Transformation but not generation of motor images is disrupted following stimulation over the left inferior parietal lobe

Motor imagery (MI) involves the generation, maintenance, and transformation of motor images; yet, the neural underpinnings of each stage are not well understood. Here, we investigated the role of the left inferior parietal lobe (IPL; recruited during MI) in the stages of MI. Healthy participants (N=20) engaged in a MI task (making judgments about hands presented on a screen; hand laterality judgment task) over two days. During the task, active (Day A; 120% resting motor threshold) or sham (Day B; placebo) transcranial magnetic stimulation (TMS) was applied to the IPL on 50% of trials at 250, 500, or 750ms post-stimulus onset, corresponding to different stages of MI. A/B days were randomized across participants. Linear mixed effects (LME) modelling conducted on reaction time and accuracy revealed that longer reaction times were observed when TMS was delivered at 750ms after trial onset, and more greatly for active vs. sham stimulation. This effect was exacerbated for palm- vs. back-view stimuli and for right vs. left hands. Accuracy overall was decreased for active vs. sham stimulation, and to a greater extent for palm- vs. back-view stimuli and for right vs. left hands. Findings suggest that the left IPL is involved in image transformation. Overall, this work informs on the neural underpinnings of the stages of MI.