Motor‐evoked potentials reveal a motor‐cortical readout of evidence accumulation for sensorimotor decisions

Abstract

This is the accepted version of the paper.This version of the publication may differ from the final published version. Permanent repository link AbstractMany everyday activities require time-pressured sensorimotor decision making.Traditionally, perception, decision and action processes were considered to occur in series, but this idea has been successfully challenged, particularly by neurophysiological work in animals. However, the generality of parallel processing requires further elucidation. Here, we investigate whether the accumulation of a decision can be observed intrahemispherically within human motor cortex. Participants categorised faces as male or female, with task difficulty manipulated using morphed stimuli. Transcranial magnetic stimulation, applied during the reaction-time interval, produced motor-evoked potentials (MEPs) in two hand muscles that were the major contributors when generating the required pinch/grip movements. Smoothing MEPs using a Gaussian kernel allowed us to recover a continuous time-varying MEP average, comparable to an EEG component, permitting precise localisation of the time at which the motor plan for the responding muscle became dominant.We demonstrate decision-related activity in the motor cortex during this perceptual discrimination task, suggesting ongoing evidence accumulation within the motor system even for two independent actions represented within one hemisphere.

Neurodynamic Evidence Supports a Forced-Excursion Model of Decision-Making under Speed/Accuracy Instructions

“…Accordingly, we found that faster and more accurate responses in easy trials were explained by higher accumulation rates in both experiments. These patterns were observed in both neural signals and their simulated accumulation profiles and, consistently with previous studies (O’Connell et al, 2012; Hadar et al, 2016), support the role of MEP and CPP signals as neural correlates of the decision variable, with corticospinal excitability likely receiving a time-lagged but continuous input from CPP/decision-generating regions.…”

“…This is particularly the case for neural signals which inherently have a low signal-to-noise ratio, such as ERPs and in particular the MEP signal. Somewhat limited signal quality is however typical for experiments of this nature (O’Connell et al, 2012; Hadar et al, 2016), and we used large numbers of trials in both experiments, producing demonstrably interpretable neural signals. We would argue that the correspondence between model predictions and neural data, both here and elsewhere, is remarkable, given a class of models originally conceived to have a largely behavioral scope (Luce, 1986).…”

“…The width of the Gaussian kernel defined by the full width half maximum was set at 5% of median RT (i.e., around 20 ms), previously suggested as an appropriate compromise between temporal resolution and signal-to-noise ratio (Hadar et al, 2016). This MEP signal was computed for both stimulus and response-locked MEP latencies, and from the responding muscle, the nonresponding muscle and the MEP amplitude difference between them (Fig.…”

“…Signals displaying the accumulation predicted by these models have been identified in electrophysiological data from nonhuman primates (Shadlen and Newsome, 1996, 2001; Gold and Shadlen, 2000) and recently also in humans (Donner et al, 2009; O’Connell et al, 2012; Hadar et al, 2016). However, when instructions or payoffs change, neural accumulation profiles appear inconsistent with a changing boundary, the traditional model-based explanation of the SAT (Heitz and Schall, 2012, 2013; Hanks et al, 2014).…”

“…Experiment 1 used transcranial magnetic stimulation (TMS) to track corticospinal excitability, a downstream signal presumed to be under continuous influence from the decision variable (Bestmann et al, 2008; Duque et al, 2010; Hadar et al, 2016; Klein-Flugge and Bestmann, 2012). In experiment 2, we recorded the event-related potential (ERP) centroparietal positivity (CPP; O’Connell et al, 2012; Kelly and O’Connell, 2013; Twomey et al, 2016), a large, late positivity recorded over parietal regions.…”