Sentence:  The authors set about answering this fundamental question through a detailed investigation of the patient 's <WSD>ability</WSD> , tactually , ( 1 ) to perceive figure and ( 2 ) to locate objects in space , with his eyes closed ( or turned away from the object concerned ) . 
Meaning of the word marked with <WSD> tags:  the quality of being able to perform; a quality that permits or facilitates achievement or accomplishment