Abstract
In the past decade hand kinematics has been reliably adopted for investigating cognitive processes and disentangling debated topics. One of the most controversial issues in numerical cognition literature regards the origin – cultural vs. genetically driven – of the mental number line (MNL), oriented from left (small numbers) to right (large numbers). To date, the majority of studies have investigated this effect by means of response times, whereas studies considering more culturally unbiased measures such as kinematic parameters are rare. Here, we present a new paradigm that combines a “free response” task with the kinematic analysis of movement. Participants were seated in front of two little soccer goals placed on a table, one on the left and one on the right side. They were presented with left- or right-directed arrows and they were instructed to kick a small ball with their right index toward the goal indicated by the arrow. In a few test trials participants were presented also with a small (2) or a large (8) number, and they were allowed to choose the kicking direction. Participants performed more left responses with the small number and more right responses with the large number. The whole kicking movement was segmented in two temporal phases in order to make a hand kinematics’ fine-grained analysis. The Kick Preparation and Kick Finalization phases were selected on the basis of peak trajectory deviation from the virtual midline between the two goals. Results show an effect of both small and large numbers on action execution timing. Participants were faster to finalize the action when responding to small numbers toward the left and to large number toward the right. Here, we provide the first experimental demonstration which highlights how numerical processing affects action execution in a new and not-overlearned context. The employment of this innovative and unbiased paradigm will permit to disentangle the role of nature and culture in shaping the direction of MNL and the role of finger in the acquisition of numerical skills. Last but not least, similar paradigms will allow to determine how cognition can influence action execution.
Highlights
Humans usually represent numbers on a mental number line (MNL), oriented from left-to-right
% Time of Maximum Left Deviation (%TMLD): the percentage of time at which the index trajectory was at maximum distance toward the left from the midline (%);
As concerns right kicks in response to large number (S8), the experimental manipulation did not affect Movement Time (MT) compared to the baseline (p = 0.232), suggesting that either anatomical constraints limited the degrees of freedom during action execution, or that compensative strategies were adopted in order to maintain a constant movement duration
Summary
Humans usually represent numbers on a mental number line (MNL), oriented from left-to-right. The “free response” task used by Daar and Pratt (2008) differs from the “forced-choice” tasks previously and largely used to study spatial-numerical association In these kind of tasks, participants were forced to emit a lateral response, usually to press a left- or a right-side key. A growing number of studies are using motion capture and detailed kinematic analyses to parameterize behavior and to deeply examine questions relating to cognitive processing in naturalistic protocols (for reviews, see Castiello, 2005; Krishnan-Barman et al, 2017) From this fascinating perspective, an essential improvement of the actual knowledge would be obtained by combining a “free response” task with a kinematic analysis of movement, which may allow to understand how the responses are executed (Rugani and Sartori, 2016). Given that cognitive representations of perceptual and semantic information cannot be fully understood without considering their impact on actions (Gallese and Lakoff, 2005), the existing knowledge on MNL will be advantaged by studies that analyze the motor action while responding to a number
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