CICASP | Center for International Collaboration and Advanced Studies in Primatology

A new study published in the journal PLoS ONE has important implications for how humans develop space-based representations of “orders” through a simple but well-controlled experiment on our evolutionary closest relatives, chimpanzees.

The Natural Order

In the last couple of decades, there has been a growing number of reports on space-based representation of numbers and serial order in humans. In the present study, to explore evolutionary origins of such representations, Dr. Ikuma Adachi of CICASP examined whether our closest evolutionary relatives, chimpanzees, map an acquired sequence onto space in a similar way as do humans. The subjects had been trained to perform a number sequence task in which they touched a sequence of “small” to “large” Arabic numerals presented in random locations on the monitor. This task was presented in sessions that also included test trials consisting of only two numerals (1 and 9) horizontally arranged. On half of the trials 1 was located to the left of 9, whereas on the other half 1 was to the right to 9. The Chimpanzees’ performance was systematically influenced by the spatial arrangement of the stimuli; specifically, they responded quicker when 1 was on the left and 9 on the right compared to the other way around. This result suggests that chimpanzees, like humans, spontaneously map a learned sequence onto space.

Access the full article at PLoS ONE.