Mental Rotation and Rotational Invariance in the Rhesus Monkey (Macaca
mulatta)
C. Köhler, K.-P. Hoffmann, G. Dehnhardt & B. Mauck
Brain Behav. Evol. 66:158-166, 2005
- 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.
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