Frontiers in Psychology (Apr 2015)
Evaluating proposed dorsal and ventral route functions in speech perception and phonological short-term memory: Evidence from aphasia
Abstract
Introduction Researchers have reported patients who perform substantially better on lexical than sublexical perception tasks (e.g., Miceli, Gainotti, Caltagirone, & Masullo, 1980). These findings challenge claims that lexical perception depends on sublexical perception (e.g., McClelland & Elman, 1986), leading to proposals of separate sublexical (dorsal) and lexical (ventral) routes in speech perception (dual route models; e.g., Hickok & Poeppel, 2000; Majerus, 2013). However, prior studies have not closely matched the discriminability of the sublexical and lexical stimuli. Further, dual route models claim that phonological short-term memory (pSTM) depends on perceptual dorsal route regions, including a sensory-motor interface that translates sensory representations into motor representations used in rehearsal (e.g., Hickock & Poeppel, 2000; Ravizza et al., 2011). In contrast, traditional models of pSTM assume a phonological maintenance buffer separate from perceptual regions (Martin & Breedin, 1992). The current study addressed shortcomings of the prior perception studies using sublexical and lexical stimuli closely matched in discriminability. Performance on these tasks was related to pSTM to determine the extent to which impairments in pSTM are predicted by speech perception deficits. Method Thirteen individuals with aphasia completed auditorily presented speech perception tasks using natural speech tokens (syllable discrimination, word discrimination, lexical decision, and picture-word matching) and a pSTM task (digit matching). For word discrimination, subjects made same/different judgments to pairs of monosyllabic words differing by one distinctive feature in the initial or final phoneme (e.g., pat-bat). For syllable discrimination stimuli, the initial or final phoneme of the word was deleted (e.g., /pæ/-/bæ/). Words from the word discrimination task were used in the lexical decision task and non-words were created by changing a single distinctive feature of one phoneme (e.g., bat-bap). For picture-word matching, patients made same/different judgments to word and picture pairs that matched or differed by one distinctive feature of the initial or final phoneme (e.g., word: “bear”, picture: pear). To provide a comparison to prior studies, a word discrimination task was also created where words differed by three distinctive features in the initial or final phoneme (e.g., bat-sat). For digit matching, participants judged whether two lists of digits were the same or different where the non-matching lists differed in the order of two adjacent digits. Results and Conclusions When the lexical and sublexical stimuli were matched in discriminability, scores were highly correlated and no individual demonstrated substantially better performance on lexical than sublexical perception (Figures 1a-c). However, when the word discriminations were easier (as in prior studies; e.g., Miceli et al., 1980), patients with impaired syllable discrimination were within the control range on word discrimination (Figure 1d). Finally, digit matching showed no significant relation to perception tasks (e.g., Figure 1e). Moreover, there was a wide range of digit matching spans for patients performing well on speech perception tasks (e.g., > 1.5 on syllable discrimination and digit matching ranging from 3.6 to 6.0). These data fail to support dual route claims, suggesting that lexical processing depends on sublexical perception and suggesting that phonological STM depends on a buffer separate from speech perception mechanisms.
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