This patient with very large, nonfunctioning pituitary tumor has no light perception in right eye. VEP results are show in parts A and B. Numbers 1 to 4 refer to recordings made at electrode positions shown in inset. Both (A) pattern and (B) flash VEPs (positive downward) are nondetectable, but (C) there remains residual PERC PSO, accompanied by shortening of latency and marked N95 loss. Pattern VEP from left eye is delayed, suggesting optic nerve dysfunction, and shows greater abnormality in right hemisphere traces than in left, in keeping with chiasmal involvement. Note N95 reduction in PERG from left eye, in keeping with ganglion cell dysfunction, but normal PSO component amplitude. (D) CT shows pituitary tumor.