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What Explains the Loss of Object Knowledge in Semantic Dementia?

Poor performance on a nonverbal test of object knowledge by patients with SD does not necessarily imply the presence of a supramodal semantic system.

These researchers administered a novel nonverbal test of object knowledge, the Feature Reality Test (FRT), to patients with semantic dementia (SD), a clinical subtype of frontotemporal lobar degeneration associated with atrophy of the anterior temporal lobes. Results of previous studies of object knowledge in SD were confounded by the use of verbal tests, because these patients have deficits in word comprehension. This study focused on multimodal object knowledge (e.g., visual and auditory attributes) without the use of language. The investigators instructed participants to select the "more real" object from image pairs that differed in the conditions of color, sound, environmental context, and motion. Twelve patients with SD and 23 nondemented controls completed the FRT and a battery of established semantic tests.

Overall, the SD patients were significantly less accurate than controls. Their FRT performance correlated with performance on other semantic tests. Patients’ accuracy increased when they could name the object, and they performed consistently across most of the attribute conditions. The authors interpret their results as evidence for a unitary, supramodal semantic system that, in patients with SD, undergoes degradation with progressive anterior temporal lobe degeneration.

Comment: From a neurologic perspective, the four conditions of the FRT are not equivalent in complexity. Color and sound judgments are distinctly modality-dependent, whereas decisions about an object’s environmental context and inherent motion rely on sensory integration and subjective experience. Therefore, patients’ mistakes regarding context and motion are hard to ascribe to deficiency in any single modality. Further, mixed disease severity in the small patient sample, exclusion of individuals from some analyses, and an excessive number of analyses may have influenced interpretation. Although the FRT is a step in the right direction for probing the neural basis of object knowledge, this test, as used here, does not definitively clarify whether the anterior temporal lobe is a point of integration for multimodal information or a unitary supramodal semantic "center" where object knowledge itself resides.

— Emily J. Rogalski, BA, and Sandra Weintraub, PhD

Ms. Rogalski is NRSA Predoctoral Trainee, Cognitive Neurology and Alzheimer’s Disease Center and Neuroscience Institute, Northwestern University Feinberg School of Medicine, Chicago. Dr. Weintraub is Professor and Director of Neuropsychology, Cognitive Neurology and Alzheimer’s Disease Center, Neuroscience Institute, and Departments of Psychiatry and Neurology; Northwestern University Feinberg School of Medicine, Chicago.

Published in Journal Watch Neurology August 8, 2006

Citation(s):

Garrard P and Carroll E. Lost in semantic space: A multi-modal, non-verbal assessment of feature knowledge in semantic dementia. Brain 2006 May; 129:1152-63.

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