Semantic memory -- the store of knowledge that allows us to produce and comprehend language, to recognize and make inferences about objects, and to understand and act in the world around us -- is critical to virtually all aspects of everyday life. A key point of controversy for current theories about the neural basis of semantic memory concerns the role of the anterior temporal lobes (ATLs), with different theorists proposing (1) that both left and right ATLs are crucial for all aspects of semantic processing, (2) that the left ATL encodes lexical representations but otherwise these regions are not involved in semantic processing, or (3) that ATL regions contribute to the recognition and retrieval of information about unique entities (e.g. Lassie, Eiffel Tower), but not to semantic processing for more general categories (e.g. dog, building). Studies of semantic memory in neuropsychological populations can provide trenchant evidence for adjudicating these hypotheses, but to date, most such work has focused on patients with degenerative brain disease where it is difficult to attribute observed patterns of impairment to focal ATL damage. In contrast, patients who undergo ATL resection to alleviate intractable epilepsy have well-delineated unilateral lesions focally restricted to the ATL -- thus post-surgical impairments in this group are unambiguously attributable to ATL pathology, and so can provide strong evidence for adjudicating the different theories. Some previous studies have shown significant and debilitating impairments to naming and to face recognition in some such patients, but the existing literature is difficult to interpret for three principal reasons. (1) No study has comprehensively assessed different verbal and nonverbal aspects of semantic memory in the same patients. (2) No study has carefully controlled or manipulated psycholinguistic factors known to strongly influence performance on semantic tasks in other patients with ATL pathology. (3) There is substantial inter-individual variability in this patient group, and the factors that influence outcomes on different cognitive measures are poorly understood. The current project proposes to redress these issues by undertaking a comprehensive assessment of verbal and nonverbal semantic abilities in a large case-series of pre- and post-surgery patients with anterior temporal-lobe epilepsy (aTLE). The assessment will use an innovative new battery of semantic tasks designed to carefully manipulate factors known to influence performance in other semantically impaired patient groups. This research will provide important new evidence regarding the neural basis of human semantic memory by testing specific predictions of three different theories that predominate in the current literature. It will also provide valuable new information for patients suffering from aTLE about the cognitive consequences of the disease and of surgical intervention to alleviate the disease.
This project proposes to undertake detailed assessment of verbal and nonverbal semantic abilities in a case series of ~100 patients with anterior temporal-lobe epilepsy (aTLE), using innovative new measures that carefully manipulate psycholinguistic factors known to influence semantic task performance in other forms of temporal-lobe pathology. The project will produce evidence critical to adjudicating among three different theories about the neural underpinnings of semantic memory;and will yield new information about the disorder that will help patients make decisions about how to manage their health.
| Lambon Ralph, Matthew A; Ehsan, Sheeba; Baker, Gus A et al. (2012) Semantic memory is impaired in patients with unilateral anterior temporal lobe resection for temporal lobe epilepsy. Brain 135:242-58 |
