Effect of high latitude on the variability of human event-related brain potentials

Abstract

European researchers (Anderson, Chambers, Myhre, Nicholson, & Stone, 1984) at 69 degrees north latitude, have reported seasonal changes in the human electroencephalogram, EEG. Other researchers from the North American mid-latitudes (Deldin, Duncan, & Miller, 1989a, 1989b), using event-related brain potentials, ERPs, have reported changes associated with ambient light. Similar changes in the event-related brain potentials of humans have not been reported from the high latitudes nor by longitudinal methods. This dissertation longitudinally investigated ERP variability in normal humans living at high latitude. One hundred and twelve residents of Fairbanks, Alaska participated in single-trial ERP testing, both auditory and visual, to evaluate criteria of validity and reliability. Subsequently, eight normal humans participated in auditory and visual ERP testing every month for twelve consecutive months. The longitudinally-studied subjects were residents of the Alaskan subarctic, where testing occurred, and each experienced naturally-occuring photoperiod variability of 3.31 hours in December to 21.98 hours in June. Dependent variables included P3 amplitude and latency in both sensory modalities. Independent variables included: age, estimated cranial volume, length of last sleep epoch, subjective wakefulness, ambient photoperiod, and ambient geomagnetic field. Distribution of the longitudinal ERP data satisfied Shapiro-Wilke criteria for normality. The GLM-ANOVA for ordered repeated measures was used and a month effect was observed in P3 amplitude and latency, as well as a month-by-sensory mode interaction. The principal components method of factor analysis evaluated the factor weights of the independent variables. Variables unique to each subject (e.g., age, cranial volume, length of last sleep) were more heavily weighted in the first factor. The environmental variables became heavily weighted in the second factor, and descriptors of the earth's natural geomagnetic activity were weighted more heavily than ambient photoperiod. Following PCA analysis, the environmental variables of geomagnetic field and photoperiod were separately investigated. No seasonal variability in the ERP characteristics was detected when using a photoperiod-based definition of season. However, variability in auditory P3 amplitude was observed when groupings were defined based on geomagnetic criteria.