Letter to the Editor |
Department of Neurology, New York University School of Medicine, New York, USA
Correspondence to: E-mail: david.levine{at}med.nyu.edu
Received January 12, 2006. Accepted January 20, 2006.
McDonald et al. (2006)
report that patients who complain of difficulty in reading following left occipitotemporal infarction with right homonymous hemianopia, when asked to read short passages of text, tend to fixate to the left of the position fixated by normal readers in words with four or more letters. This pattern of fixation causes more letters to fall within the defective right visual field. Fewer letters fall within the intact left visual field, so that additional fixations may be required for the word to be identified. Furthermore, the patients' fixations are longer than those of normal readers. The authors conclude that the patients' scan paths are inefficient, as one might expect patients to fixate to the right of the position fixated by normal subjects to compensate for the right hemianopia by allowing more letters to fall within the intact left visual field.
I (Levine and Calvanio, 1978) and others (Kinsbourne and Warrington, 1962) have investigated the capacity of such patients to identify multi-letter arrays, including words, in the intact left visual field. We have consistently found a reduction in the visual letter span. That is, when a group of letters is displayed simultaneously to the left parafoveal visual field, these patients are able to identify fewer letters than normal subjects. The problem is not one of acuity, as the identification of single letters is unimpaired. The problem seems to be that the occipitotemporal lesion disrupts pathways connecting the right occipital lobe to the language-dominant left hemisphere that are required for efficient visual identification of multi-letter arrays required for normal reading
Our findings suggest an explanation of the results of McDonald et al. (2006). The patients fixate to the left of the normal position because they can process fewer letters per fixation in their intact left visual field. Given their limited capacities in the left field, the patients may indeed be pursuing the most efficient strategy for reading available to them. This hypothesis can be tested by correlating the degree of leftward fixation with the reduction of visual letter span in a group of similar patients.
References
Kinsbourne M, Warrington EK. A disorder of simultaneous form perception. Brain 1962; 85: 461–86.
Levine DN, Calvanio R. A study of the visual defect in verbal alexia-simultanagnosia. Brain 1978; 101: 65–81.
McDonald SA, Spitsyna G, Shillcock RC, Wise RJS, Leff AP. Patients with hemianopic alexia adopt an inefficient eye movement strategy when reading text. Brain 2006; 129: 158–67.
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