According to recent neuro-rehabilitation research, an appropriate reafferent sensory feedback synchronized with a voluntary motor intention would be effective for promoting neural plasticity during stroke rehabilitation. Therefore, a BCI-based neuro-rehabilitation is considered to be a promising approach. To detect the motor intention, an event-related desynchronization (ERD), which can be evoked by intrinsic motor imagery, is usually used. However there exists various factors that affect ERD production, and its neural mechanism is still an open question. As a preliminary stage for developing an effective neuro-rehabilitation system, in this study, we evaluate the mutual effects of extrinsic (visual and somatosensory stimuli) and intrinsic (spontaneous motor imagery) factors on ERD production. Experimental results indicate that these three factors interact with each other in a complex relationship and probably affect a person's sense of agency.