N400-like negativities in action perception reflect the activation of two components of an action representation

Abstract

The understanding of actions of tool use depends on the motor act that is performed and on the function of the objects involved in the action. We used event-related potentials (ERPs) to investigate the processes that derive both kinds of information in a task in which inserting actions had to be judged. The actions were presented as two consecutive frames, one showing an effector/instrument and the other showing a potential target object of the action. Two mismatches were possible. An orientation mismatch occurred when the spatial object properties were not consistent with a motor act of insertion being performed (i.e., different orientations of insert and slot). A functional mismatch happened when the instrument (e.g., screwdriver) would usually not be applied to the target object (e.g., keyhole). The order in which instrument and target object were presented was also varied. The two kinds of mismatch gave rise to similar but not identical negativities in the latency range of the N400 followed by a positive modulation. The results indicate that the motor act and the function of the objects are derived by two at least partially different subprocesses and become integrated into a common representation of the observed action.

Keywords:

• Event-related potentials
• Action observation
• Tool use
• Mirror neurons
• Language comprehension

Acknowledgements

We thank Ina Koch, Conny Schmid and Sandra Boehme for the data collection, and Steven P. Tipper and Stefanie Schuch for constructive discussions concerning the research. The work was partly funded by a Wellcome Trust Programme Grant.

Notes

¹The ROIs were similar to our previous study on N400-effects in action/gesture perception (Gunter & Bach, 2004). Because our hypothesis primarily concerned laterality differences, there was no ROI for midline electrodes, but four ROIs capturing the most lateral electrodes were added (F7/F8, FT7/FT8, TP7/TP8, P7/P8). Note that the 10 chosen ROIs also include the electrode sites at which there were significant differences in the N400 time window in the other prior ERP-study on action observation (Sitnikova et al., 2003).

²To further validate the selection of latency windows, we split the analysis window into 40 ms intervals. For each of these intervals and for each of the ROIs, we investigated whether full matches and either of the single mismatches differed significantly (after correcting for multiple comparisons with respect to number of ROIs and latency windows). This showed that significant deviations of either single mismatching condition from the full matches indeed started to appear in the interval of 360 to 400 ms and remained significant throughout the selected latency window. One exception was the interval between 480 and 520 ms, which corresponded to the end of our first latency window (capturing the negative inflections) and the start of latency window 2 (start of the positive modulation).

³The selection of the latency windows was again validated with the procedure described in footnote 1. After adjusting for multiple comparisons, significant differences started to appear after 320 ms and remained present in all 40 ms intervals with the exception of the interval between 440 and 480 ms, which again corresponded to the end of our first latency window and the start of latency window 2.