In psychology, a stimulus is any object or event that elicits a sensory or behavioral response in an organism. * In perceptual psychology, a stimulus is an energy change (e.g., light or sound) which is registered by the senses (e.g., vision, hearing, taste, etc.) and constitutes the basis for perception. * In behavioral psychology (i.e., classical and operant conditioning), a stimulus constitutes the basis for behavior. In this context, a distinction is made between the distal stimulus (the external, perceived object) and the proximal stimulus (the stimulation of sensory organs). * In experimental psychology, a stimulus is the event or object to which a response is measured. Thus, not everything that is presented to participants qualifies as stimulus. For example, a cross mark at the ce