Evidence of a Vocalic Proto-System in the Baboon (Papio papio) Suggests Pre-Hominin Speech Precursors.

Language is a distinguishing characteristic of our species, and the course of its evolution is one of the hardest problems in science. It has long been generally considered that human speech requires a low larynx, and that the high larynx of nonhuman primates should preclude their producing the vowel systems universally found in human language. Examining the vocalizations through acoustic analyses, tongue anatomy, and modeling of acoustic potential, we found that baboons (Papio papio) produce sounds sharing the F1/F2 formant structure of the human [ɨ æ ɑ ɔ u] vowels, and that similarly with humans those vocalic qualities are organized as a system on two acoustic-anatomic axes. This confirms that hominoids can produce contrasting vowel qualities despite a high larynx. It suggests that spoken languages evolved from ancient articulatory skills already present in our last common ancestor with Cercopithecoidea, about 25 MYA.

Audio Visual Integration with Competing Sources in the Framework of Audio Visual Speech Scene Analysis.

We introduce "Audio-Visual Speech Scene Analysis" (AVSSA) as an extension of the two-stage Auditory Scene Analysis model towards audiovisual scenes made of mixtures of speakers. AVSSA assumes that a coherence index between the auditory and the visual input is computed prior to audiovisual fusion, enabling to determine whether the sensory inputs should be bound together. Previous experiments on the modulation of the McGurk effect by audiovisual coherent vs. incoherent contexts presented before the McGurk target have provided experimental evidence supporting AVSSA. Indeed, incoherent contexts appear to decrease the McGurk effect, suggesting that they produce lower audiovisual coherence hence less audiovisual fusion. The present experiments extend the AVSSA paradigm by creating contexts made of competing audiovisual sources and measuring their effect on McGurk targets. The competing audiovisual sources have respectively a high and a low audiovisual coherence (that is, large vs. small audiovisual comodulations in time). The first experiment involves contexts made of two auditory sources and one video source associated to either the first or the second audio source. It appears that the McGurk effect is smaller after the context made of the visual source associated to the auditory source with less audiovisual coherence. In the second experiment with the same stimuli, the participants are asked to attend to either one or the other source. The data show that the modulation of fusion depends on the attentional focus. Altogether, these two experiments shed light on audiovisual binding, the AVSSA process and the role of attention.

Audio-visual speech scene analysis: characterization of the dynamics of unbinding and rebinding the McGurk effect.

While audiovisual interactions in speech perception have long been considered as automatic, recent data suggest that this is not the case. In a previous study, Nahorna et al. [(2012). J. Acoust. Soc. Am. 132, 1061-1077] showed that the McGurk effect is reduced by a previous incoherent audiovisual context. This was interpreted as showing the existence of an audiovisual binding stage controlling the fusion process. Incoherence would produce unbinding and decrease the weight of the visual input in fusion. The present paper explores the audiovisual binding system to characterize its dynamics. A first experiment assesses the dynamics of unbinding, and shows that it is rapid: An incoherent context less than 0.5 s long (typically one syllable) suffices to produce a maximal reduction in the McGurk effect. A second experiment tests the rebinding process, by presenting a short period of either coherent material or silence after the incoherent unbinding context. Coherence provides rebinding, with a recovery of the McGurk effect, while silence provides no rebinding and hence freezes the unbinding process. These experiments are interpreted in the framework of an audiovisual speech scene analysis process assessing the perceptual organization of an audiovisual speech input before decision takes place at a higher processing stage.

A possible neurophysiological correlate of audiovisual binding and unbinding in speech perception.

