Wednesday, November 18, 2009

What counts as auditory cortex?

I'm working on revisions of an fMRI paper that investigates the hierarchical organization of auditory cortex. Not surprisingly, the STS has figured into our findings. One reviewer raised the question of whether the STS is auditory cortex. This is a legit question and one that is not easy to answer.

The reviewer adopted a conservative definition of auditory cortex (conservative on his/her own admission): auditory cortex is cortex that receives projections from the medial geniculuate complex. I kind of like this definition because if we apply it to vision (using LGN projections of course), visual cortex = V1 (and maybe MT). THAT should slow down those greedy cortex grabbers! :-)

But surely this is too conservative. No one would claim that V2 isn't part of visual cortex just because it doesn't receive thalamic projections. So if vision people can claim territory beyond LGN projection fields, we should too.

But then where do we draw the line between auditory cortex and the rest of cortex? Do we base it on cytoarchitectonics? Do we include any auditory-responsive field as auditory cortex (which would include frontal areas!)? Is the notion of unimodal sensory cortex even meaningful?

What do you think?


E.M. said...

It seems like "visual cortex" is defined functionally as "basic sensory areas that help us see," which is why V2 counts as visual cortex. Only "primary visual cortex" is defined anatomically, based on LGN projections. What do you think about a similar set of definitions for auditory?

For auditory cortex in the broad sense, I guess we could include frontal areas and anything else that responds to auditory input, but how useful would that be? I'd guess we don't define visual cortex that way because visual cortex is supposed to do "low level" processing and frontal is supposed to do "high level" processing. That seems like a good basis, but where does low level end and high level begin?

Mazeminded said...

In the end, whatever definition you choose will be arbitrary. All cortex involved in a sense will vary on a scale of the ratio of its involvement with that particular sense and its involvement integrating that information with other regions. Naturally, that would be difficult to measure and quantify, but something along those lines could be used, with an arbitrary line drawn when the percentage of primary sensory processing drops below a certain amount relative to whatever else the region does.

And do you count "reprocessing", or higher level processing, of the same sense? (In vision, hMT+ is taking snapshots of visual information and integrating them over time to perceive motion, which is then used to do all sorts of things.)

It's a tricky question, and I'd say go with whatever arbitrary definition you think is best and sensible--they will listen to you.

David Poeppel said...

I think this is not a question that merits too much thought. I agree with mazeminded -- just pick something and roll with it.

Yale Cohen shows lots of interesting auditory activity in frontal neurons, and we gain little understanding by worrying about whether we should call those regions 'auditory.' We are, presumably, in the business of figuring out how things work, and naming stuff is not part of that exercise.

In terms of parallels between vision and audition, and projection schemes from the geniculate, it's also worth bearing in mind that it's the inferior colliculus that is thought of (loosely) as an auditory analogue to V1, not the putative primary auditory cortex ... It's another disanalogy between vision and hearing.

Anonymous said...

The question seems to be a matter of convention. Using what data would one refute the assertion "STS is part of the auditory cortex"? There's just too much ambiguity here. First, the definition of auditory cortex (need to make an arbitrary choice of criteria; e.g., functional, projection based, cytoarchitectonic). Then, need to put a limit on "part of". If it is rarely associated with auditory input, that would count against being "part of". But the boundary is fuzzy. Finally, what parts of STS are we talking about. In short, I don't find the reviewers point worth too much merit, but on the other hand, maybe it's best to avoid the statement "STS is part of auditory ctx" altogether.

Much of this is a matter of convention/habit in any case. Sanides and Galaburda identified a region in the parietal operculum with cytoarchitectonics of auditory cortex. But the neuroimaging literature, for some reason, tends to focus on the supratemporal plane (colleagues of mine have argued that 'activity' in the parietal operculum is a result of "smoothing").