Visual field effects in letter matching : an exploration of hemispheric, attentional, and strategic biases

It is easier to determine the identity of a pair of mixed-case letters when they appear on opposite sides of fixation (across-display) than when presented on the same side (within-display). This across-display advantage is taken as evidence by some that the hemispheres can each process letter identity independently (computational complexity theory, Banich, 1998). One unexplored feature of the across-display advantage is that it typically occurs in conjunction with a left-field advantage. If a within- display advantage is obtained instead, it typically occurs in conjunction with a right-field advantage. One reason for studying this relationship further is because both field (left vs. right) and display (across vs. within) advantages may index hemispheric differences in attentional orienting. According to the asymmetrical orienting hypothesis, the left hemisphere orients to right space, generating both right-field and within-display advantages for some tasks. The right hemisphere orients most efficiently to left space, generating a weak left-field advantage, but it can also orient to right space, contributing to an across-display advantage in other tasks. These interpretations were tested in a total of 8 experiments, involving 242 participants. Experiments 1 &2 confirmed the correlation between left-field and across-display advantages in mixed-case letter rmtching. Experiment 3 revealed similar left-field and across-display advantages in same-case letter matching when a task irrelevant dimension (color) was added. Experiments 4-6 tested matching tasks and response modes favoring the left hemisphere (localization, color reporting, rmtching on rhyme) in an effort to dissociate the field and display effects. This was unsuccessful. Finally, in Experiments 7 and 8, participants were instructed to mentally compare letters in a strict order. This lead to a mimicking of the standard data pattern, or its elimination, depending on which instructions were followed. These findings do not support either theory because strategic effects should not affect parallel processing or hemispheric differences in attentional orienting. The implications are discussed with regard to hemispheric interactions, attentional orienting, and the more general problem of estabfohing priority in visual processing. A qualitative model is presented to account for the data, involving both top-down strategic biases (left-to-right comparison) and bottom-up processing biases (opposite-direction orienting).Medicine, Faculty ofGraduat