Listeners’ understanding of comprehensive speech in a non-native language

It can therefore be argued that the majority of speech comprehension studies has solely focussed on localising anatomical brain areas while few studies went beyond listeners’ perception of speech in their first language and considered listeners’ understanding of comprehensive speech in a non-native language (Inui et al., 1998; Kim, Relkin, Lee, & Hirsch, 1997; Nakai et al., 1999; Perani et al., 1996). The neuroimaging study by Nakai et al (1999) investigated Japanese speakers’ listening comprehension of their native language (Japanese), a comprehensive non-native language (English) as well as of a non-comprehensive non-native language (Hungarian). Nakai et al. (1999) were particularly interested in detecting distinct activations of separate language regions that respond to processing comprehensive and non-comprehensive languages. In contrast to prior research (e.g. Perani et al., 1996), they found no expansive responses of the IFG and angular gyrus when listeners passively listened to their native language. Nonetheless, similar to previous investigations (Mazoyer et al., 1993; Perani et al., 1996), Nakai et al. (1999) found both comprehensive and non-comprehensive languages to elicit activations from the posterior part of the STG. In line with prior research that found that the IFG is activated to a high degree during the passive listening of words (Mazoyer et al., 1993; Perani et al., 1996), and during perception of speech that is complex in syntax (Inui et al., 1998), Nakai et al (1999) observed the IFG to respond to the comprehensive languages, Japanese and English. These languages were also reported to activate the supplementary motor area (SMA) and the pre-motor area (PMA) indicating a role of these regions in perceived comprehensibility (Nakai et al., 1999). Finally, all languages were observed to elicit responses in the transverse temporal gyri and the PAC (Nakai et al., 1999).

However, the task in Nakai et al.’s (1999) study was a passive listening task and did not measure the neural correlates of participants’ active comprehension of their speech material. Additionally, their experiment dealt with sentence comprehension and did not consider listening comprehension at the word level. Moreover, they did not focus on revealing the possible linguistic benefit of a particular speech modification for listeners with varying levels of proficiency within one language (Nakai et al., 1999). Furthermore, measurements were taken from four participants only. Similar to Nakai et al (1999), the functional magnetic resonance imaging (fMRI) study by Inui et al (1998) investigated Japanese listeners’ speech comprehension. However, they did not investigate their notion at word-level as their speech material included sentences only (Inui et al., 1998).

It can therefore be said that no neuroimaging study to date has investigated the neural basis of native and non-native listeners’ comprehensibility of speech that includes a speech modification such as vowel space expansion, by using an active listening comprehensibility task with word stimuli that were produced in a naturalistic setting with a communicative purpose. Moreover, no neuroimaging study has studied the neural mechanism of the possible perceptual and cognitive advantage that vowel space expansion might provide listeners. Vowel space expansion is contributed to by changes in the first two formants, F1 and F2, and has been shown by prior behavioural studies to enhance listeners’ perception of speech (Ferguson & Kewley-Port, 2007; Uther, Knoll, & Burnham, 2007). It has been found that this kind of speech modification yields a large speech intelligibility benefit for native speakers and for early learners of a second language (L2) and to lead to a small intelligibility benefit for late L2 learners (Bradlow & Bent, 2002); however, this has not been investigated by a neuroimaging study either.