Left or right, that is the question: use of egocentric frame of reference and the right-eye advantage for understanding gestural signs in bottlenose dolphins (Tursiops truncatus).

How do bottlenose dolphins visually perceive the space around them? In particular, what cues do they use as a frame of reference for left-right perception? To address this question, we examined the dolphin's responses to various manipulations of the spatial relationship between the dolphin and the trainer by using gestural signs for actions given by the trainer, which have different meanings in the left and right hands. When the dolphins were tested with their backs to the trainer (Experiment 1) or in an inverted position underwater (Experiments 2 and 3), correct responses from the trainer's perspective were maintained for signs related to movement direction instructions. In contrast, reversed responses were frequently observed for signs that required different sounds for the left and right hands. When the movement direction instructions were presented with symmetrical graphic signs such as " × " and "●", accuracy decreased in the inverted posture (Experiment 3). Furthermore, when the signs for sounds were presented from either the left or right side of the dolphin's body, performance was better when the side of the sign movement coincided with the body side on which it was presented than when it was mismatched (Experiment 4). In the final experiment, when one eye was covered with an eyecup, the results showed that, as in the case of body-side presentation, performance was better when the open eye coincided with the side on which the sign movement was presented. These results indicate that dolphins used the egocentric frame for visuospatial cognition. In addition, they showed better performances when the gestural signs were presented to the right eye, suggesting the possibility of a left-hemispheric advantage in the dolphin's visuospatial cognition.