Complex sentence: Closed-eye hallucinations and closed-eye visualizations (CEV) are a distinct class of hallucination, which generally only occur when one's eyes are closed, or when one is in a darkened room.
Split into simpler sentences: Closed-eye hallucinations and closed-eye visualizations (CEV) are a distinct class of hallucination. These types of hallucinations generally only occur when one's eyes are closed or when one is in a darkened room.