Examination of the split fovea theory in a case of pure left hemialexia

Abstract

To address the extent to which the visual foveal representation is split, we examine the case of a patient, M.B., suffering from a left mesial occipital lesion and presenting a pure left hemialexia and a right hemianopia with a spared area of the macula. Reading performance on tachistoscopically presented four-letter words and pseudowords in the spared area of the right visual field was significantly better than reading performance in the intact left visual field. Reading performance in the spared area of the right visual field was also significantly better than reading performance of stimuli centred on the fovea. Moreover, a length effect was found only in the left half of pseudowords centred on the fovea, but not in the right half (up to five letters). These differences in reading efficiency between the left and right halves of the foveal region militates in favour of the split fovea theory and cannot be explained by the bilateral projection theory.

Acknowledgments

We thank M.B. and J.S. for their patience. We also thank Zofia Laubitz for her help. M. Lavidor is a member of the Research Training Network on Language and Brain, funded by the European Community.

Notes

¹ Absence of a cueing effect has also been found in a patient, R.C.G., who has problems reading the beginnings of words (Arduino, Daini, & Silveri, 2005). However, R.C.G.'s deficit is different, because it occurs with words presented in the RVF as well as with words presented in the LVF (stimulus-centred deficit).

² The results of an experiment conducted on M.B. could help us to differentiate between the hypothesis that the RH has problems identifying and/or transferring letters and the crowding hypothesis. We presented normal four-letter strings and four-letter strings with more space between the letters (e.g., w o r d) to the LVF and RVF. The crowding hypothesis would predict better performance with the spaced letters. However, we found that spacing the letters did not improve M.B.'s performance. Moreover, the LVF recognition of the initial letter of spaced-letter strings was superior to the recognition of letters in other positions, just as it was with non-spaced letter strings.