Visual laterality and the acuity gradient: potential artifacts and control procedures

As visual stimuli are moved away from the fixation point toward either side, perceptibility falls off rapidly. This acuity gradient can influence visual half-field differences if the two sides (or ends) of a stimulus do not provide equal amounts of task-relevant information. In general, performance should be better in the visual field with the more informative side of the stimulus closer to the fixation point. Kirsner and Schwartz (1986, Brain and Cognition, 5, 354-361) discuss implications for experiments using words. The present article considers the assumptions involved in an explanation of visual half-field differences in terms of the visual acuity gradient and offers guidelines for the design of experiments using letters, faces (and other nonverbal stimuli), and words. Special attention is given to the interpretation of Visual Field X Task Variable interactions and to the use of words with letters arranged vertically. It is concluded that, with sufficient care in their design and interpretation, visual laterality experiments are an important converging operation for the study of cerebral hemisphere asymmetry.