Visual laterality patterns for pure- versus mixed-list presentation.

In Experiment 1, an overall left visual field advantage for nonverbal form recognition was found in a pure list of forms, but an overall right visual field form recognition advantage was found when the form trials were randomly intermixed with word recognition trials. Form complexity also influenced the form recognition laterality pattern, but the complexity effects were independent of (i.e., additive with) those produced by randomly mixing forms with words. Experiment 2 found that the mixed-list laterality pattern was unchanged by a pretrial cue indicating whether a word or form would follow. Experiments 3 and 4 demonstrated that holding two nouns in memory on each trial in a pure list of forms has much the same effect on laterality pattern as mixing forms with words but that the combined effect of these two variables is no larger than the effect of either variable alone. The entire pattern of results suggests that (a) laterality patterns are caused by the interaction of several factors, (b) the effects of random mixing and concurrent verbal memory are both caused by selective left-hemisphere activation, and (c) the form-complexity effects are caused by some other mechanism--perhaps subtle difference in stimulus codability.