| Working memory(WM)refers to cognitive processes that manipulate and maintain sensory information for a short term to guide goal-directed behavioral actions,and involve a broadly distributed neural network in the brain.One of the most important questions in neuroscience is how coordinated,goal-directed behavior arises from distributed activity of billions of neurons in the human brain.Behavioral actions of animals are relatively straightforward,relying on some direct interactions between sensory input and output systems,while behavioral actions of human beings are more flexible as human beings have much more complicated neural networks in their brains.In cognitive processes,such flexibility indicates that the human brain has the ability to execute complex actions through coordination of sensory and motor systems,following information on changes in the environment.Extensive research has reported that well-organized brain networks involving both lower-level sensory and higher-level association cortices,are responsible for the coordination of a wide range of cognitive processes including WM.However,it is still poorly understood how these cortical areas are temporally involved in the process of tactile uni-modal and tactile-visual cross-modal WM.The present dissertation aimed to explore the psychological mechanisms and neural correlates of tactile uni-modal and tactile-visual cross-modal WM in healthy participants.It includes the following parts.In the first part,I reviewed previous studies on tactile(uni-modal/cross-modal)WM.In the second part,specific aims and research hypotheses were proposed based on previous research.Further,I discussed multi-modal techniques(structural magnetic resonance imaging based(MR-based)single-pulse transcranical magnetic brain stimulation(sp-TMS)and diffusion tensor imaging tractography(DTI))and experimental paradigms(delayed-matching-to-sample paradigms)used in the dissertation.The third part is the main body of the dissertation,which consists of five studies.In Study I,causal roles of both the primary somatosensory cortex(SI)and the posterior parietal cortex(PPC)were investigated in a tactile uni-modal WM task.Individual MR-based sp-TMS was applied,respectively,to the ipsilateral SI(ipsilateral to tactile stimuli),contralateral SI(contralateral to tactile stimuli)and contralateral PPC(contralateral to tactile stimuli),while human participants were performing a tactile uni-modal delayed-matching-to-sample task(1-s delay).Time points of sp-TMS were 300,600 and 900ms after the onset of the tactile sample stimulus(duration:200ms).Compared with the ipsilateral SI,application of sp-TMS over either the contralateral SI or the contralateral PPC at those time points significantly impaired the accuracy of task performance.Meanwhile,the deterioration in accuracy did not significantly vary with the stimulating time points.Together,these results indicated that the SI and PPC processed tactile information cooperatively in the same hemisphere,starting from the early delay of the tactile uni-modal WM task.In Study ?,we searched for causal evidence linking activity in the bilateral SI,contralateral PPC,and contralateral dorsolateral prefrontal cortex(DLPFC)with behavioral performance in a tactile uni-modal WM task.Participants performed delayed matching-to-sample tasks with a 2-second delay,while MR-based sp-TMS was applied over these cortical areas at 100,200,300,600,1600,and 1900ms after the onset of vibrotactile stimulation(200ms duration).In our experiments,sp-TMS over the contralateral SI at 100 and 200ms in the early encoding period deteriorated the accuracy of task performance,over the ipsilateral SI at 1600 and 1900ms in the late maintenance delay,over the contralateral DLPFC at 100 and 1600ms also induced such deteriorating effects in the tactile uni-modal WM task.Furthermore,deteriorating effects caused by sp-TMS over the contralateral DLPFC at the maintenance stage(1600ms)were correlated with effects caused by sp-TMS over the ipsilateral SI at the same stage,indicating that information retained in the ipsilateral SI during the late delay may be associated with information retained in the DLPFC.Taken together,these results suggested that both the contralateral and ipsilateral SIs were involved in tactile uni-modal WM,and the contralateral DLPFC bridged the contralateral SI and ipsilateral SI for goal-directed actions.In Study ?,we investigated causal roles of the bilateral SI and MI in tactile uni-modal WM using MR-based sp-TMS and DTI technique.Results showed that a sensorimotor network integrated information about vibrotactile stimuli during both encoding and maintenance periods.Study IV aimed to investigate causal roles played by the bilateral SI and contralateral PPC in tactile-visual cross-modal working memory(WM).Individual MRI-based sp-TMS was applied to the bilateral SI and contralateral PPC,while participants were performing a tactile-visual cross-modal delayed matching-to-sample task with 1-s delay.Time points of sp-TMS were 300ms,600ms,and 900ms after the onset of the tactile sample stimulus in the task.The accuracy of task performance and reaction time were significantly impaired when sp-TMS was applied to the contralateral SI at 300ms.Significant impairment on performance accuracy was also observed when the contralateral PPC was stimulated at 600ms.These results indicated that the contralateral SI played an essential role in the early stage of cross-modal associations,and that information processing during the cross-modal task relied on the SI first and then the PPC.In Study ?,we searched for causal evidence linking activity in the bilateral SI,and contralateral dorsolateral prefrontal cortex(DLPFC)with behavioral performance in a tactile-visual cross-modal WM task.Participants performed delayed matching-to-sample tasks with a 2-second delay,while MR-based sp-TMS was applied over these cortical areas at 100,200,300,600,1600,and 1900ms after the onset of vibrotactile stimulation(200ms duration).In our experiments,sp-TMS over the contralateral SI at 100 and 200ms in the early encoding period deteriorated the accuracy of task performance,and over the ipsilateral SI at 1600 and 1900ms in the late maintenance delay also induced such deteriorating effects in both tactile uni-modal and tactile-visual cross-modal WM tasks.Furthermore,in the cross-modal task,when sp-TMS was applied over the contralateral DLPFC at 300ms,significant deterioration on task performance was observed,which also showed significant correlations with deterioration induced by sp-TMS applied at the same time point over the contralateral SI.These results indicated that the contralateral SI and DLPFC played essential roles in cross-modal information transfer.Taken together,these results suggested that both the contralateral and ipsilateral SIs were involved in tactile uni-modal and cross-modal WM,and the contralateral DLPFC bridged the contralateral SI and ipsilateral SI for goal-directed actions.In the last part of dissertation,I concluded main findings of my studies.? The contralateral SI and contralateral PPC are essential in both tactile uni-modal and cross-modal WM.In these two different types of WM,they work together but in different ways? Both contralateral and ipsilateral SIs are involved in tactile(uni-modal/cross-modal)WM,and the contralateral DLPFC bridges them for the goal-directed action? A sensorimotor network integrates vibrotactile information during tactile WM.? These five studies support the proposal that the tactile(uni-modal/cross-modal)WM network is widely distributed in the human brain. |
PDF Full Text Request