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Behavioral study. If the co-occurrence hypothesis is correct, then people who have had unusual perceptual experiences might process visual stimuli in different ways. For example, it is well known that for most people, a letter Òpops outÓ when presented in an array of digits compared with when it is presented in an array of letters, that is, it is detected faster and with less serial search (Duncan, 1980; Duncan, 1983; Egeth, Jonides, & Wall, 1972; Jonides & Gleitman, 1972; Merikle, 1980; Schneider & Shiffrin, 1977; von Wright, 1972). The cortical segregation of letter and digit recognition naturally accounts for this so-called alphanumeric category effect because letter distractors, which are represented in the same cortical region as the letter target, would cause more interference than would digit distractors, which are represented elsewhere. So if letter segregation is due to the co-occurrence of letters with other letters rather than with other types of stimuli, then people who regularly process letters and digits together might show a reduced effect. We tested this prediction experimentally by comparing the category effect in postal employees who process Canadian postal codes (in which letters and digits alternate, e.g., V5A 1S6) with subjects who do not.
An alphanumeric category effect is evident for all subject groups as evidenced by the longer reaction times in detecting a letter among letters compared a letter among digits. As predicted however, the Canadian-mail sorters showed a smaller effect than postal worker controls who did not sort mail (measured both by the absolute difference in and ratio of response times in the letter-among-letters (LL) and letter-among-digits (LD) conditions). The sorters were, however, faster than controls, presumably due to their extensive experience with speeded tasks. So to ensure that these results were not the result of a floor effect, we excluded the three slowest postal worker controls (out of 16) for one analysis and used college graduates whose RTs were faster for another. In both cases, the control group showed a larger category effect than the sorters, even though both groups were faster than the sorters in the LD condition. These cross-over interactions eliminate any obvious interpretations based on scaling artifacts and confirm the prediction of the co-occurrence hypothesis.
These results appeared in Nature (Polk & Farah, 1995b).
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