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Dispersion Theory concerns the constraints that govern contrasts, the phonetic differences that can distinguish words in a language. Specifically it posits that there are distinctiveness constraints that favor contrasts that are more perceptually distinct over less distinct contrasts. The preference for distinct contrasts is hypothesized to follow from a preference to minimize perceptual confusion: In order to recover what a speaker is saying, a listener must identify the words in the utterance. The more confusable words are, the more likely a listener is to make errors. Because contrasts are the minimal permissible differences between words in a language, banning indistinct contrasts reduces the likelihood of misperception. The term ‘dispersion’ refers to the separation of sounds in perceptual space that results from maximizing the perceptual distinctiveness of the contrasts between those sounds, and is adopted from Lindblom’s Theory of Adaptive Dispersion, a theory of phoneme inventories according to which inventories are selected so as to maximize the perceptual differences between phonemes. These proposals follow a long tradition of explaining cross-linguistic tendencies in the phonetic and phonological form of languages in terms of a preference for perceptually distinct contrasts. Flemming proposes that distinctiveness constraints constitute one class of constraints in an Optimality Theoretic model of phonology. In this context, distinctiveness constraints predict several basic phenomena, the first of which is the preference for maximal dispersion in inventories of contrasting sounds that first motivated the development of the Theory of Adaptive Dispersion. But distinctiveness constraints are formulated as constraints on the surface forms of possible words that interact with other phonological constraints, so they evaluate the distinctiveness of contrasts in context. As a result, Dispersion Theory predicts that contrasts can be neutralized or enhanced in particular phonological contexts. This prediction arises because the phonetic realization of sounds depends on their context, so the perceptual differences between contrasting sounds also depend on context. If the realization of a contrast in a particular context would be insufficiently distinct (i.e., it would violate a high-ranked distinctiveness constraint), there are two options: the offending contrast can be neutralized, or it can be modified (‘enhanced’) to make it more distinct. A basic open question regarding Dispersion Theory concerns the proper formulation of distinctiveness constraints and the extent of variation in their rankings across languages, issues that are tied up with the questions about the nature of perceptual distinctiveness. Another concerns the size and nature of the comparison set of contrasting word-forms required to be able to evaluate whether a candidate output satisfies distinctiveness constraints.

The Motor Theory of Speech Perception is a proposed explanation of the fundamental relationship between the way speech is produced and the way it is perceived. Associated primarily with the work of Liberman and colleagues, it posited the active participation of the motor system in the perception of speech. Early versions of the theory contained elements that later proved untenable, such as the expectation that the neural commands to the muscles (as seen in electromyography) would be more invariant than the acoustics. Support drawn from categorical perception (in which discrimination is quite poor within linguistic categories but excellent across boundaries) was called into question by studies showing means of improving within-category discrimination and finding similar results for nonspeech sounds and for animals perceiving speech. Evidence for motor involvement in perceptual processes nonetheless continued to accrue, and related motor theories have been proposed. Neurological and neuroimaging results have yielded a great deal of evidence consistent with variants of the theory, but they highlight the issue that there is no single “motor system,” and so different components appear in different contexts. Assigning the appropriate amount of effort to the various systems that interact to result in the perception of speech is an ongoing process, but it is clear that some of the systems will reflect the motor control of speech.

The study of second language phonetics is concerned with three broad and overlapping research areas: the characteristics of second language speech production and perception, the consequences of perceiving and producing nonnative speech sounds with a foreign accent, and the causes and factors that shape second language phonetics. Second language learners and bilinguals typically produce and perceive the sounds of a nonnative language in ways that are different from native speakers. These deviations from native norms can be attributed largely, but not exclusively, to the phonetic system of the native language. Non-nativelike speech perception and production may have both social consequences (e.g., stereotyping) and linguistic–communicative consequences (e.g., reduced intelligibility). Research on second language phonetics over the past ca. 30 years has resulted in a fairly good understanding of causes of nonnative speech production and perception, and these insights have to a large extent been driven by tests of the predictions of models of second language speech learning and of cross-language speech perception. It is generally accepted that the characteristics of second language speech are predominantly due to how second language learners map the sounds of the nonnative to the native language. This mapping cannot be entirely predicted from theoretical or acoustic comparisons of the sound systems of the languages involved, but has to be determined empirically through tests of perceptual assimilation. The most influential learner factors which shape how a second language is perceived and produced are the age of learning and the amount and quality of exposure to the second language. A very important and far-reaching finding from research on second language phonetics is that age effects are not due to neurological maturation which could result in the attrition of phonetic learning ability, but to the way phonetic categories develop as a function of experience with surrounding sound systems.

The theory of speech perception as direct derives from a general direct-realist account of perception. A realist stance on perception is that perceiving enables occupants of an ecological niche to know its component layouts, objects, animals, and events. “Direct” perception means that perceivers are in unmediated contact with their niche (mediated neither by internally generated representations of the environment nor by inferences made on the basis of fragmentary input to the perceptual systems). Direct perception is possible because energy arrays that have been causally structured by niche components and that are available to perceivers specify (i.e., stand in 1:1 relation to) components of the niche. Typically, perception is multi-modal; that is, perception of the environment depends on specifying information present in, or even spanning, multiple energy arrays. Applied to speech perception, the theory begins with the observation that speech perception involves the same perceptual systems that, in a direct-realist theory, enable direct perception of the environment. Most notably, the auditory system supports speech perception, but also the visual system, and sometimes other perceptual systems. Perception of language forms (consonants, vowels, word forms) can be direct if the forms lawfully cause specifying patterning in the energy arrays available to perceivers. In Articulatory Phonology, the primitive language forms (constituting consonants and vowels) are linguistically significant gestures of the vocal tract, which cause patterning in air and on the face. Descriptions are provided of informational patterning in acoustic and other energy arrays. Evidence is next reviewed that speech perceivers make use of acoustic and cross modal information about the phonetic gestures constituting consonants and vowels to perceive the gestures. Significant problems arise for the viability of a theory of direct perception of speech. One is the “inverse problem,” the difficulty of recovering vocal tract shapes or actions from acoustic input. Two other problems arise because speakers coarticulate when they speak. That is, they temporally overlap production of serially nearby consonants and vowels so that there are no discrete segments in the acoustic signal corresponding to the discrete consonants and vowels that talkers intend to convey (the “segmentation problem”), and there is massive context-sensitivity in acoustic (and optical and other modalities) patterning (the “invariance problem”). The present article suggests solutions to these problems. The article also reviews signatures of a direct mode of speech perception, including that perceivers use cross-modal speech information when it is available and exhibit various indications of perception-production linkages, such as rapid imitation and a disposition to converge in dialect with interlocutors. An underdeveloped domain within the theory concerns the very important role of longer- and shorter-term learning in speech perception. Infants develop language-specific modes of attention to acoustic speech signals (and optical information for speech), and adult listeners attune to novel dialects or foreign accents. Moreover, listeners make use of lexical knowledge and statistical properties of the language in speech perception. Some progress has been made in incorporating infant learning into a theory of direct perception of speech, but much less progress has been made in the other areas.

The seeming ease with which we usually understand each other belies the complexity of the processes that underlie speech perception. One of the biggest computational challenges is that different talkers realize the same speech categories (e.g., /p/) in physically different ways. We review the mixture of processes that enable robust speech understanding across talkers despite this lack of invariance. These processes range from automatic pre-speech adjustments of the distribution of energy over acoustic frequencies (normalization) to implicit statistical learning of talker-specific properties (adaptation, perceptual recalibration) to the generalization of these patterns across groups of talkers (e.g., gender differences).