Physiological causes:  Newly born infants during the first few months of their life, because during this period the corticospinal tract fibers are not yet completely myelinated or functioning.  Normal adults under conditions in which the cerebral cortical function is considerably depressed, such as during general anesthesia, coma, or even deep sleep.  Upper Motor Neuron Lesion, higher than the motor area, or descending motor tracts.