When Hearing Lips and Seeing Voices Becomes Perceiving Speech: Auditory-Visual Integration in Lexical Access

When Hearing Lips and Seeing Voices Becomes Perceiving Speech: Auditory-Visual Integration in Lexical Access Rachel Ostrand (rostrand@cogsci.ucsd.edu) University of California, San Diego, Department of Cognitive Science 9500 Gilman Drive, #0515, La Jolla, CA 92093-0515 USA Sheila E. Blumstein (sheila_blumstein@brown.edu) Brown University, Department of Cognitive, Linguistic, and Psychological Sciences Box 1821, Providence, RI 02912 USA James L. Morgan (james_morgan@brown.edu) Brown University, Department of Cognitive, Linguistic, and Psychological Sciences Box 1821, Providence, RI 02912 USA Abstract In the McGurk Effect, a visual stimulus can affect the perception of an auditory signal, suggesting integration of the auditory and visual streams. However, it is unclear when in speech processing this auditory-visual integration occurs. The present study used a semantic priming paradigm to investigate whether integration occurs before, during, or after access of the lexical-semantic network. Semantic associates of the un-integrated auditory signal were activated when the auditory stream was a word, while semantic associates of the integrated McGurk percept (a real word) were activated when the auditory signal was a nonword. These results suggest that the temporal relationship between lexical access and integration depends on the lexicality of the auditory stream. Keywords: lexical access; McGurk Effect; auditory-visual integration; lexical-semantic network Introduction Speech comprehension is a complex, multi-staged process. Language input to the perceiver consists of information from several different sources which can augment the auditory speech stream, including visual information from the speaker’s mouth and lip movements, knowledge about the speaker’s accent and pronunciations, eye and head movements to highlight referents, and tone of voice and body language. While speech perception is most obviously driven by the auditory signal entering the listener’s ears (Erber, 1975), visual information from a speaker’s mouth and lip movements can affect and even significantly alter the perception of speech (Fort et al., 2010; Green, 1998; Summerfield, 1987), especially in noisy or degraded environments (Erber, 1975; Grant & Seitz, 2000; Sumby & Pollack, 1954). To be able to derive this processing contribution from visual information, the auditory and visual signals must be integrated into a single representation. The present work seeks to determine when such integration occurs during speech processing; in particular, whether it occurs before or after access to the lexical-semantic network. McGurk and MacDonald (1976) first reported the McGurk Effect, in which incongruent audio and visual stimuli combine to induce in listeners the perception of a stimulus different than that of the actual sound input they have received. This effect is remarkable because of its illusory status – the listener perceives a token that is distinct from the sound signal, even with a perceptually good auditory exemplar. In this case, it is clear that the auditory and visual signals are integrated at some point during speech processing. Theories of lexical retrieval in speech comprehension posit a mental lexicon as a repository of stored lexical items. This comprehension lexicon is an interconnected network of words, each containing the phonological, syntactic, and semantic information necessary for speech processing. To understand spoken speech, the incoming speech signal must activate its entry in the lexicon to retrieve the meaning of an input word (Aitchison, 2003; Collins & Loftus, 1975). This look-up process, using phonological input as a search key for its corresponding meaning, is known as lexical access. The present study investigates which components of the incoming speech stream influence this search process. In the case of McGurk Effect stimuli, for which participants perceive a stimulus different from that presented by the auditory stream alone, the differing auditory and visual inputs were necessarily integrated at some point during speech processing. However, it is unclear whether this integration happens before, after, or coincidently with lexical access. That is, does the lexical representation which is ultimately activated for processing the speech input correspond to the auditory input alone, or to the combined auditory-visual percept, which may differ from that of the auditory signal? The study presented here investigates whether this combined percept is simply a perceptual illusion that fails to access the lexicon, or if the integrated percept is treated as input to the lexicon, thereby causing activation of its own semantic associates. To create these integrated audiovisual-percepts, a video of a speaker mouthing an item is dubbed with an auditory track differing in the initial consonant’s place of articulation.