Shared versus separate processes for letter and digit identification

Abstract

Letters and digits, although similar in many respects, also differ in potentially significant ways. Most importantly, letters are elements of an alphabetic writing system, whereas digits are logographs. In this article, we explore whether letters and digits are identified by a single character identification process that makes no fundamental distinction between the two types of characters, or whether instead letter and digit identification processes diverge at least in some respects. We present evidence from an acquired dyslexic patient, L.H.D., who is impaired in both letter and digit identification. Working within a theoretical framework specifying the levels of representation implicated in letter identification, we systematically compare L.H.D.'s letter and digit processing. The results provide evidence that letter and digit identification implicate the same levels of representation, and further that L.H.D.'s identification errors for both letters and digits arise at the same point in processing. On the basis of these results, we argue for a shared process that mediates identification of both letters and digits. Finally, we discuss relevant previous results in light of this conclusion.

Keywords:

• Reading
• Letters
• Digits
• Pure alexia
• Acquired dyslexia
• Numerical

The second author was supported by an National Science Foundation (NSF) Integrative Graduate Education and Research Traineeship (IGERT) grant to the Johns Hopkins Department of Cognitive Science. We are grateful to L.H.D. for her cooperation and patience over many sessions of testing, and we thank Jeremy Purcell for creating the figure of L.H.D.'s lesion. In addition the pure alexia database compiled by Amalie Hjorth Ditlevsen was invaluable as a source of pure alexia articles.

Notes

1 Although L.H.D. worked in the banking industry, her responsibilities focused on strategic planning and did not involve unusual amounts of exposure to digits (as, for example, might have been the case for a bank teller or accountant). Rather, her premorbid experience with digits and letters appears to have been typical for an educated professional.

2 L.H.D. showed no awareness of differences in composition among lists, and it is quite unlikely that she acquired explicit knowledge of the two categories. (Indeed, after constructing and administering the tasks, and scoring, analysing, and otherwise pondering the results, the two authors remain unable to sort the consonants into categories.)

3 It is worth noting that all three of the cross-category errors occurred on early trials of pure set lists, perhaps while out-of-set letters retained some residual activation from reading that occurred before presentation of the list. One of the errors (a J to V substitution) occurred on the first trial of a Pure Set 2 list, immediately following a mixed-set list in which the next-to-last trial included a V. The remaining two errors involved reading the LL in HLLRJB as KK on Trial 3 of the first list presented in the experiment. No cross-category errors occurred after Trial 3 in any of the four 25-trial pure set lists.

4 The reported word and pseudoword reading results demonstrate impaired reading, but do not point specifically to a letter identification deficit. The report that letter-naming errors frequently involved substitution of visually similar letters is suggestive of a letter identification deficit, but no further details are provided. Finally, only 12 letters were tested in the upper–lower-case matching task, and the nature of the task could have artificially inflated the error rate. The patient was required to draw lines from a column of upper-case letters to a column of lower-case letters on a page, matching each upper-case letter with its lower-case counterpart. As the task progressed, the page presumably became increasingly cluttered with lines, and an error for one letter (e.g., a line drawn from S to z) could have the consequence that the correct lower-case match to a subsequent upper-case letter (e.g., Z) had already been chosen as a response. The patient's letter identification difficulty might therefore have been less significant than her score of 8/12 suggested.