With increased age, some individuals experience cognitive declines that are more severe than what is observed in healthy cognitive aging. This decline may be related to mild cognitive impairment (MCI) and Alzheimer’s disease (AD). Much of the current research on dementia attempts to detect subtle cognitive and memory declines before behavioral and cognitive symptoms are more apparent. Intense research interest has focused on MCI, a condition that includes impairment in some areas of cognitive functioning but is not severe enough to warrant the diagnosis of dementia. MCI may represent a transitional stage between healthy aging and AD and is considered a risk factor for AD development. The purpose of the present thesis was to examine if EEG and ERPs can be used as reliable predictors of cognitive changes in aging and MCI.
Study 1 was designed to examine if there is evidence of changes in the EEG between cognitively healthy older adults and people with MCI. The purpose of this study was to examine differences in EEG activity between healthy older adults and people with MCI during early and late portions of a longer-than-normal resting-state recording. Resting state recordings typically last 1-3 minutes. It would be advantageous to run a longer testing session because this would provide more data, but such a procedure might be problematic because it might result in increased drowsiness in the latter half of testing. If this drowsiness affected those with MCI more than healthy adults, this might produce artifactual differences between groups. If increased drowsiness occurs as the duration of the recording becomes longer, an increase of low-frequency EEG activity should be observed, particularly in the delta band. Resting state EEG was recorded in 20 healthy older adults and 20 people with MCI who rested with their eyes closed. The EEG recording was divided into two three-minute halves. People with MCI exhibited a significant increase in theta power density over posterior regions of the scalp compared to healthy older adults. Power density for all frequency bands did not change over the two halves of the recording. That is, there was little evidence of drowsiness in the second half of the recording. Taken together, these findings indicate that longer resting-state EEG recording can be reliably employed without increased risk of drowsiness.
Study 2 examined whether there is evidence of a dysfunction in the salience network in older adults. Previous research suggests that older adults may be less able to compute the level of salience of unattended stimulus inputs. The transient detector system is a specialized network of brain areas for detecting sudden changes in the intensity of an auditory stimulus. The output of this system, as reflected by the auditory ERP components N1 and P2, provides a measure of the level of salience of the stimulus. Twenty younger adults and healthy older adults participated in this study. A single auditory stimulus was presented rapidly, every 1.5 s, or very slowly, every 12 s, in different conditions. When the stimuli were presented rapidly, group differences were not observed for the amplitudes of N1 and P2, peaking at 100 and 180 ms, respectively. When stimuli were presented very slowly, their amplitudes were greatly enhanced for younger adults but did not increase for older adults. The failure to observe a large increase in the amplitude of N1 and P2 in older adults for very slowly presented stimuli provides strong evidence of a dysfunction of the salience network in this group.
There is evidence that both the functioning of salience network and the frontoparietal network deteriorate in cognitively healthy older adults. These networks might further deteriorate in people with MCI. In study 2, when stimuli were presented slowly, the P2 was delayed and peaked at a time that is more consistent with a P3a. The P3a is elicited by a potentially highly salient, but unattended stimulus input that interrupts the functioning of the frontoparietal network, resulting in a switch of processing priorities away from current task demands and toward the processing of the stimulus input. In study 3, auditory stimuli were again presented either rapidly or very slowly to 20 healthy older adults and 20 people with MCI. The amplitude of N1 did not differ between the two groups in either the fast or slow conditions. Thus, there is little evidence that people with MCI have a deficit in computing the salience of unattended auditory stimuli. When stimuli were presented slowly, the P2/P3a was significantly smaller in people with MCI compared to healthy older adults. The attenuated P2/P3a in people with MCI may reflect a reduced frontoparietal ability to determine processing priorities. In people with MCI, priority of processing may not be switched from the ongoing cognitive task to the potentially much more relevant auditory input.
In the results of studies 2 and 3, there was ambiguity regarding whether the positivity observed in the slow condition reflected P2 or P3a activity. A more definite P3a had been elicited in oddball paradigms. In the oddball paradigm, the participant is presented with a sequence of frequently presented homogenous standard stimuli. At rare and unpredictable times, a deviant is presented, the deviant representing a change in a feature of the standard. Deviants that represent a large change from the standard may elicit a P3a. Two experiments were run in which at least one of the deviants had previously been shown to elicit a large P3a in younger adults. Study 4 consisted of two experiments. In Experiment 1, the deviants represented either decreases or increases in the intensity of the standard. The deviant that represented an increase in intensity has been found to elicit a large P3a in previous studies. In Experiment 2, six different deviants were presented. The deviants included a white noise burst and environmental sounds, both of which have elicited a large P3a in previous studies. Across both experiments, the MMN/DRN and P3a did not differ between healthy older adults and people with MCI. Previous studies have indicated that the P3a is reduced in amplitude in healthy older adults compared to younger adults. The results of study 4 indicate that the P3a was not further reduced in people with MCI. This is in contrast to study 3 in which the P2/P3a was reduced in people with MCI.
This could be because of the use of the oddball paradigm in study 4. Detection of the deviant would be carried out, at least in part, by the change detection system while in study 3, the presentation of a single stimulus would have been detected only by the transient detection system. Operations of the frontoparietal network controlling processing priorities can be interrupted by sufficient output from either the transient or change detector systems. This results in a switch of processing from an ongoing task to the processing of the potentially more relevant stimulus input. When this interrupt is sent from the change detection system, the operations of the frontoparietal network do not appear to deteriorate in people with MCI compared to what is observed in cognitively healthy older adults.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/42615 |
| Date | 01 September 2021 |
| Creators | Kamal, Farooq |
| Contributors | Taler, Vanessa |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
| Rights | Attribution-NonCommercial-NoDerivatives 4.0 International, http://creativecommons.org/licenses/by-nc-nd/4.0/ |
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