One bird with two stones: Abnormal word length effects in pure alexia and semantic dementia

Abstract

In pure alexia (PA)—an acquired reading disorder consequent on posterior left-hemisphere stroke—the hallmark is a pronounced and abnormal impact of word length on reading speed. Some patients with semantic dementia (SD)—a neurodegenerative condition affecting semantic memory—have also been reported to show an abnormal word length effect (AWLE) in reading, even though they are not thought to have the basic visual-processing deficits hypothesized to underlie this phenomenon in PA. In the current study, an AWLE in reading was consistently observed in both PA and SD patients, but further manipulations demonstrated marked differences between the groups in the conditions that produce the length effect, its specific manifestation, and the pattern of other deficits accompanying it. All of the results are compatible with the twin hypotheses that the AWLE in reading arises from a visual-processing deficit in PA but from reduced top-down lexical/semantic support for word identification in SD. In other words, the AWLE in the two patient groups appears to be a common symptom arising from different underlying deficits: one bird with two stones.

We are indebted to the many patients and their families for their willingness to devote considerable time and effort to this experiment. We are also grateful to Dr B. Weekes, who allowed us to use his stimuli (Experiment 4) and to Professors J. R. Hodges, R. J. S. Wise, and M. A. Lambon Ralph, and Dr E. Warburton for kindly granting us access to the patients. Our thanks go to Jo Drake, John Keane, and Jane Powis for help in data collection (Experiments 1 and 2), and to Anna Woollams for assistance with additional reading data for the patients with semantic dementia. TBC was funded by a Commonwealth Scholarship, and MV was supported by a Fogarty Senior International Fellowship and the Medical Research Service of the U.S. Department of Veterans Affairs. KSG and KP acknowledge the support of the Medical Research Council in the UK.