Examining potential hemispheric differences in talker effects in spoken word recognition

Variability in talker identity, one type of indexical variation, has demonstrable effects on the speed and accuracy of spoken word recognition. Furthermore, variability in visual word recognition, such as changes in font, appears to affect processing differently, depending on which cerebral hemisphere initially processes the input. The present study examined whether such hemispheric differences exist in spoken language as well. In two long-term repetition-priming experiments, the authors examined responses to targets that were primed by stimuli that matched or mismatched on the indexical variable of interest, talker identity. The results demonstrate that indexical variability can, under some circumstances, be shown to affect participants’ perception of spoken words differently in the two cerebral hemispheres. Hemispheric differences in spoken word recognition 3 Both written and spoken forms of language are communicated over a highly variable signal. For example, in written language the letters comprising words can appear in different cases (UPPERCASE and lowercase) and different fonts (e.g., Chicago and Times). In spoken language, the identity of the talker and speaking rate represent two different sources of variability. Nonetheless, despite such variations, people typically process written and spoken language quickly and accurately. In the visual domain, Marsolek and colleagues (Marsolek, 1999; Marsolek, Kosslyn, & Squire, 1992) have argued that two relatively independent subsystems support our ability to recognize abstract and specific aspects of the input, and that these subsystems operate more efficiently in the left and right hemispheres, respectively. According to this view, an abstract-category subsystem, which uses a feature-based processing strategy, subserves category recognition and operates more effectively in the left cerebral hemisphere (LH). A specific-exemplar subsystem, which uses a more holistically-based processing strategy, subserves exemplar recognition and operates more effectively in the right cerebral hemisphere (RH). Indeed, recent evidence is consistent with the claims that dissociable neural subsystems underlie abstract and specific recognition of objects (Burgund & Marsolek, 2000; Marsolek, 1999; Marsolek & Burgund, 2003), word forms (Marsolek, 2004; Marsolek et al., 1992; Marsolek, Schacter, & Nicholas, 1996; Marsolek, Squire, Kosslyn, & Lulenski, 1994; but see Koivisto, 1995), pseudoword forms (Burgund & Marsolek, 1997), and letter-like forms (Marsolek, 1995). Hemispheric differences in spoken word recognition 4 The strongest support for this two-systems hypothesis comes from studies employing the long-term repetition-priming paradigm. The standard long-term repetition priming effect refers to any facilitation in the processing of a stimulus as a consequence of encoding the same (or a highly-related) stimulus in an earlier episode (Bowers, 1999). In this paradigm, participants are presented with a block of stimuli to which they must respond (the study phase). After a short distracter task, participants are presented with another block of stimuli (the test phase). In this second block, some of the stimuli from the first block are repeated. Typically, performance for repeated stimuli is better than performance for new (i.e., non-repeated) stimuli. For example, in the lexical decision task, participants are typically faster and more accurate in categorizing letter strings as words when they were studied in an earlier phase of the experiment. In the stemcompletion task, participants are more likely to complete a word stem (e.g. BEA____) as a previously studied word (e.g. BEACON) compared to an unstudied word (e.g. BEAGLE). However, if the first and second presentations (prime and target, respectively) mismatch on some dimension (e.g. letter-case in visual words; talker-identity in spoken words), the priming effect may be attenuated. This attenuation in priming is referred to as specificity (or a specificity effect). Marsolek and colleagues have reported qualitatively distinct patterns of visual longterm priming in the two cerebral hemispheres. Using the stem-completion task, these authors observed that long-term priming for words is insensitive to study-to-test changes in letter-case (i.e., UPPER and lower) when stems are presented to the LH (the right visual field) and sensitive to these changes when presented to the RH (the left visual Hemispheric differences in spoken word recognition 5 field) (Burgund & Marsolek, 1997; Marsolek, 2004; Marsolek et al., 1992; Marsolek et al., 1994). That is, the magnitude of the repetition priming effect is greater when the stems presented during the test phase appear in the same letter-case in which the words from which the stems were derived had appeared in the earlier phase of the experiment. However, this specificity effect is only obtained when the stems are presented to the RH. A similar pattern