The word stimulus is extremely powerful because it acts as a catalyst that initiates, or provokes, some sort of reaction. Moreover, stimulation implies a causal relationship between a beginning and an end result. Although this word is frequently used in highly specialized rhetoric, such as in physiological and psychological vernacular, in its most general sense it can be defined as “an agency or influence that stimulates to action or (const. to) that quickens an activity or process.” 
In the field of cognitive sciences, stimulus and stimulation are fairly common terms used to describe the instigator, and actual instigation, of a particular behavior. With respect to sensation and perception, a stimulus is typically characterized as a fundamental component of the transition from environmental information to perception. Despite the myriad of theories that attempt to explain the complex relationship between stimulus and perception, all generally agree on the transduction process of stimulation. For example, a stimulus represents a specific piece of contextual information, either internally or externally. This information is initially received by the senses (i.e. vision, gustation, olfaction, etc.), resulting in a sensation. Once the stimulus is translated into sensory input, it then is subjected to perception. The analysis process of perception is the final level of interpretation, which subsequently produces significantly individualized experiences. Essentially, this process of understanding a stimulus is expressed as a cascading series of activations that culminate with a perceived notion of the original stimulus.
Whereas the “atomistic” approach of the cognitive sciences mentioned above attempts to compartmentalize the transduction of stimuli to perception, the Gestalt school of thought emphasizes the holistic and self-organizing tendencies involved in cerebral processing. Moreover, this theory explains our primary awareness as being constructed of whole experiences (i.e. stimuli) that are comprised of indivisible structures and configurations. This includes whole concepts as well as physical objects.  For example, an object such as a ball would not be interpreted as an aggregate of lines and curves, but rather it would be recognized based on its figure and whole form. To account for these types of perceptions, Gestalt philosophy utilizes several critical principles to organize external stimuli. One such method of organization is the law of Prägnanz, which incorporates the perceptual principles of proximity, similarity, good continuation, closure, and common fate.  Together or separately, these principles elucidate how stimuli are cohesively interpreted as experiential units.
An alternative way of understanding a stimulus is with regard to the manner in which it is presented. McLuhan’s notion of hot and cold mediums is a good example of this. He defines a cold medium as something that requires participation and is “low definition,” and a hot medium as something that requires little participation and is “high definition.”  Based on these two categories, it seems that stimulation could depend on the level of definition just as much as the level of participation. In terms of the level of participation of the audience, a cold medium would presumably result in greater stimulation because it requires more stimulation/activity than a hot medium does. However, on the other hand, looking at the definition quality could conversely suggest that the “high definition”, in comparison to “low definition”, offered by a hot medium results in a similar level of stimulation. Seeing as how a cold medium contains incomplete information, it would seem more likely that a hot medium could spur greater stimulation due to the fact that it presents complete information, or more complex stimuli to arouse the senses.
In addition to his discussion of the types of media, McLuhan also investigates the role of the body, and more specifically the central nervous system, as the primary recipient of stimulation brought about by such media. He expands on the notion that any organism, whether simple or complex, is dependent on its innate ability to maintain equilibrium and remain in homeostasis. The body is designed to respond to stimulation in order to insure its survival. McLuhan suggests that “numbness” occurs in response to a threatening stimulus and allows the individual to block perception. The central nervous system responds to irritants by impeding the perceptual analysis of a stimulus in attempt to preserve itself. The numbness incited by this type of stimulation is therefore responsible for what is referred to as “autoamputation”. This accounts for the body’s effort to isolate the source of irritable stimulation and then amputate the offending sensation. The network of systems in the body operates concomitantly to protect and sustain the delicate equilibrium of the central nervous system. Therefore, by amputating the location of irritation, the body is free to numb itself and relocate the offending stimulation to an alternative external location. 
Another duality that separates media is Massumi’s concept of the analog and the digital. While the nuances of each of these two categories distinguish the elements of various media, they function cooperatively as a process. He suggests that digital is sandwiched between analog in terms of the transfer of information. For instance, input is described in analog, then it becomes digitized, and finally this produces analog output. This process mediates the transition of information to a sensation and perception. The stimuli produced by an activity causes what Massumi refers to as attention and inattention/distraction.  The stimulation of certain activities is what results in the body’s synthesis of sensation and perception.
