Saturation and adaptation in the rod system.

A background that is briefly flashed to a dark-adapted eye saturates the rod system. This transient saturation occurs with backgrounds that are as much as 2 log units dimmer than those producing saturation under steady viewing. Rod threshold is highest when the background is first turned on, and falls as adaptation proceeds. The nature of the adaptive processes are studied by presenting flashed backgrounds on pre-adapting fields. The data can be interpreted in terms of two adaptive processes: the first is multiplicative, and occurs rapidly; the second is subtractive, and occurs more slowly.