More summary of class discussion during our last meeting...
Pobric, Jefferies, & Lambon Ralph. 2007. PNAS, 104:20137-41
I got an email from Matt recently with some interesting comments and suggestions on our semantic memory posts and reading selection, so hopefully if I get anything wrong in my summary here, he will correct me.
This paper took a novel approach to determining whether the ATL is involved in semantic knowledge. They used TMS to suppress function in the left ATL. They targeted a region in the middle temporal gyrus, 10mm posterior to the tip of the temporal pole. Subjects were then asked to perform three different kinds of naming tasks, (1) basic category picture naming (dog), (2) subordinate category picture naming (poodle), and (3) number naming. They also used two kinds of judgment tasks, (1) synonym judgment, and (2) number magnitude similarity (pick the number closest in value to the target).
TMS to left ATL was found to affect subordinate category naming (slower RTs), but not basic category or number naming, and it also affected synonym but not number judgment (again showing up in RTs). It is concluded that "...the ATL plays a necessary role in semantic cognition in healthy participants." p. 20139.
This is an interesting paper. I like the approach, and there is some interesting theoretical discussion. But I'm not convinced it shows that the ATL plays a necessary role in semantic cognition. Here's why:
1. Both tasks were verbal: picture naming and synonym judgment (on sets of words). The claim being put forward is that the ATL is involved in semantic cognition generally. It would have been nice to see an effect in a non-verbal task, for example, some version of the Pyramids and Palm Trees task, which is used regularly in SD studies. (Of course, given the claims that David and I have put out there regarding the role of posterior temporal areas and lexical-semantic access, I would not expect their results to be restricted to verbal material, but we need to see the data.)
2. A TMS control condition was not utilized. It would have been nice to see TMS delivered to some other area (and one with the same amount of discomfort) to ensure that the observed effects are not some non-specific effect of TMS interacting with the particular stimuli/tasks chosen. Pobric et al. did make an explicit argument for using stimulus controls rather than a TMS control, but I didn't buy it frankly. Relatedly, I'm also not thrilled with the use of a single control task. So number-related behaviors are affected, but does this generalized to ALL non-conceptual semantic abilities? Because of the design, the interpretation of the whole study relies on how well the number tasks generalize to all non-conceptual semantic abilities, and this worries me.
Overall though, I think this study is a great start. I'm looking forward to the next one from this group.
Post a Comment