of priming has also been obtained for object identification. In Marsolek’s (1999) study, participants named objects (e.g. piano) presented in either the left or right visual field during a test phase after having viewed same-exemplar and different-exemplar objects during an initial encoding phase. The data showed that repetition priming was exemplar-abstract when test objects were presented directly to the LH, but exemplar-specific when tests objects were presented directly to the RH. In other words, equivalent priming was obtained between different exemplars (e.g. two different exemplars of a piano) when test objects were presented to the LH but priming was reduced in this case when the stimuli were presented to the RH. In the auditory domain, research has revealed specificity effects on spoken word processing and recognition (see Luce & McLennan, 2005, for review). In particular, indexical variability affects the speed and accuracy of spoken word recognition.1 Indexical variation arises from differences in speaking rate, differences among talkers, differences in affective states, and so on (Abercrombie, 1967; Pisoni, 1997). Previous research has demonstrated that surface details associated with indexical variability (e.g. talker-identity) are preserved in some form in memory and have consequences for subsequent perception (see Goldinger, 1996, 1998; Pisoni, 1997, for reviews). From a Hemispheric differences in spoken word recognition 6 theoretical point of view, representation and processing effects of indexical variation are a serious challenge to current real time processing models of spoken word recognition, all of which essentially ignore surface variability (see Luce & McLennan, 2005). Much of the representational work on indexical variability has been conducted using the long-term repetition-priming paradigm. Church and Schacter (1994) and Schacter and Church (1992) found effects of talker variation in implicit tasks such as stemcompletion and identification of low-pass filtered words. Performance in both tasks was better when stimuli were repeated by the same talker. Goldinger (1996) observed talker effects in both implicit (perceptual identification) and explicit (recognition) tasks and found that talker effects varied with level of processing (with the strongest effects at the lowest levels). Luce and Lyons (1998) observed significant talker effects in an explicit recognition memory experiment but not in an implicit priming experiment, demonstrating that repetition priming for spoken words might not always be sensitive to changes in the surface characteristics of the stimuli. Luce, McLennan, and Charles-Luce (2003) have proposed that the failure of Luce and Lyons to obtain specificity effects may have been due, at least in part, to the rapidity of the response. According to their time-course hypothesis, specificity effects may take time to develop. McLennan and Luce (2005) recently obtained results in support of their time-course hypothesis. In three long-term repetition-priming experiments, the authors manipulated the speed with which participants processed the stimuli and observed that indexical variability affects spoken word recognition only when processing is relatively slow and effortful. Hemispheric differences in spoken word recognition 7 In summary, within the auditory domain there is important evidence of specificity effects on word processing. In addition to the studies just discussed, a number of other studies have also obtained specificity effects with other paradigms (Bradlow, Nygaard, & Pisoni, 1999; Fujimoto, 2003) and specific populations (Houston & Jusczyk, 2003, in infants; Sommers, 1996, in elderly adults). However, unlike in the visual domain, no published study to date has explored whether hemispheric presentation affects the likelihood of obtaining indexical specificity effects in spoken word processing.2 Nevertheless, the motivation for the present study is not limited to an attempt to obtain hemispheric differences in specificity effects that would parallel findings in the visual domain. Rather, independent evidence provides compelling reasons to believe that such hemispheric differences potentially exist in the auditory domain as well, and in spoken language processing in particular. More specifically, research studies using cognitive neuroscience techniques, including fMRI, ERP, and investigations of populations with various disorders provide evidence that indexical and linguistic information may be represented and processed differently in the two cerebral hemispheres. Shestakova et al. (2002) conducted an ERP investigation of speech perception across different speakers and found a greater mismatch negativity (MMN) effect in the LH than in the RH, providing evidence for more abstract (phoneme) representations in the LH (more specifically, in the left temporal cortex). Furthermore, patients with RH damage perform worse than patients with LH damage in voice discrimination tasks (Van Lancker & Canter, 1982). Moreove