Which theory focuses on the proportionality of stimulus perception?

Weber's Law is the correct answer because it specifically addresses the relationship between the intensity of a stimulus and the resulting perception of that stimulus. According to Weber's Law, the smallest noticeable difference (the just noticeable difference, or JND) between two stimuli is proportional to the intensity of the initial stimulus. This means that as the intensity of a stimulus increases, so does the amount by which it must change for a difference to be noticed. This proportionality is a key principle in understanding sensory perception and helps explain how we perceive changes in the environment.

The other theories mentioned, while related to perception, have different focuses. Fechner's Law builds on Weber's Law and describes the relationship between actual stimulus intensity and perceived intensity, establishing a logarithmic function. Stevens' Power Law describes how perceived magnitude of a stimulus scales as a power function of the stimulus intensity, which can vary across different types of perception. Signal Detection Theory, on the other hand, is concerned with the ability to discern between signal and noise in uncertain conditions rather than specifically addressing proportionality in stimulus perception. Thus, Weber's Law is the most direct theory regarding the proportionality of stimulus perception.