| Summary: | 碩士 === 國立中央大學 === 認知與神經科學研究所 === 103 === Activations in the left posterior parietal cortex (LPPC) have been consistently reported in functional neuroimaging studies of episodic memory retrieval. However, it remains unclear whether this area is causally related to episodic memory retrieval. The correlational nature of neuroimaging studies makes it difficult to assert a causal relation between the LPPC activity and episodic retrieval. Furthermore, patient studies give rise to inconsistent results with regards to whether parietal lesions result in memory deficits. There have been a few noninvasive brain stimulation studies addressing this issue but the results were not consistent, possibly due to the fact that their designs were not optimal.
A defining characteristic of episodic memory is recollection. Two experiments reported in this thesis hence employed a source memory task to index recollection and applied the noninvasive stimulation methods of transcranial direct-current stimulation (tDCS) and transcranial magnetic stimulation (TMS) to examine whether LPPC activity is causally related to episodic memory retrieval. In Experiment 1, twenty-four participants were randomly assigned to the anodal or the cathodal group and performed source memory tests on three separate days. During the retrieval phase, the participants received anodal or cathodal stimulation over the LPPC, sham on the LPPC, and stimulation on primary motor cortex (M1) as a control condition. The results indicated that source memory accuracy, but not old/new recognition performance, decreased significantly in the cathodal group when subjects received stimulation on the LPPC region compared with sham and M1 conditions. No such effect was found in the anodal group. The results of Experiment 1 supported the claim that the LPPC is causally related to episodic retrieval. In neuroimaging studies, the activation of the left angular gyrus (lAnG) within the LPPC was found to be associated with recollection process during retrieval, but the spatial resolution of tDCS was not able to stimulate such specific brain region without affecting adjacent regions. Hence, TMS, a more focal brain stimulation technique, was used in the second experiment. In Experiment 2, to further investigate the causal involvement of the lAnG in the recollection process, continuous theta burst stimulation (cTBS), which is an inhibitory repetitive TMS stimulation method, was employed. Twelve subjects performed a source memory task in which they had to make two source judgments. The cTBS was administered over the lAnG or vertex before the retrieval stage. The results showed no differences in the performances of both old/new recognition and source accuracy across the lAnG stimulation, sham, and vertex control conditions. In Experiment 3, to test whether the null results of Experiment 2 were due to insufficient cTBS intensity, eleven participants of Experiment 2 were recruited in Experiment 3. They received the cTBS with the same protocol used in Experiment 2 and then engaged in a magnitude comparison task and a visual conjunction search task. The results showed that performance in both tasks were lower after the cTBS over the lAnG relative to the sham condition, suggesting that the cTBS protocol used in Experiment 2 can influence at least some cognitive functions.
Taken together, the tDCS study provided supportive evidence that the LPPC is causally related to episodic retrieval, whereas the cTBS study failed to reveal the causal relationship between the lAnG and recollection process. The null results of the cTBS study most likely was not due to insufficient cTBS intensity, as Experiment 3 showed that conjunction search and magnitude comparison were modulated by cTBS of the same protocol. These results may imply that the role of the lAnG in episodic retrieval is subtle or secondary. Another possible implication is that the appropriate lAnG location that are highly correlated with recollection should be identified individually so that a more consistent result can be revealed. Future studies also need to consider if textual and pictorial stimuli activate different regions within the angular gyrus in order to further clarify the causal relationship between the lAnG and episodic retrieval.
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