The signal detection theory is an alternative to the approaches of classical psychophysics. It emphasizes that the judgment about the presence or absence of a stimulus depends on two processes:
(a) an initial sensory process, which involves subject's sensitivity to the intensity of a stimulus, and
(b) a decision making process, which reflects psychological factors within the individual. The signal detection theory explains the role of psychological factors in judging whether a stimulus is present or absent.
Whether we perceive a single stimulus or a difference between two similar stimuli, our perception of events depends on several factors. The intensity of stimulation, described in absolute threshold and differential threshold, is one of the factors. Background factor in our perceptual field is another important condition, which influences our perceptual process. Background factor refers to the presence of other stimuli in which the perceived stimulus is observed or embedded.
In other words, how are the other stimuli happening in respect to our perceived stimulus is called the background factor. For example, it is easier to hear a sound in a quite room than in a noisy room. Here noise is in the background of our perception of the sound. The efficiency of the person's sensory system, which is involved in sensation, is also another factor in perception. For example, a normal child is experiencing so many varieties of visual sensations, which a visually handicapped child is deprived of. A young boy listens to different voices distinctly and clearly as compared to an old man. Besides these physical and biological factors, several psychological factors like learning, motivation, interest, and expectancy also influence our perception. For example, we fail to hear a loud noise very close to us probably because we do not want to hear it. The signal detection theory holds that the physical, biological, and psychosocial factors of the individual determine his perception to a large extent.