Detailed Phonetic Memory for Multi-Word and Part-Word Sequences

Humans recognize previously heard spoken words better when the words are repeated in the same voice and at the same speaking rate than when they differ in these or other dimensions (e.g., Goldinger, 1996). Such findings have usually been taken as evidence that perceived instances of either words or sublexical units are stored in a detailed form in memory, and that collections of these episodic memory traces comprise or are somehow linked to mental lexical representations. A different possibility is that, while detailed information about the speech signal is remembered, this information is not explicitly segmented or organized into discrete, traditionally assumed units such as words. Instead, episodic memories may involve sequences of connected speech that vary in length depending on the nature of the listening situation, perhaps comprising entire phrases or utterances. Such a possibility is more in line with the temporal and context-dependent nature of speech perception and production than traditionally defined exemplar models, since it predicts that more local information is always stored and (potentially) considered as part of a larger context. Indeed, there is evidence from corpus studies of pronunciation (Binnenpoorte, Cucchiarini, Boves, & Strik, 2005), phonological analysis (Bybee, 2002 for a review), and word-monitoring experiments (Sosa and MacFarlane, 2002) that at least very frequent multi-word expressions are in some sense accessed as “units” in the same way that words are usually assumed to be. However, it is unknown whether variable-length stretches of connected speech in general exhibit properties of exemplar-based storage or perception, since previous studies on memory have considered only isolated word productions. In this study, word identification and recognition memory accuracy were measured for sequences of words extracted from a large multi-speaker corpus of German sentence productions (Kohler, 1996). Sequences varied in their length (0.5-3 words, in 0.5-word steps), onset phase with respect to word onset (beginning either at a word onset or halfway between a word onset and offset), and speaker (target sequences were repeated with the same or a different voice). Sequences were extracted at random from the corpus, without considering word or sequence frequency or probability or syntactic constituent boundaries. In Experiment 1, listeners (n=19) heard 384 sequences (selected separately from the corpus for each listener to maximize coverage) in two blocks and identified the words they perceived. Blocks contained the same 192 word (or part-word) sequences extracted from the same sentence contexts; half of the words in the second block were identical repetitions of first-block stimuli and half were produced by a different speaker. Experiment 2 also involved two blocks. The first block was identical to that of Experiment 1; in the second block, listeners (n=16) categorized sequences as new or previously encountered. Half of repeated sequences (one fourth of the trials in the second block) were identical repetitions, and half involved a new speaker. Word identification results (Experiment 1) were consistent with previous findings concerning the effects of local acoustic context on phonetic perception, and the influence of lexical information on speech segmentation (e.g., Mattys, White, and Melhorn, 2005). By several measures of accuracy and consistency, listeners were better at identifying words in longer sequences, approaching ceiling performance for sequences longer than about 2.5 words. Sequence onset phase was also important, with robustly better identification where stimulus onsets and/or offsets coincided with word onsets/offsets. Responses were more consistent across repetitions of a sequence by the same speaker than repetitions by different speakers, especially at shorter sequence lengths. Recognition memory results (Experiment 2) differed from these patterns in several critical ways. Recognition accuracy increased linearly with sequence length over the range of stimuli considered, even after identification accuracy reached ceiling performance. There was no effect of sequence onset phase on memory, and there was a significant difference in the size of the benefit of stimulus and word onset/offset coincidence between identification and memory tasks (no benefit in the memory task), indicating that lexical Laboratory Phonology 11 151