The effects of procedural variations on lateralized Stroop effects

Several issues in the classic Stroop effect remain open, including (i) the stage of processing which gives rise to the effect, (ii) the effect of some procedural manipulations, (iii) the effect of hemispheric specialization and of interhemispheric interactions, and (iv) the existence of individual differences. In this paper, we investigate these issues using a series of experiments with central, lateral, and bilateral presentations of the Stroop stimuli. A total of 146 right-handed subjects took part in a multiexperiment study with relatively equal numbers of the two sexes participating in each experiment. We found that manual responses diluted but did not abolish the Stroop interference relative to vocal responses, arguing for a late processing stage account of the effect. However, separating the color patch from the color word either unilaterally or bilaterally did not significantly change the magnitude of the Stroop interference, arguing against a cost of interhemispheric transfer in the Stroop effect in normal subjects. We did, however, find evidence for hemispheric specialization in Stroop interference, greater in the left hemisphere than in the right hemisphere. Color words in the RVF produced the greatest Stroop effects regardless of the location of the color patch. In most cases, this laterality effect interacted with the sex of the subject, such that only males and a subgroup of females (i.e., those tested during a low estrogen phase in the menstrual cycle) showed evidence for greater Stroop effects when a color word projected to the left hemisphere than when it projected to the right hemisphere, regardless of the placement of the color patch.