Early versus late stages of processing and their relation to functional hemispheric asymmetries in face recognition.

To determine whether perceptual asymmetries for faces occur at early or late stages of stimulus analysis, subjects compared the members of a pair of faces which appeared in the right or left visual field, either to each other or to a previously presented sample. The first two experiments established that manual reaction times were consistently shorter to left-field presentation only when test faces were compared for identity to a memorized sample (Experiment 2), but not when they were compared to each other (Experiment 1). By varying the interstimulus interval between the sample and test (Experiment 3), it was found that a consistent left-field superiority was absent for at least the first 50 msec following sample offset, suggesting that during that time both hemispheres have access to a short-lived, visual trace that represents the results of the early, precategorical processing stages. Beyond 100 msec, or earlier if the trace is degraded by a masking field (Experiment 4), a left-field advantage is observed. Experiment 5 showed that higher order comparisons between photographs and caricatures yield a left-field superiority even in a memory-free situation. Together, the results indicate only higher order processes, such as are needed to maintain a relatively permanent memory representation or to compare caricatures with photographs, require the specialized functions peculiar to the right hemisphere. Lower-order, precategorical processes seem to be common to both hemispheres.