The effect of task set on numerosity judgement

When deciding which line has more free slots in a parking lot or which box of berries contains more berries in a fruit shop, we don’t count directly but rely on an approximate evaluation of quantities. This ability is usually called number approximation ability or numerical acuity (NA). The theoretical basis of this ability is Weber’s law, which states that the smallest noticeable difference between two stimuli is proportional to the magnitude of the stimuli. To evaluate NA, the Weber fraction is used—an index that describes the relationship between the magnitude of a physical stimulus and the perception of change. Traditionally, to find the Weber fraction, a dot-comparison task is used. However, in the literature on the topic, the same standard experimental design has always been used: participants were instructed to give an ipsilateral response to the larger array of dots. No studies have attempted to counterbalance responses or instruct participants to react to the smaller array of dots. In this study, we changed the task instructions for one of two experimental groups, asking them to select the array with a smaller number of dots. This resulted in low, almost chance-level accuracy performance across all eight ratios of numerosities presented within the group with the atypical task instructions. This outcome could be due to the inhibitory control exhibited when selecting the array with a smaller number of dots.