Mental rotation in a California sea lion (Zalophus californianus).

Mental rotation is a widely accepted concept that suggests an analogue mode of visual information-processing in certain visuospatial tasks. Typically, these tasks demand the discrimination between the image and mirror-image of rotated figures, for which human subjects need an increasing reaction time depending on the angular disparity between the rotated figures. In pigeons, tests of this kind yielded a time-independent rotational invariance, suggested as being the result of a non-analogue information-processing that has evolved in response to the horizontal plane that birds perceive from above while flying. Given that marine mammals use the water surface as the horizontal plane for orientation while diving, the ability of a California sea lion to mentally rotate two-dimensional shapes was tested. Using a successive two-alternative matching-to-sample procedure, the animal had to decide between the image and mirror-image of a previously shown sample. Both stimuli were rotated by a multiple of 30 degrees with respect to the sample. The animal's reaction time was measured by a computer-controlled touch-screen device, rewarding the animal for pressing its snout against the stimulus matching the sample. A linear regression analysis of the animal's mean reaction time against the angular rotation of the stimulus yielded a significant correlation coefficient. Thus, the present data can be explained by the mental rotation model, predicting an image-like representation of visual stimuli in this species. The present results therefore correspond well with those found for human subjects, but are inconsistent with the data reported for pigeons.