Sensory receptors are specialized neurons that respond to specific types of external stimuli, initiating the process known as sensation. This occurs when sensory input, such as light entering the eye, is detected by these receptors, causing chemical changes in the cells of the retina. These cells then convert the sensory stimulus into action potentials that are transmitted to the central nervous system, a process termed transduction.

Absolute thresholds can quantify the sensitivity of sensory systems to their relevant stimuli. This threshold is the minimal level of stimulus energy required for an observer to detect the stimulus at least 50% of the time. For example, in a visual context, this might be the faintest light visible to the human eye in a dark room or, in auditory terms, the lowest volume at which a sound is heard. This concept helps in understanding the limits of sensory perception under minimal stimulus conditions.

Subliminal messages are another intriguing aspect of sensory perception. These are stimuli that occur below the absolute threshold for conscious awareness. Despite being undetectable on a conscious level, the sensory receptors still receive and process these messages. An example of a subliminal message might be a brief, unnoticed visual or auditory stimulus embedded within media that influences behavior or thought processes subtly, such as a fraction-of-a-second advertisement in a film that is too short for conscious recognition but may still influence viewers' preferences or emotions.

The difference threshold, or just-noticeable difference (JND), measures how much a stimulus must be altered before a change in the sensory input is perceived. This threshold varies with the intensity of the stimulus; the stronger the initial stimulus, the larger the change needed to notice a difference. For instance, adding a few sheets of paper to a heavy book might not be noticeable, but the same addition to a single sheet of paper would be. This concept is crucial for understanding how sensory adaptation and perception scale relative to stimulus intensity.