A chimpanzee ( Pan troglodytes ) recognizes spoken words synthesized as sine-wave speech

The human ability to understand speech in the absence of traditional acoustic cues to phonetic content has been argued to be evidence of specialized processing. To determine whether this capability is unique to humans, perception of sine-wave speech was examined in a 23-year-old, language-trained chimpanzee named Panzee. This animal was reared from infancy by human caregivers in a speech-rich environment and identifies more than 125 spoken words using graphical symbols (lexigrams). Test trials presented one of 48 familiar words via computer in either natural or sine-wave form, with Panzee choosing a corresponding lexigram from among four alternatives. Her performance on sine-wave words was well above chance levels, in spite of receiving no reward or any other feedback when responding to these sounds. The chimpanzee also showed above-chance accuracy on trials that cumulatively represented the first instances of hearing words in sine-wave form. While she was less accurate with sine-wave words than with natural versions, human listeners hearing the same stimuli were less accurate as well. Panzee’s performance indicates that experience with spoken language and general auditory-processing mechanisms alone can be sufficient for perceiving even highly impoverished synthetic speech.