The difference in the difficulty of speech and reading exists even as alphabetic writing systems provide discrete and invariant signals and nods to vowels, sounds and consonants. When it comes to speech and conversation, it "does not come to us as a series of individual words; we must extract the words from a stream of speech." Liberman and his colleagues were training the blind to read using a reading machine that would replace each letter of the alphabet with a specific sound. However, he and his colleagues found that the replacement of the sounds for each distinct letter of the alphabet did not help with the blind to learn to read or pronounce the letters fluently. After long investigations of why this was, Liberman established that speech was not as simple as an acoustic alphabet. Therefore, speech signals are very distinct from acoustic alphabet (Fowler, 2001). These investigations showed that speech perception is different from perception of other acoustic signals, and convinced Liberman that speech perception is the result of the human biological adaptations to language. Human listeners are able to decode the repetitive variable signal of running speech and to translate it into phonemic components. This is also known as the "motor theory of speech perception". Liberman ascribed this to the human biological disposition towards speech as opposed to reading which is not ingrained genetically.