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Representing invisible displacements: Comparative experiments of human children and nonhuman primates.

Linda Anderson, Laurie Santos, Marc Hauser & Bruce Hood

Dept. Psychology, Harvard University, Cambridge, MA, 02138

By 2 years, most children can store a representation of a moving object in mind when it out of sight. However, recent work indicates that up until 2.5 years of age, children fail to find the location of a falling object that has been guided along a concealed trajectory such as an opaque tube. More specifcally, when a ball is dropped down an S-shaped opaque tube connected to a terminal box, children repeatedly search for the ball in a location that is directly beneath the poin of release (i.e., a box that is not connected to a tube), even though the ball has travelled to a different position (i.e., the box connected to the tube). This search error and perseveration suggests a naive theory of falling objects. In essence, such children appear to make a &

GLUX. In the following presentation, we ask whether this naive theory is a domain of knowledge

that is specifically human by testing a closely related species: a small New World monkey species, the cotton-top tamarin. We use the same procedure as described above for human children, thereby providing directly comparable data.

Results indicate that in the one-tube experiment described above, 8 out of 10 subjects showed a gravity error on the first trial. On subsequent trials, most of our subjects perseverated with the same response, even though such errors meant that they failed to obtain a food reward, something that they were highly motivated to obtain. Eventually, many subjects switched from their preferred choice, moving systematically from the gravity-error-box to a middle-box and lastly, to the correct-box containing the reward. Once subjects consistently selected the door containing food, we ran a transfer trial, switching the position of the tube (new starting and endins location). Only two of our subjects successfully passed this transfer test; most of the remaining subjects selected the previously rewarded box. For those subjects passing the single-tube test, we moved them onto a more complicated two-tube test; here only one tube served as a carrier, the other tube was simply a distractor. Only one of these subjects showed high success on this task, the other subject providing only weak evidence of successful tracking. For those subjects who failed on the single-tube task, we substituted a clear tube for the opaque. Now, most of the subjects successfully located the food. However, when they returned to the opaque tube condition, they failed once again. Thus, subjects were not able to transfer information garnered from the clear tube condition to the opaque condition. In summary, the capacity to represent invisible displacements seems to be in place in this species, as evidenced by the few subjects who passed the single-tube test. More interesting is the presence of a naive theory in a species that is highly motivated to find food that has fallen.