Abstract
Different hypotheses have been put forward to explain the interaction between size perception and spatial position. To explore the evolutionary roots of these phenomena, we tested long-tailed macaques' performance in a two-choice discrimination task on a touchscreen and contrasted two hypotheses. First, a hierarchy association in which large objects are associated with top positions, due to a link between power, dominance and importance with top position. Second, a naive Aristotelian association in which large objects are associated with bottom positions, due to the experience that larger objects are heavier and thus more likely to be found at the bottom. Irrespective of training regime (positively reinforcing the small (Touch-Small) or large (Touch-Large) stimulus), the monkeys had a bias to touch the bottom compared to the top location. Individuals in the Touch-Small group took significantly longer to acquire the task, but subsequently made fewer mistakes. When presented with two stimuli of equal medium size, the Touch-Large group had a clear bias to touch the lower stimulus, while the Touch-Small group touched both locations at equal rates. Our findings point to an innate bias towards larger stimuli and a natural preference for the lower position, while the extent of interaction between size and position depends on executive control requirements of a task.
Highlights
Different hypotheses have been put forward to explain the interaction between size perception and spatial position
People who were required to perform a parity judgement task on numbers presented at different locations on a PC screen categorized higher numbers faster when they were presented at the top compared to the bottom of the screen, while lower numbers were categorized faster when they were presented at the bottom compared to the top of the screen [1]. This effect is known as spatial-numerical association of response codes and is one type of spatial compatibility effect. These findings are not explained by skills or processes that depend on formal education, experience or abstract thinking: cultural habits, such as writing direction, do contribute to the horizontal SNARC effect [3,4], a number of studies have shown spontaneous mapping of numbers or quantities to horizontal space in preverbal human infants [5,6,7,8] and some animal species
While we are not aware of a similar experiment with long-tailed macaques (Macaca fascicularis), the species studied in this paper, observations of locomotive behaviour of long-tailed macaques in the wild indicate that males—who are higher ranking than females—usually occupy higher vertical positions in trees unless they are following females during the mating season, in which case they can be found at lower heights
Summary
Different hypotheses have been put forward to explain the interaction between size perception and spatial position. This effect is known as spatial-numerical association of response codes (the so-called vertical ‘SNARC effect’) and is one type of spatial compatibility effect (for an overview, see [2]) These findings are not explained by skills or processes that depend on formal education, experience or abstract thinking: cultural habits, such as writing direction, do contribute to the horizontal SNARC effect [3,4], a number of studies have shown spontaneous mapping of numbers or quantities to horizontal space in preverbal human infants [5,6,7,8] and some animal species (e.g. rhesus macaques [9] and pigeons [10]). Participants made fewer errors when the stimulus with the higher value appeared at the top and the stimulus with the lower value at the bottom; they made more errors when the stimulus with the higher value appeared at the bottom and the stimulus with the lower value at the top
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