Visual field differences in sequential letter classification tasks.

A review of the literature on visual field asymmetries indicated that although the constructs of strategy, processing, and attention had been invoked to account for results there was little objective evidence to support these views. The Posner letter classification tasks provide a methodology which enables such constructs to be tested empirically, and accordingly were employed in this research. Sequential double letter classifications were used because they provide a stable visual match advantage (Kroll, 1975), permitting an evaluation of retention interval effects without the complications of code change. Despite such stability on cognitive dependent variables, visual field effects differed between 9 sec (Experiment I) and those of less than one second (Experiment III). A change in coding bias was induced by the use of irregular time structure (Experiments IV, V, VII) although overall visual field differences were comparable to those obtained when coding was stable (Experiments III and VI). Four of the relevant studies indicated a right field advantage for cross-case (name) matches, and non asymmetric identity or visual match judgements. Single letter stimuli showed a left field advantage for identity matches, and a right field effect for cross-case classifications. The overall pattern of results indicated that visual field differences arose from the time of test stimulus presentation onwards. These findings were incompatible with models of visual field differences which have been advanced hitherto. An integration of strategy, attention and processing hypotheses was advanced and suggestions made for further research.