Visual letter matching: hemispheric functioning or scanning biases?

Finding two mixed-case letters that share the same name is easier to do when the letters are presented in opposite visual fields than when they are both in the same field. By contrast, finding a match between two same-case letters is easier when they are in the same field. These visual field effects have been attributed to the ability of the corpus callosum to coordinate the work of the cerebral hemispheres [Banich, M. T. (1998). The missing link: The role of interhemispheric interaction in attentional processing. Brain and Cognition, 36, 128-157; Weissman, D. H., & Banich, M. T. (2000). The cerebral hemispheres cooperate to perform complex but not simple tasks. Neuropsychology, 14, 41-59]. The present study considers the alternative hypothesis that attentional scanning biases may be at work. Experiment 1 examined the effects of explicit instructions to scan items in a specific order; Experiment 2 examined influences of implicit location biasing; Experiment 3 considered the possibility that same-case letter matching is different because a perceptual grouping mechanism can be used in that task. In each experiment, we first interpreted the results within the hemispheric framework before considering the alternative accounts. We concluded that two scanning biases may be in effect: (1) an automatic bias favoring items in locations relatively distant from the current focus of attention and (2) a learned bias to scan letters in a left-to-right direction.