Abstract: Introduction: The clinical condition characterized by objective memory disturbances in the absence of other cognitive deficits is called mild cognitive impairment (MCI). MCI is one of the risk factors of Alzheimer’s disease (AD). The study of cognitive impairment in patients with MCI, particularly memory deficiency, may aid in obtaining clues of early stage of AD. Many studies have reported that working memory plays an important role in learning, calculating, reasoning, and verb comprehension in cognitive processing; hence, it is important to study working memory processing in MCI. The aim of the present study was to investigate the characteristics of EEG power and coherence in patients with MCI during working memory processing.
Method: Subjects that included 35 MCI patients according to the DSM-IV criteria (mean age 61.6 years, SD 7.2) and 34 health controls (mean age 59.1 years, SD 5.4) were selected from the community. All subjects performed a simple calculation and recall task with 3 levels of working memory load, with a simultaneous recording of the EEG signal. The spectral EEG power was computed over delta (1.0–3.5 Hz), theta (4.0–7.5 Hz), alpha1 (8.0–8.5 Hz), alpha2 (9.0–13.0 Hz), beta1 (13.5–18.0 Hz), and beta2 (18.5–30.0 Hz) frequency bands and was compared between the rest stage and the working memory processing stage by a 2 × 3 MANOVA. Post hoc test analyzed the differences between each 2 levels of working memory load during the task processing. The EEG and the interhemisphere EEG coherence of frontal (F3-F4), central (C3-C4), parietal (P3-P4), temporal (T5-T6) and occipitals (O1-O2) were compared between MCI patients and health controls.
Results: (1) Spectral EEG power analysis: The spectral EEG power over delta, theta, alpha1, alpha2, and beta1 bands was significantly higher in the working memory stage than that in the rest stage. The group (MCI and controls)-state (3 task levels) interaction was not significant. Post hoc analysis indicated that there was no significant difference between each 2 levels during the working memory stage. The spectral EEG power of MCI patients over delta, theta, alpha1, alpha2, and beta1 bands was significantly higher than that of the normal controls during the task processing. (2) Coherence analysis: The interhemisphere coherence in the rest state showed no significant difference between MCI patients and the normal controls. Theta (4.0–7.5 Hz), alpha1 (8.0–8.5 Hz), and alpha2 (9.0–13.0 Hz) frequency bands were selected to investigate the differences in interhemisphere coherence between MCI and controls during the working memory processing stage. There was significantly higher inter-hemisphere coherence in central (C3-C4), parietal (P3-P4), temporal (T5-T6) sites of MCI patients than that of the normal controls during working memory processing.
Conclusion: Working memory processing was related to delta (1.0–3.5 Hz), theta (4.0–7.5 Hz), alpha1 (8.0–8.5Hz), alpha2 (9.0–13.0 Hz), and beta1 (13.5–18.0 Hz) frequency bands. MCI patients may have impairment in the central, parietal, and temporal cortex. During working memory processing, MCI patients may mobilize a compensatory mechanism that serve to maintain processing effectiveness while concealing underlying reduction in processing efficiency.

Key words: Spectral power, coherence, mild cognitive impairment, working memory