Selective interference of grasp and space representations with number magnitude and serial order processing

van Dijck, Jean-Philippe Fias, Wim Andres, Michael [UCL]

It has been proposed that the metrics of space, time and other magnitudes relevant for action are coupled through a generalized magnitude system that also contribute to number representation. Several studies capitalized on stimulus- response compatibility effects to show that numbers map onto left-right representations and grasp representations as a func- tion of their magnitude. However, the tasks typically used do not allow disentangling magnitude from serial order process- ing. Here, we devised a working memory (WM) task where participants had to remember random sequences of numbers and perform a precision/whole-hand grip (Experiment 1) or a uni-manual left/right button press (Experiment 2) in response to numbers presented during the retention interval. This task does allow differentiating the interference of number magni- tude and serial order with each set of responses. Experiment 1 showed that precision grips were initiated faster than whole- hand grips in response to small numbers, irrespective of their serial position in WM. In contrast, Experiment 2 revealed an advantage of right over left button presses as serial position increased, without any influence of number magnitude. These findings demonstrate that grasping and left-right movements overlap with distinct dimensions of number processing. These findings are discussed in the light of different theories explaining the interactions between numbers, space and action.