AUTHOR=Files Benjamin T. , Tjan Bosco S. , Jiang Jintao , Bernstein Lynne E. 

TITLE=Visual speech discrimination and identification of natural and synthetic consonant stimuli

JOURNAL=Frontiers in Psychology

VOLUME=6

YEAR=2015

URL=https://www.frontiersin.org/journals/psychology/articles/10.3389/fpsyg.2015.00878

DOI=10.3389/fpsyg.2015.00878

ISSN=1664-1078

ABSTRACT=<p>From phonetic features to connected discourse, every level of psycholinguistic structure including prosody can be perceived through viewing the talking face. Yet a longstanding notion in the literature is that visual speech perceptual categories comprise groups of phonemes (referred to as <italic>visemes</italic>), such as /p, b, m/ and /f, v/, whose internal structure is not informative to the visual speech perceiver. This conclusion has not to our knowledge been evaluated using a psychophysical discrimination paradigm. We hypothesized that perceivers can discriminate the phonemes within typical viseme groups, and that discrimination measured with <italic>d-prime</italic> (<italic>d’</italic>) and response latency is related to visual stimulus dissimilarities between consonant segments. In Experiment 1, participants performed speeded discrimination for pairs of consonant-vowel spoken nonsense syllables that were predicted to be <italic>same, near</italic>, or <italic>far</italic> in their perceptual distances, and that were presented as natural or synthesized video. <italic>Near</italic> pairs were within-viseme consonants. Natural within-viseme stimulus pairs were discriminated significantly above chance (except for /k/-/h/). <italic>S</italic>ensitivity (<italic>d’</italic>) increased and response times decreased with distance. Discrimination and identification were superior with natural stimuli, which comprised more phonetic information. We suggest that the notion of the viseme as a unitary perceptual category is incorrect. Experiment 2 probed the perceptual basis for visual speech discrimination by inverting the stimuli. Overall reductions in <italic>d’</italic> with inverted stimuli but a persistent pattern of larger <italic>d’</italic> for <italic>far</italic> than for <italic>near</italic> stimulus pairs are interpreted as evidence that visual speech is represented by both its motion and configural attributes. The methods and results of this investigation open up avenues for understanding the neural and perceptual bases for visual and audiovisual speech perception and for development of practical applications such as visual lipreading/speechreading speech synthesis.</p>