experience.