Visual speech speeds up the neural processing of auditory speech van Wassenhove, V., Grant, K. W., & Poeppel, D. (2005) Proceedings of the National Academy of Sciences, 102(4), Jaimie Gilbert Psychology 593 October 6, 2005

Audio-Visual Integration Information from one modality (e.g., visual) can influence the perception of information presented in a different modality (e.g., auditory) Speech in noise McGurk Effect

Demonstration of McGurk Effect Audiovisual Speech Web-Lab -index.html Arnt Maasø University of Oslo McGurk_english.html

Unresolved questions about AV integration Behavioral evidence exists for vision altering the perception of speech, but… When does it occur in processing? How does it occur?

ERPs can help answer the “when” question EEG/MEG studies have demonstrated AV integration effects using oddball/mismatch paradigms These effects occur around ms A non-speech ERP study with non-ecologically valid stimuli demonstrated earlier interaction effects (40-95 ms) (Giard & Peronnet, 1999) Does AV integration for speech occur earlier than ms?

There’s a debate about the “how” question… Enhancement Audio-visual integration generates activity at multi-sensory integration sites, information possibly fed back to sensory cortices VS. Suppression Reduction of stimulus uncertainty by two corresponding sensory stimuli reduces the amount of processing required

The Experiments 3 experiments were conducted Each had behavioral and EEG measures Behavioral: Forced choice task EEG: Auditory P1/N1/P2 26 participants Experiment 1: 16 Experiment 2: 10 Experiment 3: 10 (of the 16 who participated in Experiment 1)

The Stimuli Audio /pa/ Audio /ta/ Audio /ka/ Visual /pa/ Visual /ta/ Visual /ka/ AV /pa/ AV /ta/ AV /ka/ Incongruent AV with Audio /pa/ + Visual /ka/ 1 Female face & voice for all stimuli In Exp. 1 & 2, each stimuli presented 100 times; total of 1000 trials

Experiment 1 Exp. 1 Stimuli presented in blocks of audio, or blocks of visual, or blocks of AV (congruent and incongruent) Participants knew before each block which stimuli were going to be presented

Experiment 2 Exp. 2 Stimuli presented in randomized blocks containing all stimuli types (A, V, Congruent AV, Incongruent AV) to reduce expectancy Task for both experiments: choose which stimuli was presented; for AV--choose what was heard while looking at the face

Experiment 3 Presented 200 Incongruent AV stimuli Task: choose what syllable you saw, neglect what you heard In all experiments, correct response to Incongruent AV = /ta/

Waveform Analysis Retained 75-80% of recordings after Artifact Rejection and Ocular Artifact Reduction Only correct responses were analyzed 6 electrodes used in analysis: FC3, FC4, FCz, CPz, P7, P8 Reference electrodes: Linked mastoids

Results This study’s answer to “How” Suppression/Deactivation Hypothesis AV N1 & P2 amplitude were significantly reduced compared to Auditory-alone peaks Performed separate analysis to determine if summing the responses to unimodal stimuli would result in the amplitude reduction present in the data—this was not the case; therefore the AV waveform is not a superposition of the 2 sensory waveforms, but reflects actual multisensory interaction.

Results: Experiment 1 N1/P2 Amplitude AV < A (p <.0001) N1/P2 Latency AV < A (significant, but confounded by interaction) Modality x Stimulus Identity P < T < K (p <.0001) Latency effect more pronounced in P2, but can occur as early as N1

Results: Experiment 2 N1/P2 Amplitude AV < A (p <.0001) N1/P2 Latency AV < A (p <.0001) Modality x Stimulus Identity (p <.06)

Results: comparison of Exp. 1 & Exp. 2 Similar results for Exp. 1 & 2; Temporal facilitation varied by Stimulus Identity but amplitude reduction did not; No evidence for attention effect (i.e., for expectancy affecting waveform morphology)

Temporal facilitation depends on visual saliency/signal redundancy More temporal facilitation is expected to occur if: The audio and the visual signals are redundant The visual cue (which naturally precedes the auditory cue) is more salient (Figure 3)

Results: Experiment 3/Incongruent AV Stimuli Incongruent AV stimuli in Exp. 1 & 2: no temporal facilitation Amplitude reduction present and equivalent to reduction seen for Congruent AV stimuli Experiment 3: Both temporal facilitation and amplitude reduction occurred

Visual speech effects on auditory speech Perceptual ambiguity/salience of visual speech affects processing time of auditory speech Incorporating visual speech with auditory speech reduces the amplitude of N1/P2 “independent of AV congruency, participant’s expectancy, and attended modality” (p. 1184)

Ecologically valid stimuli Suggest that AV speech processing is different from general multisensory integration due to the ecological validity of speech

Possible explanation for amplitude reduction Visemes provide information regarding place of articulation If this information is salient and/or redundant with auditory place of articulation cues (e.g., 2 nd and 3 rd formants), the auditory cortex does not need to analyze these frequency regions, resulting in fewer firing neurons

Analysis-by-Synthesis Model of AV Speech Perception Visual speech activates internal representation/prediction This representation/prediction is updated as more visual information is received over time Representation/prediction is compared to the incoming auditory signal Residual errors to this matching process are reflected by temporal facilitation and amplitude reduction effects Attended modality can influence temporal facilitation

Suggest 2 time scales for AV integration 1: feature stage 25 ms Latency facilitation (sub-)segmental analysis 2: perceptual unit stage 200 ms Amplitude reduction Syllable level analysis Independent of feature content and attended modality

Summary AV speech interaction occurs by the time N1 is elicited ( ms) Processing time of auditory speech varies by the saliency/ambiguity of visual speech Amplitude of AV ERP reduced when compared to amplitude of A-alone ERP

Questions Dynamic visual stimulus and ocular artifact If effects of AV integration are influenced by attended modality, would modality dominance also influence these effects? Are incongruent AV/McGurk stimuli ecologically valid?