Languages differ in the sounds that make up their phonemic inventories. These sounds, or phonemes, are represented abstractly in the mind of a speaker, making up the underlying representations of lexical items. Additional sounds may exist in a given language, surfacing via a derivative process of phonological rule application, where they are called allophones. There are also many speech sounds that are unrepresented entirely, as separate categories, within a particular language. In either case, learners of a second language have difficulty perceiving and producing the sounds that do not exist or have differing phonological status in their native language. English and Spanish present us with an ideal case study of these differences. Both languages contain the [d], [ð], and [027E] sounds, but they differ in how they organize them. While /d/ is a phoneme in both languages, in Spanish [ð] is an allophone of /d/, while /027E/ is a separate phoneme. Conversely, [027E] is an allophone of /d/ in English while /ð/ is a separate phoneme. There is a large amount of literature showing that sounds that contrast in one's language are more perceptible than those that do not. Boomershine et al., (2008) showed this to be true of the [d], [ð], and [027E] sounds in native Spanish and English speakers, where Spanish speakers more easily perceived the differences between [d]/[027E] than [d]/[ð]. English speakers, on the other hand, had difficulty distinguishing [d] from [027E], but no issue with [d] and [ð]. The studies in this dissertation extend this work by first replicating the results of Boomershine et al., (2008) with a group of monolingual English speakers as well as a group of native Spanish speakers on a forced choice perception task. Additionally, I add a group of native English-speaking advanced learners of Spanish and show that their perception of the relevant sounds is more like that of the native Spanish speakers on a number of behavioral metrics. In a second study, I test the same three speaker groups in an Electroencephalography study (EEG) using the mismatch negativity protocol (MMN) which has been previously shown to probe auditory categorical perception (Naatanen et al., 1978). If the MMN is sensitive to phonemic contrasts, as has been claimed, the expectation is that, similarly to the behavioral perception results, speakers should show larger mismatch responses to phonemic contrasts than to allophonic contrasts. I also explore the possibility of using the MMN to probe category formation in the learner group. However, while there are subtle differences in the EEG data for each speaker group, the results are contra predictions. Instead, all three language groups pattern similarly on each sound in the EEG study, with larger MMNs being elicited by the [027E]/[d] comparison than the [ð]/[d] comparison. This mismatch response suggests that the MMN is not probing phonological status but is sensitive to phonetic category. A discussion of methodology and the validity of using the MMN in phonology research is included.
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