Memory activation enhances EEG abnormality in mild cognitive impairment☆
Introduction
Progressive memory loss is considered one of the earliest signs of Alzheimer's disease (AD) [14]. The term amnestic mild cognitive impairment (MCI) has been introduced to define a group of patients with a high probability of conversion to AD [12], [17], [18]. MCI is characterized by isolated memory impairment that does not interfere with activities of daily living. MCI is heterogeneous in nature; some patients develop AD, others another type of dementia and yet others reverse to their premorbid level of functioning. Identifying MCI patients likely to develop dementia is important as in the future this may make early therapeutic intervention possible, thereby reducing health care costs and improving quality of life [10], [12].
Electroencephalography (EEG) has potential as a tool in the diagnostic and treatment work-up of AD, as it is widely available, non-invasive and cost-effective. EEG with eyes closed, as conventionally used in daily clinical routine, revealed no differences between cognitively unimpaired controls and MCI patients [5], [6], [22], [27]. In fact, the EEG only becomes abnormal in at least mildly demented patients [27]. There is evidence to suggest that the yield of EEG for clinical as well as scientific purposes can be enhanced by using activation paradigms, such as haptic tasks [4], category-decision tasks [15] and olfactory stimulation [16]. We hypothesized that memory activation paradigms would be most successful, as memory impairment is the defining criterion for MCI. This concept is supported, firstly, by a known association between oscillations in the alpha and theta bands and memory performance [8], [9]. Secondly, a significant difference between MCI patients and patients with subjective memory complaints was recently found in a study employing a working memory task, using an EEG measure called ‘synchronization likelihood’ [20].
In this exploratory study, we investigated EEG power changes in MCI patients and healthy age-matched controls during three different memory activation paradigms in a comparison with conventional EEG.
Section snippets
Subjects
Twenty-six patients with MCI according to the criteria of Petersen et al. [17], consecutively referred to the outpatient memory clinic of the Leiden University Medical Center, were selected for study. Patient histories were reviewed and diagnoses established in multidisciplinary consensus meetings. The diagnosis of MCI was made if the patient met the following criteria: (1) memory complaint, (2) normal activities of daily living, (3) normal general cognitive function, (4) abnormal memory for
Clinical characteristics
Age, sex and years of education did not differ between groups (Table 1). Patients with MCI scored significantly lower on the CAMCOG, MMSE and WMS scales than controls. The GIT-age corrected IQ did not differ between groups, indicating preserved general intelligence in the MCI group.
EEG
In the conventional ‘eyes closed’ condition absolute power did not differ between MCI patients and controls for any band or scalp location.
In the conventional ‘eyes open’ condition the power change as compared to
Discussion
This exploratory study shows that memory activation revealed EEG differences between MCI patients and controls, while conventional EEG conditions (‘eyes closed’ and ‘eyes open’) did not. We found that picture memory activation caused a decrease in lower alpha power in both MCI patients and controls. However, this decrease was significantly smaller in MCI patients. Furthermore, a correlation was found between lower alpha reactivity and picture memory performance, in that worsening performance
Acknowledgements
We thank E.L. Tiesma, P.J. van Someren, J.G. van Vliet-de Regt, M.J. Stijl-Pek, J.C. Albus-de Meza, A.J.C. van der Kamp-Huyts, F.I. Kerkhof and P. Massaar for their help in acquiring the EEG data. We are grateful to A.W.E. Weverling-Rijnsburger for her help in patient recruitment.
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We state that there are no potential conflicts of interest, including any financial, personal or other relationships with people or organizations that could inappropriately influence the current study. The study has been approved by the local Medical Ethical Committee.