Functionally lateralised information processing and its allocation to response: Inferences from three measures of letter identification performance following lateralised visual input

Identification errors were analysed for single letters presented in each visual hemi-field. The kinds of errors occurring reflected both how the letters were made difficult to see, and the informational relationships among the possible stimulus letters. The particular pattern of errors was different in the two hemifields, with the direction of that difference suggesting the letters were better differentiated following left half-brain input. But this left-sided superiority in letter differentiation was only sometimes reflected in faster or more accurate responses overall. Instead, the extent of left-sided performance superiority in RT and accuracy depended in a complex way on the degree of difficulty of the task. This influence of task difficulty on the RT and accuracy measures of functional laterality was considered to reflect a greater or lesser allocation to response of the lateralised letter-differentiation process revealed by the errors analysis.