 Another concern that has been raised about Koko's ability to express coherent thoughts through signs is that interpretation of the gorilla's conversation was left to the handler, who may have seen improbable concatenations of signs as meaningful.For example, when Koko signed "sad" there was no way to tell whether she meant it with the connotation of "How sad".Following Patterson's initial publications in 1978, a series of critical evaluations of her reports of signing behavior in great apes argued that video evidence suggested that Koko was simply being prompted by her trainers' unconscious cues to display specific signs, in what is commonly called the Clever Hans effect.