A color-word matching task was used to investigate the basis of Stroop interference. Subjects were shown a pair of stimuli: an ink color (e.g., a red bar) and a colored word (e.g., RED printed in red or blue) and decided whether the two items had the same meaning (meaning decisions) or whether they had the same surface color (visual decisions). In Experiment 1, the two stimuli were shown simultaneously, and conflicting visual information of the word (e.g., RED printed in blue, against a red bar) led to interference in meaning decisions, whereas conflicting verbal information (e.g., BLUE printed in red, against a red bar) produced no interference in visual decisions. In Experiment 2, as an increasing time interval was imposed between presentation of the color bar and the colored word, interference in meaning decisions diminished, whereas interference in visual decisions was established. These results suggest that semantic competition, not response competition, is the major source of Stroop and Stroop-like interference.One of the most popular tasks for studying the relationship of word reading, object naming, and selective attention is the Stroop colorword task (Stroop, 1935). In the Stroop task, subjects are asked to name the color of a color patch or the color of a colored word. Relative to naming a color patch, subjects often show a large delay in naming the color in which a color word is printed (e.g., blue in red ink). The Stroop effect is highly robust and has been demonstrated for a variety of stimulus materials and experimental tasks (see MacLeod, 1991, for a comprehensive review). The mechanism underlying the Stroop effect is, however, far from clear, despite more than six decades of intensive investigation.There are two classic accounts of the Stroop phenomenon (MacLeod, 1991). The relative-speed-of-processing hypothesis (e.g., Morton & Chambers, 1973;Posner & Snyder, 1975) attributes Stroop interference to the fact that word reading is faster than color naming (e.g., Cattell, 1886;Theios & Amrhein, 1989). According to this account, when two potential responses, one from reading a word and one from naming an ink color, compete to be the response to be selected for output, the consequence is a delay in response production, especially when the response from the more rapid process is the one to be ignored. Note that the relative-speed-of-processing hypothesis places the locus of interference at the stage of response output.The second account, the automaticity hypothesis (e.g., LaBerge & Samuels, 1974;Shiffrin & Schneider, 1977), attributes Stroop interference to the fact that word reading is more automatic than color naming (e.g., Cattell, 1886;Posner & Snyder, 1975