This work examines the link between low-level cortical acoustic processing and higher-level cortical phonemic processing. Specifically, using functional magnetic resonance imaging, it looks at 1) whether or not the vowels [ɑ] and [i] are distinguishable in regions of interest defined by the first two resonant frequencies (formants) of those vowels and 2) whether or not that neural discrimination ability changes based on anatomical region. The formant-frequency based regions of interest are found to respond differentially to [ɑ] and [i] with the response to [ɑ] statistically significantly greater than the response to [i] in the averaged [ɑ] formant-frequency based region. Unexpectedly, the response to [i] is numerically but not statistically significantly greater than the response to [ɑ] in the averaged [i] formant-frequency based region. Additionally, there is not a significant interaction of this pattern with anatomical region, although early cortical auditory regions appear to show the pattern while later ones do not. Further investigation into the results leads to the hypotheses that they could be due to task-specific neural processing strategies and that the link between lower and higher-level cortical auditory processing is more complex than originally hypothesized.

Vowel perception primarily depends on the overall shape of the speech spectrum, which is imposed by the positions of the primary speech articulators. Voiced vowels also have a harmonic fine structure due to the activity of the vocal folds, and these harmonics give rise to synchronized activity in the brainstem. This synchronous firing may be useful for speech perception in noise and speaker discrimination, although it is unknown if the synchronized neural response to the harmonic increases perceptual audibility of the harmonic. The focus of the current study was to examine the relationship between the audibility of harmonics and the brainstem response to harmonics. The individual harmonics were found to be encoded in the brainstem, determined using brainstem frequency-following response recording, and the individual harmonics were audible to the individual, as determined using the pulsation threshold technique; however there was minimal relationship between the frequency-following response and perception of harmonics.

One family of theories regarding vowel perception suggests onset and offset formant-frequencies are important for identification and that the shape of the transitions themselves are not otherwise perceptually important. The present study determined just-noticeable-differences in deviations from linear formant trajectories. Diphthong-like stimuli were manipulated by inserting a point of inflection into the otherwise linear transition. Several parameters were manipulated including vowel duration, location of the inflection point in time, and fundamental frequency. Data from the first experiment indicate that listeners are largely insensitive to deviations from linearity of formant trajectory but that large enough deviations could eventually be detected. The size of these deviations seems dependent on the range of onset-offset formant frequencies. However, a second experiment in which only the first half of stimuli was presented thereby affecting the frequency range of the stimuli, gave different results. Results from these experiments along with several hypotheses are presented.

The present study examined the perception of English tense-lax vowel pairs by adult Saudi English as a second language (ESL) learners. More specifically, it looked at the effect of experience and length of exposure to the target language on the perception of these vowels. Thirty-eight male Saudi participants were recruited for the purpose of this study and were divided into two groups based on their level of proficiency and length of residence in the US, referred to as the experienced group and the inexperienced group, respectively. The research instrument contained four tense-lax vowel pairs, including /i/-/ɪ/, /e/-/ɛ/, /u/-/ʊ/, and /o/-/ɔ/. In order to control for the effect of the environment, all of the vowels were embedded in /hVd/ contexts as illustrated by the following examples: heed (/i/), hid (/ɪ/), hayed (/e/), head (/ɛ/), hawed (/ɔ/), hoed (/o/), hood (/ʊ/), and who'd (/u/). Native-speaker recordings of each vowel token were used to test participants’ perception accuracy. The recordings included both male and female voices. According to the statistics provided by the t-tests, there were significant differences in the perception of five out of the eight vowels that were examined in this study. In all five cases, the experienced group had a significantly better perception accuracy. The vowel that showed the highest magnitude of difference between the experienced and inexperienced students was the tense mid front vowel /e/ with an effect size of 1.5. It was followed by the lax high back vowel /ʊ/ with an effect size of 1.08. The third in magnitude of difference was the lax mid front vowel /ɛ/ (effect size = 1.02), followed by the lax mid back vowel /ɔ/ (effect size = 0.79), and finally the tense mid back vowel /o/ (effect size = 0.72). On the other hand, vowels that did not show significant differences between the two groups were the tense and lax high front vowels /i/ and /ɪ/ and the tense high back vowel /u/. However, although the experienced group showed a significantly higher level of perception accuracy in five out of the eight tense-lax vowel contrasts, the participants in this group reached the 80% accuracy level with only two vowels: /i/ and /e/. On the other hand, the inexperienced group did not show mastery of any of the eight vowels as their perception accuracy scores were below 80%. In terms of related theories, some of the results supported the Perceptual Assimilation Model (Best, 1994), the Speech Learning Model (Flege, 1995), and the Markedness Differential Hypothesis (Eckman, 1977), whereas other findings provided contradictory evidence. Overall, this study concluded that experience and exposure had a positive effect on Saudi ESL speakers’ perceptions of the tense-lax vowel contrasts in English. Even though this effect was not the same for all vowel contrasts, it carried the implication that perception accuracy can be facilitated through systematic training and practice.

