For CN IV, the expected eye movement during testing is:

Cranial Nerve IV, also known as the trochlear nerve, is primarily responsible for controlling the superior oblique muscle of the eye. This muscle is crucial for specific eye movements, particularly in allowing the eye to move downwards and laterally, which is essential for activities such as looking down while reading or looking down at an object. When the trochlear nerve functions properly, it will facilitate this downward and outward movement.

Testing for CN IV typically involves observing the eye for movements in conjunction with the other cranial nerves that also control eye movement, such as CN III and CN VI. A patient is instructed to follow an object, and the expected movement when the trochlear nerve is functioning correctly is downward and slightly outward, consistent with the action of the superior oblique muscle.

In this context, while downward and nasal may seem to imply a vertical and inward movement, it does not accurately reflect the distinct action that the trochlear nerve is designed to facilitate, as downward movement must include the lateral component when considering the role of the superior oblique muscle.