Visual word recognition: Evidence for a serial bottleneck in lexical access

Abstract

Reading is a demanding task, constrained by inherent processing capacity limits. Do those capacity limits allow for multiple words to be recognized in parallel? In a recent study, we measured semantic categorization accuracy for nouns presented in pairs. The words were replaced by post-masks after an interval that was set to each subject's threshold, such that with focused attention they could categorize one word with~80% accuracy. When subjects tried to divide attention between both words, their accuracy was so impaired that it supported a serial processing model: on each trial, subjects could categorize one word but had to guess about the other. In the experiments reported here, we investigated how our previous result generalizes across two tasks that require lexical access but vary in the depth of semantic processing (semantic categorization and lexical decision), and across different masking stimuli, word lengths, lexical frequencies and visual field positions. In all cases, the serial processing model was supported by two effects: (1) a sufficiently large accuracy deficit with divided compared to focused attention; and (2) a trial-by-trial stimulus processing tradeoff, meaning that the response to one word was more likely to be correct if the response to the other was incorrect. However, when the task was to detect colored letters, neither of those effects occurred, even though the post-masks limited accuracy in the same way. Altogether, the results are consistent with the hypothesis that visual processing of words is parallel but lexical access is serial.

Is there a serial bottleneck in visual object recognition?

“…processing limit, as if participants can recognize only one word at a time (White, Palmer, & Boynton, 2018;White, Palmer, & Boynton, 2020). This large effect of divided attention is consistent with a serial processing bottleneck.…”

“…In addition, because only one stimulus out of two is processed, there is a negative correlation in the accuracy of the two responses: correct responses for one stimulus co-occur with incorrect (or chance) responses for the other. These predictions have been satisfied for letter-digit tasks with conflicting S-R mapping (Sperling & Melchner, 1978), for certain multiple object dual tasks (Bonnel & Prinzmetal, 1998), and for masked words (White et al, 2018;White et al, 2020). In summary, the all-or-none serial model predicts both a large magnitude effect of divided attention and a negative correlation between dual-task responses.…”

“…Here, we consider three models of serial and parallel processes as benchmarks for judging experimental evidence in favor of or against the existence of a serial processing bottleneck. These models were implemented as in White et al (2020).…”

“…For example, a fixed-capacity parallel process can predict a larger dual-task deficit when target detection uses discretized states than when target detection uses continuous information (Swagman, Province, & Rouder, 2015; and see Appendix). Similarly, a serial process can produce a smaller dual-task deficit if there is enough time within one trial to complete processing one stimulus and switch to processing a second stimulus (White et al, 2020). To interpret our findings, we consider these generalized versions of these models alongside the three benchmark models.…”

“…To determine the appropriate sample size (number of participants), we examined data from four previous dual-task experiments using RSVP and masked word stimuli (White et al, 2018;White et al, 2020). In each, participants (n = 10) performed judgments of words with similar methods as the current study.…”