The Eye Movements of Japanese Pure Alexic Patients During Single Word and Nonword Reading

Abstract

Two Japanese patients with pure alexia, SH and YH, who showed right homonymous hemianopia following a left occipital lobe lesion, demonstrated letter-by-letter (LBL) reading in pronouncing Japanese kana words and nonwords. In contrast to alphabetic letters, each Japanese kana character has an invariant and identical pronunciation whether it appears in isolation or as a component of any word and nonword string. It is important to investigate the eye movements as well as reading latency and duration in Japanese-speaking LBL readers. Relative to normal controls, these patients demonstrated a more robust string-length effect, which was characterized by larger increases in reading latency and duration as well as in the number of fixations as the string length increased. We propose that in pure alexia, parallel activation of orthographic representations is abnormally delayed but not completely abolished.

Notes

¹ These models of word reading differ with regard to processing after parallel identification of individual letters: One model assumes that these identified letters are translated into phonology within a single system that operates in a parallel manner (e.g., Plaut et al., 1996), whereas another model assumes that phonological translation is accomplished by two separate routes, in which the lexical route addresses whole-word phonology from whole-word orthography in a parallel manner and the nonlexical route converts each letter into its phonological counterpart in a serial manner based on a set of pronunciation rules (e.g., Coltheart et al., 2001).

² Japanese orthography is comprised of two scripts—morphographic kanji and phonographic kana (see Patterson et al., 1995, among others, for further description). Whereas the majority of kanji characters have multiple legitimate pronunciations, each kana character has a single transparent and invariant pronunciation.

³ It should be noted that character frequency is not synonymous with word frequency. Kanji comprises approximately 6,000 characters, with about half of these used in daily life. Each kanji character appears as a single-character noun, as a component of multiple-character kanji nouns, kanji-kana compound verbs and adjectives, or one, two, or three of these written word classes. Because word frequency, character frequency and visual complexity are correlated with one another in kanji script, it is not easy to disentangle these effects in clinical observations.

⁴ During reading, the efficiency of letter identification is assumed to deteriorate asymmetrically, with more deterioration for letters that are located on the left than on the right of the fixation point (Nazir et al., 1991). This assumption has been empirically supported by, for example, the data demonstrating that the reading latency increased as a function of the distance of the first fixated letter from the center (or just left of the center) of a string, with a more robust effect when the fixated letter is located on the right than on the left of the center of the string (O’Regan et al., 1984).

⁵ In pure alexic patients, the difficulty in identifying letters located around the final part of a string cannot be simply attributed to the decrease in visual acuity as a function of increasing retinotopic eccentricity. For example, less accurate identification of letters around the final part is also observed when a string is presented briefly to the left of the fixation point, and thus the final letter appears at the center of foveal vision (Kay and Hanley, 1991; Chialant and Caramazza, 1998). Furthermore, in some cases or conditions, poorer letter identification is observed in the central rather than the final part of a string (Reuter-Lorentz and Brunn, 1990; Chialant and Caramazza, 1998). The exact nature of the visual constraint in pure alexic patients should be further investigated.

⁶ The string-length effect on the reading latency in our pure alexic patients (i.e., 210 ms increase per additional character) seems to be small compared with that reported in patients in alphabetic writing systems (see Leff et al., 2001 for a discussion about the absolute size of word-length effect in pure alexia). This is perhaps due to the experimental set-up we employed, in which a voice key was not used to erase the presented stimuli from the display as soon as it detected the subject’s oral response. In the experimental set-up used in our study, the patient may tend to initiate articulation before completion of phonological processing of the entire string, resulting in reduction of the effect of string length on reading latency. In agreement with this assumption, the effect of string length on reading duration was sufficiently large in our patients.

⁷ The criterion of 0.3° was employed to detect a fixation on a character in a string presented in the 20P condition, in which each character subtended 0.4°. However, in some studies, fixations less than 100 ms were eliminated from the analyses due to the possibility that such short fixation times reflect oculomotor programming not specific to reading (e.g., Morrison, 1984; Kambe et al., 2001). When we did not track fixations on each character in the 20P condition and also eliminated these short fixations from the analyses—that is, when fixation was defined as at least three consecutive samples each with a velocity of eye movement of less than 10.0 degree/second, corresponding to eye movement within 1.0° during 100 ms—our normal controls showed a mean duration of fixation of 206 ms before and 197 ms during the oral response, which were similar to those obtained in previous studies, although the mean number of fixations on a single string (1.96 before and 1.80 during an oral response, for a total of 3.76 times) was still large. Upon comparison of the mean number of fixations between the normal controls and the pure alexic patients using this criterion, we obtained the same statistical results as described in the main text: significant main effects of subject group and string length on the number of fixations both before and during the oral response, and a significant interaction between subject group and string length on the number of fixations during the oral response.