A member of the Abu Dhabi ruling family accused of torturing an Afghan grain merchant may have been disoriented and impaired by drugs he used for health problems, his defense argued in court on Monday. A video that came to light in May appears to show the defendant, Sheik Issa bin Zayed al-Nahyan, torturing the merchant with whips, cattle prods and a plank with a protruding nail, then driving over him with an S.U.V. Sheik Issa, a half brother of the Emirati president, has pleaded not guilty. A forensics expert testified Monday that he was taking a combination of dozens of drugs, including medicine for heart and back conditions.