On doing two things at once: Time sharing as a function of ideomotor compatibility.

A relationship between stimulus and response of quot;ideomotor compatibilityquot; was defined as occurring to the extent that the stimulus resembles normally occurring sensory feedback from the response (e.g., saying a word in response to hearing it said). It was proposed that the stimulus of highly ideomotor-compatible combinations should effectively select the response without burden-ing limited-capacity decision processes of the central nervous system. Accord-ingly, (a) perfectly efficient time sharing of two simultaneous decision tasks was predicted when both tasks were highly ideomotor compatible, and (b) inefficient time sharing was expected when both tasks were not highly ideo-motor compatible. Eight Ss served in High and eight in Low Ideomotor Compatibility conditions that required rapid independent decision responses (spoken and manual) to two simultaneous stimuli (auditory and visual). The predicted effect of the ideomotor-compatibility variation on time-sharing efficiency was clearly confirmed. Can two independent decision tasks be performed simultaneously with perfect ef5-ciency, that is, with no loss of speed or accuracy for each relative to its performance in isolation? This should be possible only if the two tasks do not share in the use of any limited-capacity information-process-ing systems. Avoidance of common limited-capacity systems is evidently possible when neither of two simultaneous tasks requires a decision based on stimulus information (Adams, 1966, p. 190f.). However, when time sharing of decisions has been required, some residual inefficiency of simultaneous performance has generally been observed even when care has been taken to assure that the two tasks use different input modalities and output effectors (e.g., Schvaneveldt, 1969; see also the review by