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What is Ability to perceive shape and size is lost due to the lesion of and explain in detail?
Though both tractus cuneatus & tractus gracilis are correct, since it is a single answer question, the best answer would be tractus cuneatus, as we commonly use hands to recognize size and shape of objects, more than the foot.
Somatosensory cortex lesion- proprioception and tactile sensations are lost while pain and temperature sensations are preserved. 
Tractus cuneatus- formed by fasciculus cuneatus (spinal cord) and nucleus cuneatus (medulla). Pass through the lateral portion of the dorsal column. 
Carries sensations of the dorsal column- proprioception and tactile sensations from the upper part of the body.
Tractus gracilis- formed by fasciculus gracilis (spinal cord) and nucleus gracilis (medulla). Pass through a medial portion of the dorsal column. 
Carries sensations of the dorsal column- proprioception and tactile sensations from the lower part of the body.