"All of the physiological parameters measured are controlled unconsciously by the autonomic nervous system, and reflect functions such as the orienting reflex. The orienting reflex is a response to small changes in the environment, whose strength depends on the intensity, novelty and significance of the new stimulus. Repeated presentation of a stimulus therefore makes it more familiar, leading to a smaller reflex, and to a decrease in the associated responses. On the other hand, any change in the stimulus evokes a reflex that produces larger responses.
Thus, a stimulus that has already been encountered will be more significant than one that has not, because it is more familiar, and this will be reflected in increased skin conductance. The researchers reasoned that falsely recognized items would be less significant than those encountered before, and would therefore be associated with decreased skin conductance. This is exactly what they found."