Do macaques count from left to right? Number-Space Association and the role of emotions in macaques

The mental number line (MNL) suggests that humans spatially represent smaller numbers on the left and larger numbers on the right. Although cultural factors, such as reading direction, may influence this orientation, evidence from infants and non-human animals suggests an underlying biological basis. One explanation, known as the motivational hypothesis, posits that this spatial bias may arise from a shift from smaller to larger quantities in appetitive contexts, where larger quantities activate the left hemisphere (linked to approach processes and positive emotions), favoring the right hemispace. Conversely, a shift from larger to smaller quantities activates the right hemisphere, favoring the left hemispace. In aversive contexts, however, this orientation may diminish or even reverse. To investigate the MNL and the motivational hypothesis in nonhuman primates, we conducted five experiments on macaques. The first two experiments tested for a left-to-right MNL by presenting pairs of identical food items on the left and right sides, revealing a left-side preference for smaller quantities and a right-side preference for larger ones. The remaining experiments assessed whether aversive stimuli could affect the orientation of the MNL, using spontaneous tasks involving both negative and neutral stimuli. Our findings did not support the hypothesis that aversive stimuli can modify the MNL in macaques.