| Summary: | The characteristics of speech that determine its greater disruption of serial recall relative to non-speech (the irrelevant sound effect) are investigated (c.f. Tremblay et al., 2000). Degraded non-words disrupted serial recall less than clear non-words. Tasks show that both vowels and consonants of degraded non-words were misperceived, with initial consonants misperceived to a greater degree. Measures that followed showed that clear sequences of non-words, with changing vowels were more disruptive than sequences with changing consonants. Degrading vowel only changing sequences reduced disruption of serial recall to a level observed with clear consonant only changing sequences, whereas degradation had no effect on disruption by consonant only changing sequences. In further experiments the acoustic complexity of speech was reduced while maintaining its intelligibility by removing fundamental frequency information. Whispered speech disrupted serial recall to the same degree as voiced speech. Alternating voiced and whispered speech sounds within a sequence did not reduce serial recall performance relative to a sequence of voiced-only speech sounds. Results indicate the formant structure of speech sounds and not fundamental frequency information is the important carrier of acoustic change. Reversing the fine structure of whispered speech damaged its intelligibility whilst preserving acoustic complexity and these sounds were as disruptive of serial recall as normal whispered speech. This indicates that vocal tract resonances (formants) of speech and not its intelligibility determine its disruptive power. The relative disruptiveness of speech and non-speech sounds was then examined. Sounds were matched for acoustic complexity, but their 'speech-likeness' was destroyed. Speech disrupted serial recall more than did non-speech. Results indicate that the biological nature of speech renders it more disruptive than non-speech. The findings refute the 'changingstate- hypothesis' which is derived from the object-oriented episodic record model. This hypothesis argues that it is the degree of acoustic variation within an irrelevant stream and not the nature of its component sounds which determines its disruption of serial memory. Biological sounds may disrupt serial memory to a greater degree since they are of behavioural relevance and provide information about the environment that may need to be attended to. The addition of an attentional mechanism to the object-oriented episodic record model that regulates the reallocation of cognitive processing resources is proposed.
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