The sense datum theory redefines stimulus and stimulation to better understand the philosophical implications of the relationship between sensation and perception. In this model, sense data, or sense content, functions as sensory stimuli (i.e. colors, sounds, smells, tastes). Similar to the Gestalt principles, the sense datum is defined more as a direct element of experience, one that is correlated to the immediate perception or awareness of something. This is the stimulation that occurs as a result of the sense datum. Along with this notion of instantaneous perception are the properties that regulate the identification of sense data. They state that sense data are private to only one percipient, they last only as long as they are being sensed, they result in individualized experiences, they lack causal relationships (i.e. one stimulus doesn’t act on any other stimulus), and they can’t be other than what they appear to be. Although these five principles are used to specify sensory stimulation, they are quite general in the philosophical sense. The argument helps to explain how stimulation is categorized into sensing, but it fails to clarify how perception is mediated by sensing and more importantly how this affects awareness. This debate ultimately raises the issue of certainty in the matter of how we can determine what we are aware of. 
Maurice Merleau-Ponty proposes another explanation for how sensation and perception relate to each other. In The Phenomenology of Perception, he highlights the nuances associated with the process of perceiving. Just as Gestalt ideologies infer, Merleau-Ponty suggests that perception occurs as a whole. This is due to the fact that all experiences are composed of sensations (stimulation) and sensed qualities (perceptions). Complicating this relationship further is the role of consciousness in perception. He proposes that our past experiences of sensations allow our consciousness to perceive objects based on assumptions more so than the actual translation of the sensation into a perception. Consequently, these prejudices are responsible for the misinterpretation of qualities as elements of consciousness, instead of stimuli for consciousness, in addition to counteracting the role of experience as something that changes in terms of time and space with relation to varying stimulation. 
In general, stimulation occurs to excite a single sensory organ so that the body can perceive the experience of a stimulus. However, a phenomenon exists in which the body is able to activate a sensory modality but elicit a perception of another. This is called synaesthesia. In cognitive terminology, this is an extremely rare condition that can be observed in individuals that report a variance in sensation and perception, such as a person who tastes sound, or hears colors. Although this is indeed a condition that affects the interpretation of stimulation, synaesthesia also is used to describe media. Kandinsky is accredited with trying to use his paintings to evoke multiple perceptions other than those that were to be expected.  Challenging established media have produced several revolutionary developments in the understanding of stimulation. Synaesthesia is effect, helps to redefine a stimulus so that it can elicit something contrary to the ordinary.
1. OED, “stimulus”
2. Osborn, “Gestalt” (Grove Art Online)
3. Galotti, pg. 43-47
4. McLuhan, pg. 22-23
5. McLuhan, pg. 41-44
6. Massumi, pg. 138-139
7. Borchert, pg. 813-818 (Encyclopedia of Philosophy)
8. Merleau-Ponty, pg. 5-9
9. Barron-Cohen and Harrison, “synaesthesia” (Grove Art Online)
Baron-Cohen, S., and Harrison, J. E. Synaesthesia: classic and contemporary readings. Oxford University Press, 2007, http://www.groveart.com/.
Borchert, Donald M. Encyclopedia of philosophy. Farmington Hills, MI: Macmillan Reference USA/Thomson Gale, 2006.
Galotti, Kathleen M. Cognitive psychology: in and out of the laboratory. Belmont, CA: Wadsworth/Thomson Learning, 2004.
Massumi, Brian. Parables for the virtual: movement, effect, sensation. Durham: Duke University Press, 2002.
McLuhan, Marshall. Understanding media: the extensions of man. Cambridge: The MIT Press, 1998.
Merleau-Ponty, Maurice. Results: Phenomenology of perception. New York: Routledge, 2005.
Osborn, Harold. Gestalt. Oxford University Press, 2007, http://www.groveart.com/.
Oxford English Dictionary, “stimulus”