Sentence:  There are two types of catch trials: false positive and <WSD>false negative</WSD>. ... <WSD>False-negative</WSD> catch trials are meant to determine if a subject has fallen asleep or has an otherwise reduced ability to respond during a test. If a subject misses a brighter stimulus than has previously been seen, it is assumed that this represents a bona fide <WSD>false-negative</WSD> catch trial. 
Meaning of the word marked with <WSD> tags:  A result of a test that shows as absent something that is present.