Mental Rotation of Perspective Stimuli in a California Sea Lion (Zalophus californianus)

Abstract

The time it takes humans to discriminate rotated objects from their mirror images increases linearly with the rotation angle. This phenomenon is probably due to an analogue mode of visual information processing during which an object’s mental representation is rotated in a time-consuming process called mental rotation. As the speed of mental rotation in humans depends on rotation axis, we tested the ability of a California sea lion to mentally rotate perspective line drawings of three-dimensional objects about four axes. In a matching-to-sample experiment the animal was presented with the image and a mirror image of a block sample that had previously been shown upright. Both image and mirror image were rotated by a multiple of 60° about the object’s x-, y-, z-axis, or a skew axis (an axis oblique to these standard orthogonal axes). The animal’s choice and reaction times were recorded using a computer-controlled touch-screen device. Mean reaction times and errors generally increased with angular disparity supporting the model of mental rotation for three-dimensional objects. Linear regression analysis of mean reaction times yielded high correlation coefficients only for three axes. The slope of reaction time functions indicated the highest mental rotation speed for the skew axis. This contrasts with the priority of mental rotation axes in humans suggesting that due to special ecological demands a different mode of orientation invariance evolved in marine mammals.