What is in a reach? Domain-general spatial modulation of motor responses by number representations

Abstract

Gaze, pointing, and reaching movements are thought to provide a window to internal cognitive states. In the case of numerical cognition, it has been found that the left-right deviation of a reaching movement is modulated by the relative magnitude of values in a number comparison task. Some have argued that these patterns directly reflect the representation of a logarithmically compressed mental number line (direct mapping view). However, other studies suggest that the modulation of motor outputs by numerical value could be a more general decision-making phenomenon (response competition view). Here we test the generality of interactions between the motor system and numerical processing by comparing subjects' reach trajectories during two different nonverbal tasks: numerosity comparison and facial expression comparison. We found that reaching patterns were practically identical in both tasks -reach trajectories were equally sensitive to stimulus similarity in the numerical and face comparisons. The data provide strong support for the response competition view that motor outputs are modulated by domain-general decision processes, and reflect generic decision confidence or accumulation of evidence related to mental comparison. When asked to point to the larger of two numbers, one presented to the left and another to the right, subjects trajectories take a medial route when the numerical distance is close, and a more direct one when it is far (Santens, Goossens, & Verguts, 2011;Song & Nakayama, 2008). Such results imply that motor computations are penetrated by cognitive processing -the task of comparing numbers interacts with the path of the motor response. Modulations of motor responses by cognitive processes are not exclusive to number but are observed across a variety of domains, including intertemporal discounting, random dot motion, and phonology (Chapman et al., 2010a;Dshemuchadse, Scherbaum, & Goschke, 2013;Freeman & Ambady, 2011;Freeman, Ambady, Rule, & Johnson, 2008;Friedman, Brown, & Finkbeiner, 2013;Gallivan et al., 2011;Koop & Johnson, 2013;McKinstry, Dale, & Spivey, 2008;Spivey, Grosjean, & Knoblich, 2005;van der Wel, Sebanz, & Knoblich, 2014). jnc.psychopen.eu | 2363-8761 Although prior research suggests that reach trajectories are affected by cognitive processes across a variety of domains, a strong test of the domain-generality of these effects would be to compare performance with stimuli from different domains within a common task. Such a test is important for determining whether number-based modulations of motor responses are domain-specific (Chapman et al., 2014;Dotan & Dehaene, 2013;Song & Nakayama, 2008) or domain-general (Alonso-Diaz, Cantlon, & Piantadosi, 2015Santens et al., 2011). Surprisingly, to date, no studies have pitted domains against each other in equivalent reaching tasks to assess the effect of domain on dynamic motor behavior. In the literature on number processing, interactions between stimulus values and motor responses have only been tested with numerical stimuli alo...

A threshold-free model of numerosity comparisons

“…There is a considerable amount of work exploring number representations and their effects on movement plans [ 17 , 54 – 56 ]. The novelty of our work is a formalization of how psychophysical representations of number may be projected to reach.…”

“…index finger, is moved in physical space [ 25 , 54 , 57 ]. In our model the displacement of the index finger is not forced to follow precise aspects of the number representation; it is the state of the inference and confidence that is being reflected along the reaching trajectory (see a similar discussion in [ 56 ]). In a broader sense, movement may only be a window to a domain-general process, such as confidence, rather than domain-specific aspects of cognition (log.…”