grasp reflex. n. An involuntary bending of the fingers in response to tactile or tendon stimulation on the palm, producing an uncontrollable grasp, and associated with frontal lobe brain lesions.he reflex is a pathologic manifestation of diseases of the premotor cortex. In young infants the tonic grasp reflex is normal: the child can grasp the examiner's fingers so firmly that he or she can be lifted into the air.