Magnitude shifts spatial attention from left to right in young rhesus monkeys

Humans represent numbers and quantities as increasingly orientated from left to right. Traditionally, these associations were considered cultural by-products. Recent comparative and developmental evidence challenged this idea. Newborns and animals have shown spatial-numerical association, but evidence on spatial-quantity association and the link between discrete and continuous magnitudes is sparce. Here, we explore whether the magnitude could shift the spatial bias from left to right in young rhesus macaques (Macaca mulatta). We designed a touch-screen task which required to remember and select a dot on which a target stimulus appeared. At test, monkeys faced arrays of 2, 4, 6 or 10 dots and maintained high performance through changes in arrays’ location, spacing and length. Remarkably, facing 2-dot arrays monkeys remembered better the left target; facing 6- or 10-dot arrays they remembered better the right targets. In a continuous transfer test, in which the array of dots was replaced with a long bar, they still remember better the right than left locations. Our results demonstrate that rhesus macaques transfer a learned rule from discrete to continuous magnitudes, proving that in animals the association with space is not limited to numbers but comprises other magnitudes, like in humans.