Pobric, G., Jefferies, E., and Ralph, M. A. L.
Curr Biol, 20:964–968, 2010
DOI, Google Scholar
Abstract
Semantic cognition permits us to bring meaning to our verbal and nonverbal experiences and to generate context- and time-appropriate behavior. It is core to language and nonverbal skilled behaviors and, when impaired after brain damage, it generates significant disability. A fundamental neuroscience question is, therefore, how does the brain code and generate semantic cognition? Historical and some contemporary theories emphasize that conceptualization stems from the joint action of modality-specific association cortices (the “distributed” theory) reflecting our accumulated verbal, motor, and sensory experiences. Parallel studies of semantic dementia, rTMS in normal participants, and neuroimaging indicate that the anterior temporal lobe (ATL) plays a crucial and necessary role in conceptualization by merging experience into an amodal semantic representation. Some contemporary computational models suggest that concepts reflect a hub-and-spoke combination of information–modality-specific association areas support sensory, verbal, and motor sources (the spokes) while anterior temporal lobes act as an amodal hub. We demonstrate novel and striking evidence in favor of this hypothesis by applying rTMS to normal participants: ATL stimulation generates a category-general impairment whereas IPL stimulation induces a category-specific deficit for man-made objects, reflecting the coding of praxis in this neural region.
Review
This is a short TMS experiment investigating the role of the left anterior temporal lobe (ATL) in semantic processing of stimuli. Semantics here is practically defined as the association to a high-level category defining an object. The task was simply to name the object shown on a picture. Involvement of ATL in this task is indicated by patients with semantic dementia who forget the meaning of categories/objects, i.e., they cannot associate a perceived object with its category/class (example: they see a sheep and don’t know what it is – do they still know what a sheep is, if you tell them that it is a sheep?).
The experiment is supposed to differentiate between 3 hypothesis: 1) object meaning results from a distributed representation of a stimulus between all modalities, 2) object meaning is only generated in ATL, other areas provide only sensory input and 3) part of the object meaning is generated already in single modal areas and ATL acts as an amodal integration hub. These hypothesis are only verbally described and indeed it seems difficult to differentiate between 2) and 3).
The experiment shows that 10min of repetitive TMS can increase response times of subjects in the picture naming, but not a number reading task, if TMS was applied to the left ATL. In a post-hoc analysis the authors then devided the shown pictures into living-nonliving and low-high manipulable objects and again looked for interactions with TMS stimulation. They found that stimulation of left IPL, an area associated with manipulable objects, had an effect on nonliving and high-manipulable objects while having no effect on the others. Stimulation of ATL, however, had a (smaller) effect on all categories. Furthermore, stimulation in occipital lobe had no effect with respect to taks or stimulus at all. The authors conclude that this is evidence for hypothesis 3) above.
A major concern with the study is that the main result has been obtained with a post-hoc analysis and the authors did not even specify more precisely which pictures they used in this analysis, e.g., we don’t know which objects were among them. Furthermore, the results do not really allow to make any conclusions about the connectivity of the different regions. Hypotheses 2) and 3) cannot be discerned with the given results. Even hypotheses 1) could still be true, if one assumes that ATL is a region mainly for producing verbal output of a category – something necessary for the task, but not necessarily involved in associating with a category. However, Katharina mentioned that ATL was also implicated in experiments with other output modalities (e.g. drawing). So, what stays, if one believes the post-hoc analysis, is that TMS on ATL disrupts picture naming in general while TMS on IPL disrupts picture naming selectively for nonliving, high-manipulable objects. We cannot rule out any of the hypotheses above completely.