Although she is normally not aware of its existence, Ingrid discovers that she has a blind spot in her vision by closing one eye and focusing on a fixed point. What explains why Ingrid has this blind spot in her vision?

A) She suffers from retinal degeneration.
B) She is experiencing sensory adaptation.
C) No photoreceptors are located where the optic nerve leaves the eye.
D) There is insufficient light to activate her rods.

Answer: C
Rationale: Each eye has an optic nerve, a cluster of neurons that gathers sensory information, exits at the back of the eye, and connects with the brain. This nerve presents a challenge to the brain. Because it travels through the back of the eye it creates an area in the retina with no rods or cones, called the optic disc. The result is a blind spot—a space in the retina that lacks photoreceptors.