Manual exploratory procedures and asymmetries for a haptic search task: a comparison between capuchins (Cebus apella) and humans.

Left-hand preferences in monkeys (capuchins, macaques) for haptic discriminations have been taken to indicate that the right hemisphere is more involved than the left hemisphere in solving these tasks. We confirmed this left-hand bias in a group of 21 capuchins haptically searching for sunflower seeds located in crevices in 12 different objects (Lacreuse & Fragaszy, 1996). In an attempt to specify the relations of the left-hand bias to hand performance and hemispheric lateralisation, we analysed the manual exploratory procedures used by the subjects when exploring the objects, and compared them to those of humans confronted with the same task. All the hand-movement patterns displayed by humans were also observed in capuchins, but humans performed exhaustive explorations, whereas capuchins restricted their haptic investigations to limited portions of the objects. Both species adopted several ''exploratory procedures'' to investigate particular objects. Although capuchins showed a left-hand preference to perform the task, finer analyses of haptic exploratory procedures revealed no difference in the way the left and right hands explored the objects, nor in the efficiency with which each hand solved the task. Nor was any measurable difference in manual exploratory procedure and efficiency found in humans. The discussion emphasises the need to complement measures of manual preference with finer indices of hemispheric lateralisation, for a better understanding of functional asymmetries in primates.