Brain Advance Access originally published online on November 29, 2005
Brain 2006 129(1):158-167; doi:10.1093/brain/awh678
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Patients with hemianopic alexia adopt an inefficient eye movement strategy when reading text
1 Department of Psychology, University of Edinburgh, Edinburgh, 2 Department of Clinical Neurosciences, Royal Free Hospital and University College Medical School and 3 MRC Clinical Sciences Centre and Division of Neuroscience, Faculty of Medicine, Imperial College, Hammersmith Hospital, Du Cane Road, London, UK
Correspondence to: Dr Alexander Leff, Department of Clinical Neurosciences, Royal Free Hospital and University College Medical School, London NW3 2QG, UK E-mail: alexander.leff{at}imperial.ac.uk
Patients with an acquired homonymous hemianopia often adapt over a period of a few months to compensate for some of the impairments caused by their visual field defect. Changes in their eye movement patterns have been demonstrated as performance on visual tasks improves with time; however, these patients often complain of persistent text reading problems. Using a video-based eye-movement tracking system, we investigated the text reading behaviour of patients with established hemianopic alexia (>6 months post deficit), a condition affecting left-to-right readers, with a homonymous field defect that encroaches into their right foveal/parafoveal visual field. Word-based analyses of text reading are standard in experiments involving normal readers, but this is the first time these methods have been extended to patients with hemianopic alexia. Using this method, we compared the patients' reading scanpaths to those generated by normal controls reading the same passages, and a random model generated by matching the patients' eye movement data to random permutations of the text they read. We demonstrate that patients adopt an inefficient reading strategy, fixating to the left of the preferred viewing location of words of four letters and longer. Fixating to the left of the normal preferred viewing location not only results in less of the fixated word being processed by the language system; ensuing fixations are also more likely to land within the same word (a refixation). It is this refixation rate that is the main factor in slowing reading times in these patients. Our data suggests that patients are able to extract some useful visual information from text to aid the planning of reading scanpaths as their behaviour differs critically from the random model. Potential reasons for this patient group failing to produce an effective reading strategy are discussed.
Key Words: hemianopia; reading; eye movements; alexia; fixations
Abbreviations: HA = hemianopic alexia; ILP = initial landing position; RVF = right visual field; WL = word length
Received June 1, 2005. Revised October 3, 2005. Accepted October 10, 2005.
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