Initial stages of dark and light adaptation.

Both light and dark adaptation show rapid fluctuations in their early stages. The effects of the fluctuations may be quite large compared to the transitions customarily referred to as visual adaptation, and they serve to enhance the familiar adaptation effects. The nature of these rapid transient changes is described empirically for several experimental conditions. The transient fluctuations do not appear to have a photochemical basis.The dark-adaptation transient effect seems to show two phases: an early small reduction of sensitivity, and a subsequent large and rapid sensitivity increase, which leads into the familiar dark-adaptation curve. The early threshold rise seems to be associated with the receptor, but the rapid subsequent threshold drop appears to be associated with neural pathways higher than the receptor. The light-adaptation transient shows but one effect: a sudden loss of sensitivity overshooting the terminal sensitivity level, followed by a rapid increase back to the terminal level. Some evidence indicates that the light-adaptation transient is the combined effect of a major peripheral process and a small central process.