Laterality for Visual-Spatial Processing in Two Language-Trained Chimpanzees (Pan troglodytes)

Most reviews of laterality in nonhuman primates indicate that hemispheric asymmetries, similar to those found in humans, are not evident. With the growing evidence for cognitive processes germane to language in apes, in addition to their phylogenetic similarity to humans, they appear to be useful candidates for studies of laterality. Laterality for visual-spatial processing in two language-trained chimpanzees (Pan troglodytes) was investigated with a visual half-field paradigm. Initially, the chimpanzees were taught to manipulate a joystick that controlled the movement of a cursor on a computer monitor to a central fixation point. Subjects were then taught a visual discrimination based on the location of a short line contained within a geometric form. Testing consisted of systematic presentation rates of 15,122, and 226 ms to the left and right visual fields. For half of the trials, subjects used their left hand to respond and, alternately, used the right hand for the remaining trials. Accuracy and reaction time were the dependent measures. One subject demonstrated significantly faster reaction times to stimuli presented to the left visual field (LVF) regardless of which hand was used to respond. The other subject demonstrated a significant LVF bias when using the left hand to respond. The results are discussed in the context of current theories on the evolution of laterality and its relation to linguistic functioning.