The present dissertation aims at analyzing the interplay between initial acoustic biases and language exposure during acquisition of language in the first year of life. This is a critical period in development because it is when phonetic categories are attuned to the native language. This goal is addressed by integrating the results from two developmental studies. The first study explores the presence of asymmetries in vowel perception in infants from 4 to 12 months of age, as a function of the acoustic salience and distributional properties in the test language. The second study investigates the previous contrasting results on vowel perception in 8-month-old infants growing in bilingual environments, and in particular and their relationship with task demands and the properties of the bilingual input. The results emerging from these studies point to two developmental trends. On one hand, asymmetries in vowel perception are modulated by acoustic biases before and during perceptual reorganization, and by distributional cues after perceptual reorganization. On the other hand, discrimination abilities of 8-month-old bilinguals parallel those of monolinguals when tested with an Anticipatory Eye Movement procedure, highlighting the importance of task demands in determining infants' performance. / L'objectiu d'aquesta tesis és analitzar la dinàmica que s'estableix entre els biaixos acústics i l'experiència amb el llenguatge durant el primer any de vida. Aquest és un període decisiu durant el qual les categories fonètiques s'ajusten a les propietats de la llengua materna. Aquest objectiu s'assoleix integrant els resultats de dos estudis. En el primer estudi s'explora la presència d'asimetries en la percepció de les vocals en bebès de 4 a 12 mesos d'edat, en funció de la saliència acústica i les propietats distribucionals de la llengua. El segon estudi investiga els resultats contradictoris obtinguts amb bebès de 8 mesos d'edat bilingües, i la seva relació amb les demandes de la tasca i les propietats de l'input bilingüe. Els resultats d'aquests estudis indiquen dos tendències al desenvolupament. Per una banda, que les asimetries en la percepció de les vocals són modulades per biaixos acústics abans i durant la reorganització perceptiva i per les propietats distribucionals després de la reorganització perceptiva; per l'altra banda, els resultats mostren que les habilitats de discriminació dels bilingües de 8 mesos d'edat són corresponents a les dels monolingües quan es fa servir un procediment d'anticipació de la mirada, posant en rellevància la importància de les demandes de la tasca en determinar el rendiment dels bebès.

En los estudios sobre la percepción de los sonidos del habla es fácil encontrar diversos ejemplos que muestran que algunas características de la señal resultan más notorias que otras. En general estas diferencias no han sido incorporadas en los modelos de reconocimiento de palabras. En este trabajo se muestran evidencias de los sesgos en la percepción de vocales en adultos utilizando la técnica de los potenciales evocados. Independientemente de la lengua materna de los participantes, el fonema /i/ resulta mejor discriminado (i.e. respecto a /e/). Además se estudia el papel que la notoriedad de los estímulos juega en la representación y el acceso léxico. El juicio léxico realizado sobre no-palabras se ve acelerado cuando estas contienen como vocal crítica la /i/ (vs. /e/). / In speech perception literature, it is easy to find examples of some characteristics in the signal hich are more salient than others. However, such differences have generally not been incorporated in word recognition models. In this dissertation, evidence of adult vowel perception biases is shown by means of the event-related potentials (ERP) technique. We show that, regardless of the participants' native language, the /i/ phoneme is recognised more easily (relative to the /e/ phoneme). In addition the role that salience plays in lexica representation and access is studied. We find that lexical judgement in non-words is speeded when these contain /i/ as a critical vowel (vs. /e/).