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Acoustic theories assume that speech perception begins with an acoustic signal transformed by auditory processing. In classical acoustic theory, this assumption entails perceptual primitives that are akin to those identified in the spectral analyses of speech. The research objective is to link these primitives with phonological units of traditional descriptive linguistics via sound categories and then to understand how these units/categories are bound together in time to recognize words. Achieving this objective is challenging because the signal is replete with variation, making the mapping of signal to sound category nontrivial. Research that grapples with the mapping problem has led to many basic findings about speech perception, including the importance of cue redundancy to category identification and of differential cue weighting to category formation. Research that grapples with the related problem of binding categories into words for speech processing motivates current neuropsychological work on speech perception. The central focus on the mapping problem in classical theory has also led to an alternative type of acoustic theory, namely, exemplar-based theory. According to this type of acoustic theory, variability is critical for processing talker-specific information during speech processing. The problems associated with mapping acoustic cues to sound categories is not addressed because exemplar-based theories assume that perceptual traces of whole words are perceptual primitives. Smaller units of speech sound representation, as well as the phonology as a whole, are emergent from the word-based representations. Yet, like classical acoustic theories, exemplar-based theories assume that production is mediated by a phonology that has no inherent motor information. The presumed disconnect between acoustic and motor information during perceptual processing distinguishes acoustic theories as a class from other theories of speech perception.

Ideophones, also termed mimetics or expressives, are marked words that depict sensory imagery. They are found in many of the world’s languages, and sizable lexical classes of ideophones are particularly well-documented in the languages of Asia, Africa, and the Americas. Ideophones are not limited to onomatopoeia like meow and smack but cover a wide range of sensory domains, such as manner of motion (e.g., plisti plasta ‘splish-splash’ in Basque), texture (e.g., tsaklii ‘rough’ in Ewe), and psychological states (e.g., wakuwaku ‘excited’ in Japanese). Across languages, ideophones stand out as marked words due to special phonotactics, expressive morphology including certain types of reduplication, and relative syntactic independence, in addition to production features like prosodic foregrounding and common co-occurrence with iconic gestures. Three intertwined issues have been repeatedly debated in the century-long literature on ideophones. (a) Definition: Isolated descriptive traditions and cross-linguistic variation have sometimes obscured a typologically unified view of ideophones, but recent advances show the promise of a prototype definition of ideophones as conventionalized depictions in speech, with room for language-specific nuances. (b) Integration: The variable integration of ideophones across linguistic levels reveals an interaction between expressiveness and grammatical integration, and has important implications for how to conceive of dependencies between linguistic systems. (c) Iconicity: Ideophones form a natural laboratory for the study of iconic form-meaning associations in natural languages, and converging evidence from corpus and experimental studies suggests important developmental, evolutionary, and communicative advantages of ideophones.

Morphological defectiveness refers to situations where one or more paradigmatic forms of a lexeme are not realized, without plausible syntactic, semantic, or phonological causes. The phenomenon tends to be associated with low-frequency lexemes and loanwords. Typically, defectiveness is gradient, lexeme-specific, and sensitive to the internal structure of paradigms. The existence of defectiveness is a challenge to acquisition models and morphological theories where there are elsewhere operations to materialize items. For this reason, defectiveness has become a rich field of research in recent years, with distinct approaches that view it as an item-specific idiosyncrasy, as an epiphenomenal result of rule competition, or as a normal morphological alternation within a paradigmatic space.

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.