Investigating Priming Effects of Physical Practice on Motor Imagery-Induced Event-Related Desynchronization

Abstract

For motor imagery (MI) to be effective, an internal representation of the to-be-imagined movement may be required. A representation can be achieved through prior motor execution (ME), but the neural correlates of MI that are primed by ME practice are currently unknown. In this study, young healthy adults performed MI practice of a unimanual visuo-motor task (Group MI, n = 19) or ME practice combined with subsequent MI practice (Group ME&MI, n = 18) while electroencephalography (EEG) was recorded. Data analysis focused on the MI-induced event-related desynchronization (ERD). Specifically, changes in the ERD and movement times (MT) between a short familiarization block of ME (Block pre-ME), conducted before the MI or the ME combined with MI practice phase, and a short block of ME conducted after the practice phase (Block post-ME) were analyzed. Neither priming effects of ME practice on MI-induced ERD were found nor performance-enhancing effects of MI practice in general. We found enhancements of the ERD and MT in Block post-ME compared to Block pre-ME, but only for Group ME&MI. A comparison of ME performance measures before and after the MI phase indicated however that these changes could not be attributed to the combination of ME and MI practice. The mixed results of this study may be a consequence of the considerable intra- and inter-individual differences in the ERD, introduced by specifics of the experimental setup, in particular the individual and variable task duration, and suggest that task and experimental setup can affect the interplay of ME and MI.

Keywords: EEG; event-related desynchronization; motor imagery; physical practice; priming.

Figure 1

Group design. Experimental Group design. Periods of ME are indicated by a light gray background. Both groups began with eight trials of ME serving as pre-measurement (Block pre-ME). Then, Group MI performed six blocks of 40 trials each of MI practice (Block 1 to Block 6), while Group ME&MI performed two blocks of 40 trials each of ME practice (Block 1 and Block 2), followed by four blocks of 40 trials each MI practice (Block 3 to Block 6). Both groups finished with eight trials of ME (Block post-ME). Blocks subject to planned comparisons are indicated by black font.

Figure 2

Setup. Photograph of the experimental setup. The experimental setup consisted of a customized table with two holders in the middle and two buttons on the right side (see upper photograph). For each new trial, the experimenter placed the rectangular object (1) with a marble on top in the upper, start holder (2). The start holder was equipped with slots at different angles (−22, 0, 45, and 56°) to allow for variations in the movement. Between upper (2) and lower (3) holder a glass pane (4) with an electrochromic foil was installed. Of the two buttons, the lower button (5) was a release button sending a trigger that signaled the start of the movement in motor execution (ME) trials, while pressing the upper button (6) signaled the end of the movement in both ME and motor imagery (MI) trials. A new trial would be initiated by the experimenter when object and marble were in place, and the palm of the participant’s right hand rested on the release button (right photograph). As soon as the otherwise opaque glass pane turned transparent, participants physically executed (ME) or kinesthetically imagined (MI) the movement. The movement was to reach for and grasp the object in the start holder (1 and 2), to transport it around the glass pane (4) and to place it in the lower, target holder (3), all without losing the marble. A trial was finished when the participant pressed the end button (6), which also changed the glass pane back to opaque. Then, in preparation for the next trial, the participant rested their right palm on the release-button (5), while the experimenter placed the object anew in the upper holder (2).

Figure 3

ERD time courses and mean topographies for ME and MI. Illustrated are the mean relative ERD per group and standard error for (A) Block pre-ME, (B) Block post-ME, (C) Block 3 (corresponding to first MI block for Group ME&MI and to third MI block for Group MI), (D) Block 6 (corresponding to fourth MI block for Group ME&MI and to sixth MI block for Group MI). As trial length varied considerably, the ERD traces comprise only the first 2 s of the task phase that were available for all trials. The gray area indicates the baseline period, and the dashed black line represents the start of the trial. Topographies show the distribution of relative ERD averaged across trials and participants. Topographies are based on individual single trial durations as described in the methods. Topographies were z-transformed within each condition for illustration purposes. Note that positive z-values can result from the z-transformation; they correspond however to negative relative power values. White dots highlight the electrodes of the contralateral ROI from which the mean relative ERDs were derived for statistical analysis.

Figure 4

ERD and MT_total for all experimental blocks. (A) Mean ERD and standard error per group for Block 1 to Block 6. The black rectangle marks blocks in which groups performed different tasks, i.e., Group ME&MI performed ME, and Group MI performed MI. Blocks without statistical comparisons are displayed transparent. (B) Mean ERD and standard error per group for Block pre-ME and Block post-ME. For Group ME&MI the mean ERD derived from a subset of trials of the second ME block (Block 2) is also shown (see section “Methods” for details). (C) Mean movement time and standard error per group for Block 1 to Block 6. The black rectangle marks blocks in which groups performed different tasks, i.e., Group ME&MI performed ME, and Group MI performed MI. Blocks without statistical comparisons are displayed in transparent. (D) Mean movement time and standard error per group for Block pre-ME and Block post-ME. For Group ME&MI, the MT_total time derived from a subset of trials of the second ME block (Block 2) is also shown (see section “Methods” for details).