 Mental imagery is the representation that results from perceptual processing that is not triggered directly by sensory input. So we get mental imagery when the first stop of perceptual processing happens without direct sensory input. Motor imagery is to the output what mental imagery is to the input. So we get motor imagery when the last stop in action processing happens without directly triggering motor output. In other words, motor imagery is the representation that results from processing in the motor system (in the motor and premotor cortices) that does not trigger motor output directly.