Tuesday, September 15, 2009

A multisensory cortical network for understanding speech in noise














mcgyrus said...

Hi Greg,

I also found this paper interesting in terms of the use of the 'Understood'>'Heard' (U>H) contrast. The post-test run with a subset of participants shows that they were able to identify the 'heard' stimuli with mean 66% accuracy - pretty good going for a chance level of 25%. The 'understood' stimuli were identified with mean 87% accuracy. What this indicates to me (as the authors also note) is that the participants were responding 'Understood' in the scanner only when they could almost certainly identify the syllable, which in turn is reflected in a network of atypical activations for a speech study in the U>H contrast that could represent post-perceptual decision-making processes.

I should, however, point out that the authors do also report an activation in left anterior STS for the U>H contrast, albeit for auditory-only trials. This is very much in line with Scott et al. (2000). It's mentioned in the caption for Figure 2.

Finally, I think it is a bit too strong to interpret the bilateral activations in the SNR contrast as reflective of 'phonemic perceptibility', as the regressor hasn't been related to any behavioural measure of perception. I would rather have seen the authors modulate the SNR regressor for VCV identification scores (if these were available) instead of the more fuzzy understood/heard judgement. This would have perhaps yielded a contrast image that could be more reflective of the participant's perceptual experience, and furthermore more interpretable in terms of the left-right balance of task-related processing.

Greg Hickok said...

Regarding the left anterior STS U>H result, it is puzzling that they found it only in the auditory-only condition. I don't know how to interpret that, but I don't think it supports Scott et al.'s position. Don't you think that a region that is critical to the perception of intelligible speech should activate whether there is visual information or not? Further, it is interesting that the aSTS showed up in the contrast that you suggest highlights "post-perceptual decision-making processes." No matter how you look at it, the result does not support Scott et al.'s view.

You are right that the SNR regressor was not related to any behavioral measure of perception, so my claim is probably too strong. However, I think it is a very safe bet to assume that if you asked Ss to rate perceptibility on an x-point scale (rather than a binary judgment), SNR would be strongly correlated with perceptibility ratings. Assuming this is true, the SNR variable does reflect phonemic perceptibility. Hey that's a good idea of a study. Lee, you on it? :-)

Unknown said...

Hi Greg and mcgyrus,
These are great suggestions, and we'll certainly keep them in mind for subsequent studies. Thanks so much for the input!

Unknown said...

Greg and mcgyrus,

Thank you so much for the encouraging comments! I apologize for the delay in my post!

Greg, I generally agree with you that the SNR regressor is a good indicator of available information or where target information is represented. However, how much of the SNR regressor activity has to do with 'phonemic perceptibility' or is specific to speech is difficult to say. We'd probably see similar results for target detection of non-speech stimuli (i.e., same paradigm, but a non-speech target), but the results would probably vary qual/quantitatively in their spatial extent in earlier auditory areas, depending on which features are maintained in the non-speech target. In fact, it might be fun to use a paradigm like this to compare spatial extent of activity in early auditory areas using speech and non-speech stimuli (not terribly novel, maybe even a half-baked idea, but it might be fun). We took a stab at this using bird calls, but the data were underpowered and ill suited to clarify this point (the experiment was meant to clarify other points), so we omitted them from the manuscript.

mcgyrus, thank you for pointing out the audio-only panel in figure 2. I'd like to add that we did see the left anterior STS activity in the U>H contrast across all conditions prior to including the SNR regressor, but the activity was small (a handful of voxels) and borderline significant at FDR p<0.05. The SNR regressor accounted for enough variance to drop STS below significance across all conditions. However, we were interested in how our findings in anterior STS compared to other auditory only studies and reported those data at a relaxed threshold.
Thank you again!