We discuss these complementary types of integration in the context of schematic models of audiovisual speech processing. Although it seems like a basic point, from our perspective the available evidence suggests that multisensory processing influences perception at multiple levels (and in neuroantomically dissociable regions).
Finally, one very important aspect worth emphasizing: like all speech processing (Peelle 2012), the details of audiovisual speech processing are likely heavily influenced by the type of stimulus and task that we are doing. So, connected speech (sentences) may provide visual information that aids in processing that is simply unavailable in single words or phonemes. Similarly, phoneme studies (say, with a token of /da/) will not require the lexical competition and selection processes involved in word perception. This is not to say that any of these levels are more or less valid to study; however, we have to be cautious when trying to make generalizations, and sensitive to differences in visual information as a function of linguistic level (phoneme, word, sentence).
There are still many unresolved questions regarding the representations of visual-only speech, and audiovisual integration during speech processing. Hopefully the suggestions Mitch and I have made will be useful, and we look forward to having some more data in the coming years that speak to these issues.
References:
Giraud A-L, Poeppel D (2012) Cortical oscillations and speech processing: Emerging computational principles and operations. Nat Neurosci 15:511-517. doi:10.1038/nn.3063
Peelle JE (2012) The hemispheric lateralization of speech processing depends on what "speech" is: A hierarchical perspective. Frontiers in Human Neuroscience 6:309. doi:10.3389/fnhum.2012.00309 (PDF)
Peelle JE, Davis MH (2012) Neural oscillations carry speech rhythm through to comprehension. Frontiers in Psychology 3:320. doi:10.3389/fpsyg.2012.00320 (PDF)
Peelle JE, Sommers MS (2015) Prediction and constraint in audiovisual speech perception. Cortex. doi:10.1016/j.cortex.2015.03.006 (PDF)
Schroeder CE, Lakatos P, Kajikawa Y, Partan S, Puce A (2008) Neuronal oscillations and visual amplification of speech. Trends in Cognitive Sciences 12:106-113. doi:10.1016/j.tics.2008.01.002