Abstract
Reading is a complex visual task used frequently in daily life. Previous research has concluded that the human brain has limited capacity for processing written words: higher-level lexical information is processed serially, one word at a time. These studies used a behavioral paradigm in which pairs of words were flashed briefly, one on either side of fixation, and then replaced by post-masks at an interval set to each subject’s threshold. When focusing attention to categorize just one word, participants’ accuracy was between 80 and 90% correct. When dividing attention to categorize both words independently, accuracy dropped far enough to support a serial processing model. This result was obtained with words centered 1.1 and 1.5 degrees above/below fixation, and with words centered 2.75 degrees left/right. Those studies suggested the presence of a serial bottleneck: a point in the word recognition pathway at which parallel processing becomes serial, requiring attention to be focused on one word at a time. Here, we investigated how the results generalize to a lexical decision task (distinguishing real words from pseudowords) with letter strings presented left/right of fixation, and closer together: the inner edges of each string were 0.7 degrees from fixation. To confirm the previous results, we also conducted the same task with the letter strings 1.5 degrees above/below fixation. In the left-right presentation, there were reliable individual differences: a minority of participants exhibited severe capacity limits consistent with serial processing, while the rest exhibited varying levels of parallel processing. In the above-below presentation, the vast majority of the same participants demonstrated serial processing, replicating previous results. We conclude that, for most but not all individuals, there is a narrow window around fixation within which simultaneous lexical access is possible for two words that are arranged horizontally.