Shape and color naming are inherently asymmetrical: Evidence from practice-based interference

- Asymmetry in Stroop interference is thought to arise from differential practice.
- Participants practiced shape and color naming in counterbalanced schedules.
- Strong asymmetries in interference arose, color much more susceptible than shape.
- No reading was involved, precluding word form mapping causing the asymmetry.
- Verbal labels are directly accessible by shapes but only indirectly by colors.

Stroop interference is characterized by strong asymmetry between word and color naming such that the former is faster and interferes with the latter but not vice versa. This asymmetry is attributed to differential experience with naming in the two dimensions, i.e., words and colors. Here we show that training on visual-verbal paired associate tasks equivalent to color and shape naming, not involving word reading, leads to strongly asymmetric interference patterns. In two experiments adults practiced naming colors and shapes, one dimension more extensively (10 days) than the other (2 days), depending on group assignment. One experiment used novel shapes (ideograms) and the other familiar geometric shapes, associated with nonsense syllables. In a third experiment participants practiced naming either colors or shapes using cross-category shape and color names, respectively, for 12 days. Across experiments, despite equal training of the two groups in naming the two different dimensions, color naming was strongly affected by shape even after extensive practice, whereas shape naming was resistant to interference. To reconcile these findings with theoretical accounts of interference, reading may be conceptualized as involving visual-verbal associations akin to shape naming. An inherent or early-developing advantage for naming shapes may provide an evolutionary substrate for the invention and development of reading.