Dark adaptation curve during body temperature decrease.

To investigate the effect of body temperature decrease which occurred along with cold exposure and menstrual cycle on visual acuity, the dark adaptation curve (DAC), heart rate (HR) and tympanic membrane temperature (Tty) were measured at thermoneutral (29 degrees C) and cold (15 degrees C) in both follicular and luteal phases in menstrual cycle. Tty at thermoneutral and cold exposure in follicular phase were significantly lower than those in luteal phase. The degree of decrease in Tty by cooling in follicular phase was not significantly larger than that in luteal phase. HR was decreased significantly by cooling in both phases, while the difference between the two phases was not significant. DAC during cold exposure was lower than that during thermoneutral condition in both phases. Furthermore, the curve obtained during cooling in follicular phase was lower than that during cooling in luteal phase. However, DAC obtained by a subject who showed an increase of Tty by cold exposure in both phases was higher than that at thermoneutral. Even in thermoneutral in luteal phase, DAC during head cooling was significantly lower than that during thermoneutral. These results suggest that the brightness sensitivity in scotopic vision might be increased by a decrease of body temperature and the light threshold during dark adaptation threshold may be related to the change of Tty.