How does Oliver Sacks use imagery in A Leg to Stand On?

Example of Imagery:

"I could no longer remember having a leg. I could no longer remember how I had ever walked and climbed ... there was a gap - an absolute gap - between then and now; and in that gap, into the void, the former 'I' had vanished - the 'I' who could thoughtlessly stand, run and walk, who was totally and thoughtlessly sure of his body ..."