| Summary: | 博士 === 國立陽明大學 === 神經科學研究所 === 97 === The research focus on the cerebellum has been shifting from motor control and learning to higher cognitive functions, such as memory and language, over the past two decades. Supported by developmental, evolutional, and neuroimaging evidence, now it is widely accepted that the cerebellum participates in various function, both in motor and non-motor domain, by means of its densely reciprocal connections with the cerebral cortex. The uniformity of the cerebellar cytoarchitecture leads to the hypothesis that the computation performed by the cerebellum is exactly the same for the numerous functions associated with the cerebellum. Different functions of the cerebellum are fulfilled depending on the cortical inputs received.
Accordingly, many theories about the computation of the cerebellum have been proposed in order to explain the contributions of the cerebellum in various functions. Among these theories, event timing and sensory acquisition are two of the most preeminent and the most actively testing hypotheses. Exploiting the inherent nature of both temporal processing and sensory analysis, duration discrimination tasks are frequently employed for dissociating the role of the cerebellum between temporal processing and sensory analysis
However, most previous studies compared a duration discrimination task with a motor control or a non-temporal discrimination control task in order to see whether the activation of the cerebellum was found only in the duration task or it was observed in both the duration and the control task. If the cerebellum only yields its activation in the duration task, then the view of timing processing is supported. Otherwise, the support is inclined toward the view of sensory analysis. Nevertheless, the interaction between the cerebellum and the cerebral cortex during duration discrimination is scarcely investigated. Furthermore, for the previous studies manipulating task difficulty during duration discrimination, only tasks with different temporal difficulties were examined, the effect of the sensory difficulty on the cerebellar activation and the cerebrocerebellum interaction is barely addressed.
Given the importance of cortical inputs for the contribution of the cerebellum to neuropsychological functions, it is noteworthy to unveil how the function connectivity between the cerebellum and the cerebral cortex changes in response to both temporal and sensory difficulty during duration discrimination. Based on these findings, the role of the cerebellum between timing processing and/or sensory analysis can be better understood.
Thus, a series of experiments were done in this thesis using duration discrimination tasks. The first attempt was to replicate the findings of previous studies comparing a duration discrimination task with a non-temporal discrimination task. An auditory and a visual duration discrimination tasks were contrasted with their corresponding control tasks, i.e. auditory intensity and visual size discrimination tasks. By comparing duration discrimination and other sensory feature discrimination in both auditory and visual modalities, the results of this experiment suggests that duration and non-temporal discrimination share mostly similar neural networks.
The second experiment focused on uncovering whether duration discrimination across different sensory modalities share the same neural substrates or they engage different modality-specific networks in order to understand if the cerebellum plays a role in duration discrimination across auditory and visual modalities. The findings of this experiment support the view that hearing has the superiority for temporal processing over vision, i.e. the modality appropriateness hypothesis, since a relatively extensive neural network (the parietal, limbic cortices and cerebellum besides the frontal-temporal network) was observed in the visual than the auditory task. The results also led to the suggestion that the DLPFC, preSMA/SMA, and the basal ganglia rather than the cerebellum, are the common neural substrates for temporal processing across sensory modalities and sensory structures for explicit timing within the sub-second range.
Having a clear picture of sensory discrimination for temporal and non-temporal events and of duration discrimination across auditory and visual modalities, the last two experiments were designed to reveal the effect of temporal and sensory difficulties on the pattern of cerebrocerebellar interaction during duration discrimination. The results from these two experiments provide a comprehensive view of the role of the cerebellum in sensory discrimination of temporal events and give forth the understanding of the cerebellum in neuropsychological functions.
On the whole, the cortical computational efficiency can be achieved by an enhancement of functional connection between the cerebral cortex and the cerebellum, e.g. the PMC/SMA and DLPFC strengthened their connections with the cerebellum instead of increasing their activation per se when temporal difficulty was increased. Increasing sensory difficulty resulted in a tight coupling between the cerebellum and thalamus/hypothalamus. In addition, relatively restrictive cortical regions accompanying more extensive intra-cerebellum connections were engaged when integration of two kinds of sensory information benefits the discrimination process, in accord with the view proposed in the sensory acquisition hypothesis that specific sensory data are interpreted in the context of all data being acquired in the cerebellum.
Taken together, the work presented in this thesis demonstrates that sensory discrimination of temporal and non-temporal events engages similar neural networks. Concur with the prediction of the sensory acquisition hypothesis, the cerebellum increases its activity in response to increasing both temporal and sensory difficulty and enhances its connection with other brain regions responsible for temporal or sensory processing when demanding information processing is required. A saving of cortical computation can be realized by retrieving the well-controlled sensory information from the cerebellum.
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