Monday, January 28, 2008

Semantics and Brain - 3rd meeting: Specificity of ATL involvement in SD

This week we read over a bunch of papers looking at the distribution of atrophy and metabolic changes in semantic dementia, as well as correlation of these measures with semantic and naming tasks. Our goal was to determine whether the semantic deficits in this syndrome can be linked to ATL dysfunction, as some have claimed. As Patterson et al. (2007, NRN, 8:976-88) put it, the strength of this view "...hinges, however, on claims about the relatively focal nature of the pathology in SD." p. 980.

So how focal is it? Not so focal. Here's a few quotes and info from the papers we read this week:

"Patients with SD as a group, compared with controls, showed hypometabolism over the whole left temporal lobe... and in the right temporal pole...." Diehl et al. 2004, Neurobiology of Aging, 25:1051-6, p. 1053 (see figure below)

"...even patients in the mild SD subgroup were found to have bilateral temporal polar and amygdalar atrophy bilaterally, plus left-sided atrophy of the hippocampus, parahippocampal, fusiform, and inferior and middle temporal gyri. In the moderate SD group, the atrophy also involved the right parahippocampal, fusiform, and inferior and middle temporal gyri." Galton, et al., 2001, Neurology, 57:216-25, p. 221

Grossman et al (2004, Brain, 127:628-49) reports that, "...significant grey matter atrophy was evident in several areas of the left emporal cortex in SD." p. 635. These included (from Table 4, p. 637), Left ventral temporal, Left anterior temporal, Left posterolateral temporal, & Left parahippocampal.

Correlational analyses are any more focal:

Grossman et al. (2004, same as above) report significant correlation between naming performance and grey matter atrophy in two left ventral temporal sites (one quite posterior involving BA 19/37), right posterolateral temporal, and bilateral occipital.

The paper by Williams et al. (2005, NeuroImage, 24:1042-51) which included SD patients as well as the frontal variant FTD patients and correlated semantic scores across all patients, reports, "The analysis of correlation with the semantic test scores yielded extensive cluster in the left temporal lobe that included temporal pole... parahippocampal gyrus and fusiform gyrus ... superior and middle temporal gyrus... and inferior temporo-occipital region." p. 1046 (see figure below)
And Galton et al. (2001, same as above) report correlations between semantic tasks and volume measurements in the left fusiform.

Conclusion: data from semantic dementia does not conclusively implicate the ATL, unless by "ATL" one is willing to include minimally (e.g., see Nestor et al. 2006, NeuroImage, 30:1010-20 for the most "focal" results of all the papers we've read), limbic structures in the medial temporal lobe (hippocampus and surrounding cortices), and ventro-lateral temporal lobe structures extending (on the left) posteriorly to include about half the entire temporal lobe.

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