Stimulus, Sensory and Visual cue

In psychology, stimuli are energy patterns which are registered by the senses. In behaviorism and related stimulus–response theories, stimuli constitute the basis for behavior, whereas in perceptual psychology they constitute the basis for perception.[1]
In the second half of the 19th century, the conception had been established by psychophysics, the “scientific study of the relation between stimulus and sensation”,[2] together with the notion of the reflex arc constituting a foundational concept of scientific psychology.[3] While at this time “whatever could be controlled by an experimenter and applied to an observer could be thought of as a stimulus.”[3] In the context of perception, a distinction is made between the distal stimulus (the external, perceived object) and the proximal stimulus (the stimulation of sensory organs).[4]
Helmholtz (1821-1894) and Brunswik (1903-1955) are two scientists who were known for cue theories. Helmholtz held that the visual system constructs visual percepts through a process of unconscious inference, in which cues are used to make probabilistic best guesses about the state of the world. For Helmholtz (and most modern perceptual scientists), a visual percept is the manifestation of this process.
Brunswik formalized Helmholtz’s ideas with the lens model, which breaks the system’s use of a cue into two parts: the ecological validity of the cue, which is its correlation with a property of the world, and the system’s utilization of the cue. In these theories, accurate perception requires both the existence of cues with sufficiently high ecological validity to make inference possible, and that the system actually utilizes these cues in an appropriate fashion during the construction of percepts.
In a nutshell, visual cues are elements of a design that communicate its purpose and method of use. Common cues have developed over time out of repetitive use, natural association with other cues, and common sense (or the minds natural processes). These cues are vital to the success of a product. If a person is confused by the cues he/she is left to read pages of instructions. If the cues are wrong he/she will feel betrayed and put-off. If they are obvious, descriptive, and accurate the individual will enjoy their natural like experience.
Have you ever entered a public building and pushed on a door, then realize after your face smacks the glass you were suppose to pull? Usually, you feel kind of dumb and look around to make sure no one noticed, but most likely this could have been a result of poor cues.[5]

1 “Stimulus”. In: Richard L. Gregory (Ed.), The Oxford Companion to the Mind, Oxford, N.Y.: Oxford University Press.
2 Gescheider, G. (1997). Psychophysics: the fundamentals (3rd ed.). Lawrence Erlbaum Associates. p. ix. ISBN 080582281X.
3 a b Gibson, James J. (1960): “The Concept of the Stimulus in Psychology”. American Psychologist, 15, pp. 694–703, here p.694.
4 http://www.learner.org/discoveringpsychology/07/e07glossary.html
5 http://robjensendesign.com/2009/04/designing-visual-cues/