Visual laterality for letter comparison: Effects of stimulus factors, response factors, and metacontrol

Abstract

Right-handed subjects indicated whether two highly discriminable uppercase letters were the same or different. Letter pairs were projected to the left visual field/right hemisphere (L VF IRH) or the right visual fieldlleft hemisphere (RVFILH), or the same letter pair was presented to both visual fields simultaneously (bilateral trials). Laterality effects were not influenced by moderate blurring of the letters. However, on RVFILH trials, reaction times were faster for same pairs than for different pairs. This effect was absent on LVFIRH trials, suggesting a qualitative difference in the mode of processing for the two unilateral trial types. The pattern of results on bilateral trials was identical to that obtained on RVFILH trials. This suggests that on bilateral trials, the subjects employed the mode of processing characteristic of RVFILH trials, perhaps indicating assertion of metacontrol by the left cerebral hemisphere.

Categorization versus distance: Hemispheric differences for processing spatial information

“…This was true in an experiment that examined RT to determine that two drawings of faces were different when they were either identicalor differed on only one feature (Hellige, Jonsson, & Michimata, 1988) and also in a physical-identity letter-comparison experiment (Hellige & Michimata, 1989). One interpretation of these results is that the left hemisphere exerted metacontrol over the qualitative nature of information processing when both hemispheres received the relevant stimuli.…”

“…To do this, we employed the logic developed by Hellige (1987) and utilized in several recent experiments (e.g., Hellige, Jonsson, & Michimata, 1988;Hellige& Michimata, 1989;Helligeet al, 1989). The logic grew out of an interest in the ways in which the two hemispheres might interact when both have access to the same stimulus input.…”

“…This response procedure was used because it allows each hemisphere to have access to both responses on each trial (see ). This may be particularly important when using bilateral trials to examine whether or not one hemisphere's preferred mode of processing dominates when both hemispheres have access to the same information (e.g., Hellige & Michimata, 1989). In the event that the two buttons were not pressed with perfect simultaneity, reaction time (RT) was defined by the time of the first press.…”

“…By comparing the pattern of results on bilateral trials with the pattern on LVF-RH and RVF-LH trials, it is possible to examine whether subjects are equally influenced by the LVF and RVF stimuli in a condition for which the same information is presented simultaneously to both fields. To the extent that the pattern of results on bilateral trials matches that of one unilateral visual field but not the other, CATEGORIZAnON VERSUS DISTANCE 771 we have some evidence about which hemisphere exerts control over the task under conditions that allow either hemisphere to do so (see Hellige, 1987;Hellige, Jonsson, & Michimata, 1988;Hellige & Michimata, 1989;Hellige, Taylor, & Eng, in press). …”

Semantic Representations of Word Meanings by the Cerebral Hemispheres

“…If frontier words have categorical representations that were not observed under central presentation conditions then these representations may reflect LH metacontrol. Metacontrol is when both hemispheres have access to the same information, and there is a neural mechanism that determines the mode of processing that is characteristic of either the LH or the RH (Hellige & Michimata, 1989aLevy & Trevarthen, 1976). If the LH assumes metacontrol, and because categorical targets of frontier words were not facilitated in Experiment 1, these representations must be weak because of the LH tendency to distinguish between strength of relation.…”