The acoustic correlates of "speechlike": a use of the suffix effect

The stimulus suffix paradigm has been used to establish the importance of precategorical acoustic storage (PAS) as a theoretical construct in the investigation of attention and speech perception. Morton and Chambers concluded that sounds must have typical "speechlike" properties extracted at an early stage of processing in order to act as suffixes. In this article we use the suffix effect to investigate the conditions under which a sound is treated by the acoustic system as speechlike. On the basis of our findings we then perform other studies that reaffirm the essentially precategorical nature of the memory source termed PAS by Crowder and Morton. In Experiments 1-13 we demonstrate the complex basis on which sounds are classified. Our experiments show that a completely regular sound, in which a single pitch pulse from a naturally spoken vowel was repeatedly reproduced, still produced a substantial suffix effect. In addition a natural sound had to be quite severely filtered before the suffix effect began to vanish. However, a combination of regularity and filtering proved very effective, the two dimensions dramatically interacting in neutralizing the effect of the sound as a suffix. In two further experiments (14 and 15) we show that the classification parameters can be shifted by changing the acoustic properties of the stimulus list. However, forcing the subjects to make a linguistic classification of suffix sounds did not lead to any changes in their potency as suffixes. The classification of sounds, and thus the suffix effect, is an acoustic question, not a subjective one. The distinction between subjective and acoustic influences was further demonstrated when subjects rated a variety of sounds for their naturalness and for their similarity to the original suffix (Experiments 17-22). These measures showed themselves sensitive to the filtering operations we performed but, unlike measures of suffix effectiveness, were insensitive to regularity. Another suffix that produced a full suffix effect was shown to be rated as very nonspeechlike. Contrary to recent claims, these results reinforce our view of a distinction between central, subjectively controllable factors and a strong precategorical effect that is automatic in action and is based on the decision of whether a sound is speechlike.