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Abstract

The research described in this thesis tries to explain the origins and the struc­ ture of human sound systems (and more specifically human vowel systems) as the result of self­organisation in a population under functional con­ straints. These constraints are: acoustic distinctiveness, articulatory ease and ease of learning. The process is modelled with computer simulations, following the meth­ odology of artificial life and artificial intelligence. The research is part of a larger re­ search effort into understanding the origins and the nature of language and intelli­ gence.

The emergence of sound systems is studied in a setting called the imitation game. In an imitation game, agents from a population interact in order to imitate each other as well as possible. Imitation is a binary process: it is either successful or a failure. Agents are able to produce and perceive speech sounds in a human­like way, and to adapt and extend their repertoires of speech sounds in reaction to the outcome of the imitation games. The agents' vowel repertoires are initially empty and are bootstrapped by random insertion of a speech sound when an agent with an empty repertoire wants to produce a sound. When the agents' repertoires are not empty anymore, random insertion does not happen anymore, except with very low probability. This low­probability random insertion is done in order to keep a pres­ sure on the agents to extend their number of vowels. .

As the agents' repertoires are initially empty and their production and percep­ tion are not biased towards any language in particular, the systems of speech sounds that emerge are language­independent and can be considered predictions of the kinds of systems of speech sounds that can be found in human languages.

The main focus of the thesis is on the emergence of vowel systems. It is shown that coherent, successful and realistic vowel systems emerge for a wide range of pa­ rameter settings in the simulation. When the vowel systems are compared with the types of vowel systems that are found in human languages, remarkable similarities are found. Not only are the most frequently found human vowel systems predicted, (this could already be done with direct optimisation of acoustic distinctiveness) but also less frequently occurring vowel systems are predicted in approximately the right proportions.

Variations on the basic imitation game show that it is remarkably robust. Not only do coherent, successful and realistic vowel systems emerge for a large number of parameter settings, but they also emerge when either the imitation game or the agents are changed qualitatively. Coherent and realistic systems still emerge when the perception and production of the agents are changed. Even if the rules of the imitation game are slightly changed, coherent and realistic systems still emerge. Of course, there are circumstances under which no systems emerge, indicating that the process is non­trivial.

It is also shown that the vowel systems can emerge and be preserved in chang­ ing populations. When old agents are removed from the population, and new, empty agents are added, coherent and realistic vowel systems can still emerge, provided that the replacement rate is not too high. It is also shown in the thesis that vowel systems can be preserved in a population, even though all original agents in it have been replaced. Furthermore, it is shown that under certain circumstances it can be advantegeous to have an age­structure in the population, so that older agents learn less quickly than young ones.

Finally, some experiments with more complex utterances are presented in the thesis. An experiment with artificial CV­syllables is presented and it is shown that, although phonemically coded (as opposed to holistically coded) systems can emerge, this simulation is much harder and much more sensitive to parameter changes than the vowel simulation. This probably has to do with the fact that in the case of CV­ syllables multiple independent and partly contradictory constraints have to be satis­ fied simultaneously, whereas in the vowel simulations, only one constraint (acoustic distinctiveness) is really important. Also, the first attempts at building a system that can produce complex and dynamic utterances without any constraints on their structure are presented, and it is argued that the main obstacle to getting such a system to work is the mapping from acoustic signals back to articulatory com­ mands.

The conclusion of the thesis is that universal tendencies of human vowel sys­ tems, and probably of human sound systems in general can be explained as the re­ sult of self­organisation in a population of agents that try to communicate as well as possible under articulatory and acoustic constraints. The articulatory and acoustic constraints cause the emerging sound systems to tend towards articulatory and acoustic optimality. However, the fact that the agents communicate in a population forces them to conform to the sound system in the population and causes sub­ optimal systems to emerge as well.

@phdthesis{deboer99selfOrganisation,
  author={B. de Boer},
  title={Self-Organisation in Vowel Systems},
  year={1999},
  month={June},
  school={Vrije Universiteit Brussel AI-lab},
  url={http://www.isrl.uiuc.edu/~amag/langev/paper/deboer99selfOrganisation.html}
}

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