Thermal Activation

An event where an opsin is triggered without catching a photon. This still gives the perception of light and therefore acts as “noise in the signal”. The higher the rate of Thermal Activation, the more noise there is, and the larger a signal (a light stimulus) must be before it is noticeable. Having a low rate of thermal activation increases the dynamic range of the that signal and therefore the individual’s contrast sensitivity. Rhodopsins have much lower thermal activation than Photopsins, which is why they can be used in the much higher sensitivity Rod Cells.