Final answer:
The pupil response to light is an indicator of brainstem function.
Step-by-step explanation:
The pupil response to light is an indicator of **brainstem** function. The brainstem is responsible for controlling many autonomic functions, including the dilation and constriction of the pupil in response to light levels. The hypothalamus, located in the brainstem, plays a crucial role in driving the pupillary reflexes based on the information received from the optic nerve.