ABSTRACT
While human capacity is impressive in many areas, there are basic limitations to human performance. Eyes cannot be moved to two different locations at the same time and two different phonemes cannot be pronounced simultaneously. Human limitations go far beyond these physical constraints. Even if two activities are not physically impossible, performance does deteriorate when two different operations must occur at the same time even when input and responses are quite separate, such as in reading a book while listening to television. It seems as if the mind is constantly selecting a subset of stimuli, and this selection affects mental processes and behavior. This basic feature of performance gives rise to one of the most widely used metaphors regarding attention, the metaphor of the light beam, which selectively illuminates stimuli sources, spatial locations, and mental structures.
