Which photoreceptors have a higher threshold during the initial phase of dark adaptation?

During the initial phase of dark adaptation, rods exhibit a higher threshold compared to cones. This is primarily due to the fact that cones are less sensitive to light and are responsible for vision under well-lit conditions, functioning optimally in photopic (bright light) environments. In contrast, rods are specialized for scotopic (dim light) conditions and have a lower threshold for light detection after a period of adaptation to darkness.

When moving from a well-lit environment to a dark one, cones quickly lose their ability to respond to the decreased light levels because they require brighter conditions to function effectively. Rods, meanwhile, take longer to become sensitive as they rely on the regeneration of visual pigments (rhodopsin) that enables them to detect lower light levels. As a result, during the initial stages of dark adaptation, the sensitivity of rods increases more slowly, leading to the conclusion that they have a higher threshold compared to cones in this phase.

Ultimately, rods become more sensitive than cones after a period of time, but initially, they are at a disadvantage due to their thresholds being higher during the very early stages of dark adaptation.