Hemispheric specialization of emotion processing is explained by two theories. The right hemisphere theory indicates that all emotions are processed by the right hemisphere. The valence theory states that negative and positive emotions are respectively processed by the right and the left hemisphere. Our study examined eye use preference in nine de Brazza’s monkeys (Cercopithecus neglectus) when they need to look at positive and negative visual stimuli through a hole. The use of one eye indicates that information is processed by the contralateral hemisphere. The emotional value of stimuli appeared to alter visual laterality. Subjects with a left eye bias showed a higher degree of right hemisphere’s lateralization for negative stimuli. Some individuals even switched to the left hemisphere for positive stimuli. Subjects with a right eye bias showed a higher degree of left hemisphere’s lateralization for positive stimuli. Our results seem consistent with the valence theory.