Positive feeling is derived not only from biologically significant causes but also from biologically non-significant causes (e.g., seeing preferred stimuli). To neurobiologically examine whether or not the preference for biologically non-significant neutral stimuli is related to positive emotion which arises when monkeys see preferred stimuli, we first determined whether monkeys exhibit preference for biologically non-significant fractal stimuli. We presented 2 fractal stimuli simultaneously and asked monkeys to select one of them by eye movements and look at the selected stimulus for up to 12 sec. Monkeys exhibited different percentages of correct performances to different stimuli. The rank orders of the stimuli estimated by the percentage of correct performance were maintained throughout many sessions. The rank orders of the stimuli were not related to the exposure effect or stimulus-reward association. These results indicate that monkeys exhibited preference for fractal stimuli and that the rank orders of the stimuli estimated by the percentage of correct performance reflect the strength of monkey's preference for the stimuli. This method allows us to determine monkey's preference for biologically non-significant neutral stimuli to perform neurobiological experiments for examining whether this preference is related to positive emotion.

Key words: Fractal art stimulus, preference, positive emotion