The Relationship Between Phonological Short-Term Memory (pSTM) and Categorical Perception of Speech
In real-world listening situations, the speech signal is often perceptually ambiguous. To achieve successful speech identification, listeners must resolve this ambiguity by organising the continuously varying phonetic features into discrete phoneme categories (categorical perception). The cognitive demand associated with categorisation depends on the degree of ambiguity in the signal, and on the perceptual acuity of the individual listener. Prior research suggests that phonological short-term memory (pSTM) capacity is, in part, determined by the quality of the speech input. For example, recall is poorer for speech-in-noise, and individuals with poor pSTM may also show weak categorical perception. However, the relationship between categorical perception of phonemes and pSTM capacity remains under-studied. This thesis assesses whether the difficulty associated with phoneme categorisation has an impact on serial recall performance, using a variety of methods that advance the current evidence base. Firstly, speech-sound discrimination was experimentally disrupted in a pSTM task, by asking typical adult listeners to recall audio-morphed syllables with varying perceptual ambiguity (Chapter Two). Recall for high-ambiguity syllables from letter-letter (“B”-“P”) continua was poorer than for low-ambiguity equivalents under specific circumstances, but when top-down control could be engaged to disambiguate the sounds, the ambiguity effect was abolished. Secondly, sensitivity to phoneme contrasts was compared between adolescents (10-15 years) with phonological deficits and controls, using a measure of categorical perception with minimal extraneous cognitive demands (Chapter Three). No between-groups differences were identified on either speech or non-speech auditory perception. In an independent experiment (Chapter Four), individual differences in speech perception acuity could not predict pSTM capacity in typical adults (N = 75). Finally, a functional MRI study provided preliminary evidence that perceptual demands and pSTM demands do not load on a shared brain network (Chapter Five). Thus, by collating evidence from children and adults, and using behavioural and neuroimaging approaches, I conclude that categorical perception and pSTM are dissociable in individuals, and should be considered relatively independent functions.
Categorical Perception, Phonology, Short-Term Memory, Speech Perception
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