Audiovisual (AV) speech integration of auditory and visual streams generally ends up in a fusion into a single percept. One classical example is the McGurk effect in which incongruent auditory and visual speech signals may lead to a fused percept different from either visual or auditory inputs. In a previous set of experiments, we showed that if a McGurk stimulus is preceded by an incongruent AV context (composed of incongruent auditory and visual speech materials) the amount of McGurk fusion is largely decreased. We interpreted this result in the framework of a two-stage "binding and fusion" model of AV speech perception, with an early AV binding stage controlling the fusion/decision process and likely to produce "unbinding" with less fusion if the context is incoherent. In order to provide further electrophysiological evidence for this binding/unbinding stage, early auditory evoked N1/P2 responses were here compared during auditory, congruent and incongruent AV speech perception, according to either prior coherent or incoherent AV contexts. Following the coherent context, in line with previous electroencephalographic/magnetoencephalographic studies, visual information in the congruent AV condition was found to modify auditory evoked potentials, with a latency decrease of P2 responses compared to the auditory condition. Importantly, both P2 amplitude and latency in the congruent AV condition increased from the coherent to the incoherent context. Although potential contamination by visual responses from the visual cortex cannot be discarded, our results might provide a possible neurophysiological correlate of early binding/unbinding process applied on AV interactions.

Effects of aging on audio-visual speech integration.

This study investigated the impact of aging on audio-visual speech integration. A syllable identification task was presented in auditory-only, visual-only, and audio-visual congruent and incongruent conditions. Visual cues were either degraded or unmodified. Stimuli were embedded in stationary noise alternating with modulated noise. Fifteen young adults and 15 older adults participated in this study. Results showed that older adults had preserved lipreading abilities when the visual input was clear but not when it was degraded. The impact of aging on audio-visual integration also depended on the quality of the visual cues. In the visual clear condition, the audio-visual gain was similar in both groups and analyses in the framework of the fuzzy-logical model of perception confirmed that older adults did not differ from younger adults in their audio-visual integration abilities. In the visual reduction condition, the audio-visual gain was reduced in the older group, but only when the noise was stationary, suggesting that older participants could compensate for the loss of lipreading abilities by using the auditory information available in the valleys of the noise. The fuzzy-logical model of perception confirmed the significant impact of aging on audio-visual integration by showing an increased weight of audition in the older group.

Atypical audio-visual speech perception and McGurk effects in children with specific language impairment.

Audiovisual speech perception of children with specific language impairment (SLI) and children with typical language development (TLD) was compared in two experiments using /aCa/ syllables presented in the context of a masking release paradigm. Children had to repeat syllables presented in auditory alone, visual alone (speechreading), audiovisual congruent and incongruent (McGurk) conditions. Stimuli were masked by either stationary (ST) or amplitude modulated (AM) noise. Although children with SLI were less accurate in auditory and audiovisual speech perception, they showed similar auditory masking release effect than children with TLD. Children with SLI also had less correct responses in speechreading than children with TLD, indicating impairment in phonemic processing of visual speech information. In response to McGurk stimuli, children with TLD showed more fusions in AM noise than in ST noise, a consequence of the auditory masking release effect and of the influence of visual information. Children with SLI did not show this effect systematically, suggesting they were less influenced by visual speech. However, when the visual cues were easily identified, the profile of responses to McGurk stimuli was similar in both groups, suggesting that children with SLI do not suffer from an impairment of audiovisual integration. An analysis of percent of information transmitted revealed a deficit in the children with SLI, particularly for the place of articulation feature. Taken together, the data support the hypothesis of an intact peripheral processing of auditory speech information, coupled with a supra modal deficit of phonemic categorization in children with SLI. Clinical implications are discussed.

Degradation of labial information modifies audiovisual speech perception in cochlear-implanted children.

