Which sign is NOT associated with impending herniation?

The correct choice is associated with the absence of warning signs for impending herniation. Gradual improvement in motor skills is not typically a sign of herniation. In fact, impending herniation is characterized by worsening neurological deficits and a decline in mental status, as the brain is under pressure and potentially being displaced from its normal position.

In contrast, the other options reflect common signs associated with the dangerous condition of herniation. Contralateral weakness or paralysis indicates that one side of the brain is affected, which can occur when brain tissue is compressed. Rapidly deteriorating mental status shows that the brain's functioning is diminishing due to the increased pressure. A unilateral dilated pupil can indicate pressure on specific cranial nerves or brain structures, which is also a warning sign of potential herniation. All these symptoms demonstrate a decline in neurological function, which is directly opposite to what is described by the correct choice.