Abstract
The human mirror system has been the subject of much research over the past two decades, but little is known about the timecourse of mirror responses. In addition, it is unclear whether mirror and counter-mirror effects follow the same timecourse. We used single-pulse transcranial magnetic stimulation to investigate the timecourse of mirror and counter-mirror responses in the human brain. Experiment 1 demonstrated that mirror responses can be measured from around 200 ms after observed action onset. Experiment 2 demonstrated significant effects of counter-mirror sensorimotor training at all timepoints at which a mirror response was found in Experiment 1 (i.e. from 200 ms onward), indicating that mirror and counter-mirror responses follow the same timecourse. By suggesting similarly direct routes for mirror and counter-mirror responses, these results support the associative account of mirror neuron origins whereby mirror responses arise as a result of correlated sensorimotor experience during development. More generally, they contribute to theorizing regarding mirror neuron function by providing some constraints on how quickly mirror responses can influence social cognition.
Highlights
Mirror neurons, which fire both when performing an action and when observing another performing the same action, have been the focus of much interest and speculation since their discovery in the macaque (di Pellegrino et al, 1992)
For each muscle in every participant, mean normalized motorevoked potentials (MEPs) sizes were calculated for each observation condition and transcranial magnetic stimulation (TMS) pulse timepoint and submitted to a 2 Â 2 Â 5 repeated-measures analysis of variance (ANOVA) with muscle (FDI and ADM), observed action and timepoint (100, 150, 200, 250 and 300 ms) as within-subjects factors
For each muscle in every participant for both pre- and post-training sessions, mean normalized MEP sizes were calculated for each observation condition and TMS pulse timepoint and submitted to a 2 Â 2 Â 2 Â 3 repeated-measures ANOVA with session, muscle (FDI and ADM), observed action and timepoint (200, 250 and 320 ms) as within-subjects factors
Summary
Mirror neurons, which fire both when performing an action and when observing another performing the same action, have been the focus of much interest and speculation since their discovery in the macaque (di Pellegrino et al, 1992). Other explanations have been proposed, including the possibility that mirror response properties arise as a result of sensorimotor experience gained during development (Heyes, 2001, 2010; Keysers and Perrett, 2004). Standard functional magnetic resonance imaging (fMRI) techniques can identify, with high anatomical precision, the areas involved in action execution which respond to action observation. These techniques cannot identify whether such responses correspond to activation of a motor program that matches the observed action; operationally, it is this matching property that defines mirror responses. Methods that can determine the relative activity of specific motor programs during action observation have the potential to provide operational
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