OBJECTIVE: The aim of the present study was to examine audiovisual speech integration in cochlear-implanted children and in normally hearing children exposed to degraded auditory stimuli. Previous studies have shown that speech perception in cochlear-implanted users is biased toward the visual modality when audition and vision provide conflicting information. Our main question was whether an experimentally designed degradation of the visual speech cue would increase the importance of audition in the response pattern. The impact of auditory proficiency was also investigated. DESIGN: A group of 31 children with cochlear implants and a group of 31 normally hearing children matched for chronological age were recruited. All children with cochlear implants had profound congenital deafness and had used their implants for at least 2 years. Participants had to perform an /aCa/ consonant-identification task in which stimuli were presented randomly in three conditions: auditory only, visual only, and audiovisual (congruent and incongruent McGurk stimuli). In half of the experiment, the visual speech cue was normal; in the other half (visual reduction) a degraded visual signal was presented, aimed at preventing lipreading of good quality. The normally hearing children received a spectrally reduced speech signal (simulating the input delivered by the cochlear implant). RESULTS: First, performance in visual-only and in congruent audiovisual modalities were decreased, showing that the visual reduction technique used here was efficient at degrading lipreading. Second, in the incongruent audiovisual trials, visual reduction led to a major increase in the number of auditory based responses in both groups. Differences between proficient and nonproficient children were found in both groups, with nonproficient children's responses being more visual and less auditory than those of proficient children. Further analysis revealed that differences between visually clear and visually reduced conditions and between groups were not only because of differences in unisensory perception but also because of differences in the process of audiovisual integration per se. CONCLUSION: Visual reduction led to an increase in the weight of audition, even in cochlear-implanted children, whose perception is generally dominated by vision. This result suggests that the natural bias in favor of vision is not immutable. Audiovisual speech integration partly depends on the experimental situation, which modulates the informational content of the sensory channels and the weight that is awarded to each of them. Consequently, participants, whether deaf with cochlear implants or having normal hearing, not only base their perception on the most reliable modality but also award it an additional weight.

Binding and unbinding the auditory and visual streams in the McGurk effect.

Subjects presented with coherent auditory and visual streams generally fuse them into a single percept. This results in enhanced intelligibility in noise, or in visual modification of the auditory percept in the McGurk effect. It is classically considered that processing is done independently in the auditory and visual systems before interaction occurs at a certain representational stage, resulting in an integrated percept. However, some behavioral and neurophysiological data suggest the existence of a two-stage process. A first stage would involve binding together the appropriate pieces of audio and video information before fusion per se in a second stage. Then it should be possible to design experiments leading to unbinding. It is shown here that if a given McGurk stimulus is preceded by an incoherent audiovisual context, the amount of McGurk effect is largely reduced. Various kinds of incoherent contexts (acoustic syllables dubbed on video sentences or phonetic or temporal modifications of the acoustic content of a regular sequence of audiovisual syllables) can significantly reduce the McGurk effect even when they are short (less than 4 s). The data are interpreted in the framework of a two-stage "binding and fusion" model for audiovisual speech perception.

The effect of lip-reading on primary stream segregation.

Lip-reading has been shown to improve the intelligibility of speech in multitalker situations, where auditory stream segregation naturally takes place. This study investigated whether the benefit of lip-reading is a result of a primary audiovisual interaction that enhances the obligatory streaming mechanism. Two behavioral experiments were conducted involving sequences of French vowels that alternated in fundamental frequency. In Experiment 1, subjects attempted to identify the order of items in a sequence. In Experiment 2, subjects attempted to detect a disruption to temporal isochrony across alternate items. Both tasks are disrupted by streaming, thus providing a measure of primary or obligatory streaming. Visual lip gestures articulating alternate vowels were synchronized with the auditory sequence. Overall, the results were consistent with the hypothesis that visual lip gestures enhance segregation by affecting primary auditory streaming. Moreover, increases in the naturalness of visual lip gestures and auditory vowels, and corresponding increases in audiovisual congruence may potentially lead to increases in the effect of visual lip gestures on streaming.

Effect of rhythmic attention on the segregation of interleaved melodies.

As previously suggested, attention may increase segregation via enhancement and suppression sensory mechanisms. To test this hypothesis, we proposed an interleaved melody paradigm with two rhythm conditions applied to familiar target melodies and unfamiliar distractor melodies sharing pitch and timbre properties. When rhythms of both target and distractor were irregular, target melodies were identified above chance level. A sensory enhancement mechanism guided by listeners' knowledge may have helped to extract targets from the interleaved sequence. When the distractor was rhythmically regular, performance was increased, suggesting that the distractor may have been suppressed by a sensory suppression mechanism.

Masking release for consonant features in temporally fluctuating background noise.

