The lateralized Stroop: A meta-analysis and its implications for models of semantic processing

The prevailing theory in the literature concerning the lateralization of Stroop effects involves a speed of processing account (see MacLeod. 1991). Because the left hemisphere (LH) demonstrates an overall advantage relative to the right hemisphere (RH) on most verbal tasks, interference effects are hypothesized to be greater in the LH than in the RH. Inspection of the data, however, reveals that many studies find no difference in magnitude of Stroop interference between the hemispheres. Given findings within the lateralized semantic printing literature. this is not surprising. A meta-analysis on a subset of lateralized Stroop experiments Was conducted to determine whether or not the LH produces significantly more interference than the RH in this paradigm. Based on a number of exclusionary criteria, a total of 19 different studies were included, representing a potential 26 effect size estimates or differential interference. The effect size representing interference using congruent conditions as the baseline (d + = .06) reveals that there is no significant difference between the hemispheres in magnitude of the Stroop interference effect. The LH interference effect was d + = .73, which is significant. Likewise, the RH interference effect, d + = .67, was significant. In Summary, while there was no significant difference between the hemispheres, each hemisphere, when examined independently, did exhibit significant within hemisphere interference effects. These findings are presented in light of the lateralized semantic priming literature. (C) 2002 Elsevier Science (USA). All rights reserved.