Differences between encoding and retrieval failure in mild cognitive impairment: results from quantitative electroencephalography and magnetic resonance volumetry

Background: Memory impairments in mild cognitive impairment can be classified into encoding (EF) and retrieval (RF) failure. We explored differences between the two using quantitative electroencephalography (qEEG) and magnetic resonance imaging volumetry.Methods: We compared qEEG power spectra and connectivity between 87 participants with EF and 78 with RF, and gray matter (GM) volume differences between participants with mild cognitive impairment and 71 controls. Demographic and cognitive assessment results between groups were compared with Student’s t-tests, and obtained qEEG features were analyzed according to statistically significant differences between the groups using iSyncBrain ™. GM volume changes underlying EF and RF in amnestic mild cognitive impairment were compared on processed magnetic resonance images of cognitively normal subjects using Student’s t-tests, with age and total intracranial volume classified as nuisance covariates. We used a voxel-based morphometry analysis to demonstrate significant atrophic GM areas in the two types of patients with amnestic mild cognitive impairment. Results were considered statistically significant at p < 0.05 and were corrected for family-wise errors to avoid multiple-comparison problems.Results: qEEG showed higher frontal theta and lower beta2 band power and higher theta and lower beta connectivity in patients with EF. GM volume reductions due to EF in the left thalamus and bilateral hippocampi and reductions due to RF in the left thalamus, right superior frontal lobe, right superior temporal lobe, and right middle cingulum were observed. Conclusions: Patients with mild cognitive impairment differ functionally and structurally according to their specific memory impairments. EF findings are more consistent with the prodromal Alzheimer's disease stage than RF findings. Since this study is a cross-sectional study, prospective follow-up studies are needed to investigate whether different types of memory impairments can predict the underlying pathology of amnestic MCI. Additionally, insufficient sample size may lead to ambiguous statistical finidings in direct comparisons, a larger patient cohort could more robustly identify differences in GM volume reductions between the EF and the RF group.