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 (LVF/RH) or the right visual field/left hemisphere (RVF/LH), 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 RVF/LH trials, reaction times were faster for same pairs than for different pairs. This effect was absent on LVF/RH 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 RVF/LH trials. This suggests that on bilateral trials, the subjects employed the mode of processing characteristic of RVF/LH trials, perhaps indicating assertion of metacontrol by the left cerebral hemisphere.
Hellige, J.B., Michimata, C. Visual laterality for letter comparison: Effects of stimulus factors, response factors, and metacontrol. Bull. Psychon. Soc. 27, 441–444 (1989).