Introduction Brachial plexus birth injury (BPBI) may cause permanent disability. It is recently thought to result from the (mal)adaptive reorganizational central nervous system problems. Methods In this study, adult patients with BPBI and age-matched healthy controls were compared for the cortical activity during action observation by functional magnetic resonance imaging (fMRI). Results The cortical activity in patients was significantly weaker than in the control group (p < .05). Areas of difference were middle temporal gyrus, premotor area, and inferior parietal lobule. The signal change in these areas was significantly lower in the patient group (p < .05). Conclusions This study showed that the cortical activity in the associative motor regions was weaker in the patients while no primary region showed any difference. The results were concluded that there is a diversity in the neuroplastic changes between primary and associative motor areas. Clinically, neurorehabilitative interventions should be planned based on this diversity.