Consonant identification was measured for normal-hearing listeners using Vowel-Consonant-Vowel stimuli that were either unprocessed or spectrally degraded to force listeners to use temporal-envelope cues. Stimuli were embedded in a steady state or fluctuating noise masker and presented at a fixed signal-to-noise ratio. Fluctuations in the maskers were obtained by applying sinusoidal modulation to: (i) the amplitude of the noise (1st-order SAM masker) or (ii) the modulation depth of a 1st-order SAM noise (2nd-order SAM masker). The frequencies of the amplitude variation fm and the depth variation f'm were systematically varied. Consistent with previous studies, identification scores obtained with unprocessed speech were highest in an 8-Hz, 1st-order SAM masker. Reception of voicing and manner also peaked around fm=8 Hz, while the reception of place of articulation was maximal at a higher frequency (fm=32 Hz). When 2nd-order SAM maskers were used, identification scores and received information for each consonant feature were found to be independent of f'm. They decreased progressively with increasing carrier modulation frequency fm, and ranged between those obtained with the steady state and the 1st-order SAM maskers. Finally, the results obtained with spectrally degraded speech were similar across all types of maskers, although an 8% improvement in the reception of voicing was observed for modulated maskers with fm < 64 Hz compared to the steady-state masker. These data provide additional evidence that listeners take advantage of temporal minima in fluctuating background noises, and suggest that: (i) minima of different durations are required for an optimal reception of the three consonant features and (ii) complex (i.e., 2nd-order) envelope fluctuations in background noise do not degrade speech identification by interfering with speech-envelope processing.

Seeing to hear better: evidence for early audio-visual interactions in speech identification.

Lip reading is the ability to partially understand speech by looking at the speaker's lips. It improves the intelligibility of speech in noise when audio-visual perception is compared with audio-only perception. A recent set of experiments showed that seeing the speaker's lips also enhances sensitivity to acoustic information, decreasing the auditory detection threshold of speech embedded in noise [J. Acoust. Soc. Am. 109 (2001) 2272; J. Acoust. Soc. Am. 108 (2000) 1197]. However, detection is different from comprehension, and it remains to be seen whether improved sensitivity also results in an intelligibility gain in audio-visual speech perception. In this work, we use an original paradigm to show that seeing the speaker's lips enables the listener to hear better and hence to understand better. The audio-visual stimuli used here could not be differentiated by lip reading per se since they contained exactly the same lip gesture matched with different compatible speech sounds. Nevertheless, the noise-masked stimuli were more intelligible in the audio-visual condition than in the audio-only condition due to the contribution of visual information to the extraction of acoustic cues. Replacing the lip gesture by a non-speech visual input with exactly the same time course, providing the same temporal cues for extraction, removed the intelligibility benefit. This early contribution to audio-visual speech identification is discussed in relationships with recent neurophysiological data on audio-visual perception.

Mitral cell temporal response patterns evoked by odor mixtures in the rat olfactory bulb.

Mammals generally have the ability to extract odor information contained in complex mixtures of molecular components. However, odor mixture processing has been studied electrophysiologically only in insects, crustaceans, and fish. As a first step toward a better understanding of this processing in high vertebrates, we studied the representation of odor mixtures in the rat olfactory bulb, i.e., the second-order level of the olfactory pathways. We compared the single-unit responses of mitral cells, the main cells of the olfactory bulb, to pure odors and to their binary mixtures. Eighty-six mitral cells were recorded in anesthetized freely breathing rats stimulated with five odorants and their 10 binary mixtures. The spontaneous activity and the odor-evoked responses were characterized by their temporal distribution of activity along the respiratory cycle, i.e., by cycle-triggered histograms. Ninety percent of the mixtures were found to evoke a response when at least one of their two components evoked a response. Mixture-evoked patterns were analyzed to describe the modalities of the combination of patterns evoked by the two components. In most of the cases, the mixture pattern was closely similar to one of the component patterns. This dominance of a component over the other one was related to the responsiveness of the cell to the individual components of the mixture, to the molecular nature of the stimulus, and to the coarse shape of individual response patterns. This suggests that the components of binary mixtures may be encoded simultaneously by different odor-specific temporal distributions of activity.