Adaptive Processes in Speech Perception: Contributions from Cerebral and Cerebellar Cortices

In the sensorimotor domain, adaptation to distorted sensory input has been well-characterized and is largely attributed to learning mechanisms in the cerebellum that adjust motor output to achieve the same desired sensory outcome. Our interest in the role of the cerebellum in cognitive processes has led us to question whether it also contributes to adaptation in tasks that do not require voluntary motor output. Speech perception is a domain where there exist many examples of adaptation that are guided by both sensory and cognitive processes, without intentional motor involvement. Thus, we investigated behavioral and neural characteristics of speech perception adaptation to spectrally distorted words using a sophisticated noise-vocoded speech manipulation that mimics cochlear implants. We demonstrated that adaptation to spectrally distorted words can be achieved without explicit feedback by eithergradually increasing the severity of the distortion or by using an intermediate distortion during training. We identified regions in both the cerebellar and cerebral cortex that showed differences in neural responses before and after training. In the cerebellum, this included regions in lobes V and VI, and Crus I. In the cerebrum, this included regions in the inferior frontal gyrus, the superior temporal sulcus, and the posterior inferior/middle temporal gyrus. In some of these regions, we further found changes in the magnitude of the neural responses that corresponded to the degree of behavioral improvements in performance. To gain some insight into the nature of the interactions between cerebral and cerebellar cortices and the types of representations involved in speech perception adaptation, we conducted a simple functional connectivity analysis using cerebellar seed regions of interest. We found interactions between the cerebellum and cerebral cortex that were dependent on the location of the cerebellar region. Overall, our behavioral and functional neuroimaging results point to cerebellar involvement in speech perception adaptation, and we conclude with a discussion of the learning mechanisms and neuroanatomical pathways that may support such plasticity