Which phase of adaptation is represented by the initial (upper left) portion of a dark adaptation curve?

The initial portion of a dark adaptation curve represents cone adaptation. This phase occurs when the eyes transition from light to dark conditions primarily involving the cones, which are responsible for vision in well-lit environments. During the early stages of dark adaptation, the sensitivity of the cones is adjusted to the lower light levels, but they are limited in their ability to respond to very dim light, as they have a lower sensitivity compared to rods.

As darkness continues, the rods begin to contribute more significantly to vision, leading to the later stages of the dark adaptation curve, where rod adaptation becomes the dominant factor. Therefore, the upper left portion of the curve effectively captures the cone adaptation phase before the rods take over and enhance sensitivity in darker conditions. Understanding this phase is crucial for visual perception, as it demonstrates how our visual system transitions from photopic (well-lit) to scotopic (dark) vision.