Abstract Electrical activities of cortical neurons produce a scalp potential distribution. It was already found that this potential distribution for the alpha rhythm is smooth and stable in normal subjects, whereas it loses smoothness and is unstable in AD patients. We investigated a histogram of the normalized power variance (NPV) evaluated for 100 ms of the beta component (13–30 Hz), recorded with 21 electrodes placed according to the 10–20 method. It was evaluated in 56 normal subjects for each electroencephalogram (EEG) channel, and its mean 〈NPV〉 and standard deviation σ were calculated. When an NPV of a patient with Alzheimer's disease (AD) lies outside of a range 〈NPV〉±σ, cortical neurons related to that channel are regarded as partly impaired, where impairment is classified as being more unstable (over-active) and more inactive (under-active) than averaged normal neurons. The neuronal impairment map (NIM) was defined based on such evaluations of the 21 channels, where the NIP is characterised in red and blue regions correspond to over- and under-activity. The NIM pattern is similar to the regional cerebral blood flow (rCBF) reduction pattern measured by SPECT. The NIM is a simple tool to see local, cortical, neuronal impairment.