Sequence order resolves ambiguity in a nonlinguistic visual categorization task

When we encounter an unfamiliar word in a sentence, word order can be used to determine the grammatical category to which that word belongs and clarify ambiguity. However, it is unclear whether a similar categorization effect occurs in nonlinguistic contexts. We created three perceptually distinct categories of shape stimuli—rounded (A); squared (B); pointed (C). Participants were first taught to categorize these shapes, then participants in the experimental condition were trained to select them in a fixed sequence order (A→ B → C) while participants in the control condition were allowed to select them in any order. We then generated ambiguous stimuli by morphing shapes from different categories together. In the experimental condition, when an ambiguous morph was presented in a sequence, its subsequent categorization radically shifted based on which shape it replaced. For example, when an AB morph replaced the A shape in a sequence, it was more likely to be categorized as A. By contrast, participants in the control condition, who selected these stimuli without a fixed sequence order, showed no effect, demonstrating the critical role of sequence order and ruling out alternative explanations. These results demonstrate that, even using nonlinguistic stimuli, the position in which an ambiguous stimulus occurs in a sequence drastically impacts how it is subsequently categorized.