Tolerance is also experienced when the person notices a decreased sensation with similar doses of a substance over time. Tolerance can be measured objectively, for example when a person with high blood alcohol level can still perform given perceptual and motor tasks such as walking in a straight line. Withdrawal symptoms are physiological and psychological symptoms associated with withdrawal from a substance after prolonged administration or habituation. Withdrawal is present when a characteristic physiological pattern associated with a certain substance is experienced, or when the person uses the substance to avoid or reduce specific symptoms (Gitlow 2001).