Mental rotation and rotational invariance in the Rhesus monkey (Macaca mulatta)

The mode of visual information processing during visuospatial tasks differs across species and is supposed to depend on evolutionary and ecological factors. Humans show reaction times that increase with angular disparity when tested in mental rotation tasks. Pigeons show a time-independent rotational invariance that possibly evolved in response to the horizontal reference plane birds perceive while flying. As it was suggested that hominids may have secondarily lost the ability of rotational invariance while retreating from arboreal living and evolving upright gait where the vertical reference plane is more important, mental rotation tests with various recent primate species promise to model the evolution of the respective modes of information processing. The results of recent corresponding experiments with a mainly arboreal living primate species could not be explained by either mode, thus supporting the idea of information processing systems having gradually evolved. Here, we conducted mental rotation experiments with three Rhesus monkeys, a more terrestrial living primate species. In a two-alternative matching-to-sample procedure, we measured the monkeys' reaction times for decisions between rotated figures representing image and mirror-image of a previously shown upright sample. The results of our three monkeys were inconsistent. Linear regression analyses showed for one test animal significant correlation coefficients for mean reaction times depending on angular disparity and thus clearly indicated mental rotation. The other two test animals showed reaction times not consistent with mental rotation, whereas rotational invariance might explain the responses to the smaller angles of rotation. Our results suggest that two separately evolved information processing systems may be coexisting to a certain extent in species with correspondingly overlapping ecological demands.