A non-human primate's understanding of solidity: dissociations between seeing and acting

Studies often reveal a dissociation between what infants know as revealed by action and what they know as revealed by perception. We explored whether non-human primates exhibit a similar dissociation, focusing on what rhesus macaques know about solidity. In a series of search experiments, Hauser (2001) found that rhesus do not possess a complete understanding of solidity, searching below a solid sheaf for an invisibly displaced object. In the present experiments, we explored how rhesus would perform in expectancy violation versions of the same tasks. Subjects looked longer when an apple appeared to fall through a solid shelf and when it appeared to roll through a solid barrier. These results suggest that macaques have some understanding of solidity when tested using looking paradigms even though they do not appear to use this knowledge when searching for food. We speculate that this dissociation is similar to that demonstrated in human development.