|Author:||Tan, Gim Hoon Davynn|
|Title:||Neuromodulation on working memory among older adults and persons with mild cognitive impairment (MCI)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Brain stimulation -- Therapeutic use
Cognition disorders in old age -- Treatment
|Pages:||xv, 150 pages : color illustrations|
|Abstract:||Background: Working memory decline is one common complaint in aging and mild cognitive impairment (MCI). There are emerging evidences of using non-invasive neuromodulatory techniques including transcranial direct current stimulation (tDCS) to modulate memory function behaviourally and neurally. This provides a tool to investigate the question of how neuroplastic the aging brain is by observing how it might be responsive to the cortical modulation induced by the electrical stimulation and to further examine how the MCI condition would further impact on this neuroplasticity. Objectives: This study aimed to investigate how single-session electrical stimulation might modulate working memory function by altering the associated neural mechanisms among a group of healthy older adults and compared them with a group of persons with MCI. Our focus was placed on the match-mismatch discriminative process (denoted by N200) and attentional allocation process (denoted by P300) essential to the working memory function. Methods: Ten minutes of 1-mA anodal HD-tDCS was applied at dorsolateral prefrontal cortex on 18 healthy older adults (mean age = 63.1 ± 2.6, 10 males), 18 persons with MCI (mean age = 68.3 ± 7.1, 5 males), and 22 younger adults (mean age = 19.7 ± 1.6, 9 males). Electroencephalogram (EEG) data was recorded while a standard working memory task (digit two-back) was performed before and after HD-tDCS. The stimulation effect was examined using repeated measure ANOVAs to analyse the mean amplitude of N200 and P300 indexing key processes in two-back task and the behavioural performance between the younger and healthy older adults, and between the two older adult groups.|
Results: In the behavioural pilot study conducted to test the time-course of the after-effect of the HD-tDCS, a greater enhancement in the working memory performance in the young participants after anodal stimulation was observed approximately at 30 minutes post-stimulation, concurring with previous evidence of the HD-tDCS after-effect on motor cortex peaking at 30 minutes after stimulation. In the main study, the stimulation generally improved the accuracy and quickened the response in the younger and the healthy older adult participants but no significant change was observed in the group of persons with MCI. The stimulation induced a less negative-going frontal N200 for target stimuli in the younger adults but induced a more positive-going frontal P300 for non-target in the healthy older adults. Both changes were associated with faster response in the respective groups after stimulation. No change in the N200 was observed for non-target in the younger adults and also generally for the older adults. The P300 in the younger adults did not change after stimulation and this component for target in older adults was not modulated by the stimulation too. The frontal P300 in the group of persons with MCI also became more positive-going but for target stimuli and associated with poorer accuracy rate. Conclusion: The short-lifting electrical stimulation enhanced the discrimination process (denoted by N200) for target stimuli in the younger participants. In contrast, the same stimulation resulted in enhancing the attentional allocation process (denoted by P300) for non-target in the older participants. Despite similar post-stimulation modulation effects were revealed for the MCI participants, the increases in the P300 positivity did not seem to influence the task performance. The match-mismatch discriminative process for target in the younger participants was less negative-going after the stimulation, suggesting that target stimuli were less perceived as non-targets by them, which was found beneficial to the task performance. No significant modulation of the discriminative process was found in the older participants. The enhanced attentional allocation and hence the task performance among the older participants could be due to the intact compensatory neural capacity among the older participants in response to the excitatory effects brought by the electrical stimulation. This is in contrary to the MCI participants in which the enhanced attentional attention from the same stimulation resulted in poorer task performance. These findings tend to suggest reduced compensatory neural capacity among the MCI participants as they were likely to recruit and exhaust the compensatory mechanism earlier than their healthy counterparts given that MCI condition marks a further deterioration from normal aging. The observed compensatory neural capacity in older participants in response to the electrical stimulation points towards the potential for external stimulation such as tDCS to augment cognitive reserves in the older adults.
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