Most models of reaction time based on the formation of mental codes assume not only that separate mental codes are formed for relevant and irrelevant stimulus features, but that the codes are formed in parallel by separate processes. Whenever a stimulus is characterized by more than one dimension (i.e. whenever there is more than one stimulus set), each stimulus dimension can be understood as a separate functional stimulus (Miller, 1988), from the point of view of the perceptual system.
Thus, instead of there being a single stimulus identification process, each dimension of the stimulus is identified by its own separate process, or separate “channel.” These processes can operate completely concurrently, and they do not depend on one another for information. (There is debate, however, as to whether there is “cross-talk” between different stimulus identification processes that are going on at the same time; see, e.g., Egeth, 1977; Estes, 1972, 1982; Mordkoff, 1991; Morton, 1969).
The assumption of multiple identification processes was developed by Eriksen and colleagues (Eriksen, 1966; Eriksen & Lappin, 1965, 1967; Eriksen & Spencer, 1969) for displays with multiple elements, such as flanker stimuli. They suggested that display items that are presented in different spatial locations are identified through separate and independent processes that act in parallel. The idea that the stimulus dimensions of color and word are processed by separate “perceptual analyzers” has also been assumed in even the earliest accounts of performance in the Stroop task (e.g. Morton & Chambers, 1973; Posner & Snyder, 1975). These assumptions were brought together to form the general assumption of separate stimulus identification processes or “channels” (see Egeth, 1977; Miller, 1988): when a stimulus consists of multiple dimensions, they form separate functional stimuli, and are processed by separate stimulus identification processes.