Many models of reaction time assume that performance can be understood in terms of the formation of mental codes. That is, the process of identifying the stimulus in a task is understood as a process of forming a mental representation of the stimulus, or a stimulus code. Similarly, the process of selecting a response in a task is understood as a process for forming a mental representation of the desired response, or a response code.
Stimulus and response codes represent abstract properties of stimuli and responses in a task. For example, spatial stimuli are coded in terms of their location relative to the other possible locations in the stimulus set, rather than their absolute location in the visual field; similarly, spatial responses are coded in terms of their desired outcomes, relative to other outcomes in the response set, instead of in terms of specific actions or muscular movements.
Simon and his colleagues (Craft & Simon, 1970; Simon, Craft & Small, 1971; Simon, Small, Ziglar & Craft, 1970) demonstrated that the spatial S-R consistency effect in a Simon task depends on where you perceive the stimulus to be, and not on what ear, eye or visual hemi-field is actually stimulated. Umilta and colleagues (Umilta & Nicoletti, 1985, exp. 2 and 4; Umilta & Liotti, 1987, exp. 3) later showed that a spatial S-R consistency effect can also be found for relative stimulus positions: that is, when both stimulus positions appear on the left side of a display, but one stimulus position is farther left than the other. These experiments indicate that stimulus codes are formed based on your perception of the current irrelevant stimulus relative to other elements in the set, rather than on physical sensory stimulation or absolute stimulus position.
Other experiments have shown that response codes are also represented in terms of abstract properties, such as the desired outcome of the response. For example, when the subjects’ goal is to “press a key” in the Simon task, performance is determined by the relationship between the stimulus position and the response key position, even when subjects crossed their hands (Simon, Hinrichs & Craft, 1970; Wallace, 1971), cross their fingers (Riggio, Gawryszewski, & Umilta, 1986), or press keys using only one finger from one hand (Bauer & Miller, 1982), or using two fingers from the same hand (Heister, Ehrenstein & Schroeder-Heister, 1987). However, when pressing a key also lights up a light on the opposite side (i.e. pressing a left key lights up a light on the right side, pressing a right key lights up a light on the left side), and subjects are told to respond by “lighting up a light” rather than “pressing a key,” performance is determined by the relationship between the stimulus position and the light position. That is, when the stimulus and the light position are on the same side (but the response key is on the opposite side), subjects respond faster, while when the stimulus and the response light are on the opposite side (but the response key is on the same side as the stimulus), subjects respond more slowly (Hommel, 1993a).
The assumption of abstract code representation allows coding models to explain the appearance of consistency effects under a diverse set of stimulus and response conditions using a single explanatory mechanism (see Nicoletti & Umilta, 1984; Riggio, Gawryszewski, & Umilta, 1986; Umilta & Nicoletti, 1990). The coding model framework leaves open for debate the question of how mental codes are formed (e.g. Heister, Schroeder-Heister & Ehrenstein, 1990; Proctor, Reeve, & van Zanddt, 1992; Rubichi et al., 1997; Stoffer, 1991; Stoffer & Umilta, 1997; Weeks, Chua, & Hamblin, 1996); however, once it is taken as given that appropriate abstract mental codes are formed, these models then only need to specify how the relationship between different mental codes influences performance in order to explain the effects in all of these diverse task conditions.