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Houser and Fitch emphasize throughout the article the importance of engaging in a "comparative approach" in the research of what makes language unique on a speech production/speech perception level. The article is a review of comparative research in these specific areas of speech production and speech perception.

Speech Production

Source: (typically the larynx) acts to convert the flow of air passing through lungs into an acoustic signal.

Formant: a filter that allows energy to pass through a center frequency which suppresses frequencies higher or lower than it.

Filter: a formant pattern, created by all the formants of a vocal tract coming together to form a formant function.

Originally the function of the larynx was to protect the respiratory system, but in mammals it has evolved to produce speech sounds. A lower larynx produces a deeper sound (demonstrated by the post-pubescent human male voice as compared to the female voice). Humans have among the lowest of larynxs, originally thought to be lowest, until discovery of two species of deer. It is thought that the lowering of the larynx originally evolved to deceive other animals about the size of the animal producing the sound, which would make sense for humans as a defense tactic against large carnivorous beasts. Also, the fact that both birds and mammals can perceive formants, suggests that this is a homologous trait, and that the perception of formants did not evolve to process speech sounds. Thus is not unique to humans, but evolved for one or both of these reasons: Body size deception or bluffing, and individual identification.

Speech Perception

Can infants tell when one word ends and another begins? Infants have the capacity to compute conditional statistics. They have an orienting response to part-words and non words but not to the familiar words based on their human capacity to compute such statistics.

Did categorical perception evolve for processing speech?

Evidence/examples in this section that non-human animals parse speech signals in the same way humans do provides evidence against the claim that such capacities evolved for speech perception, instead that they evolved for more general auditory functions and where __exapted__ for the speech system.

Both animals and humans can recognize speech variations through sound discrimination and categorizing at the capacity of a neonate.