読みの精神物理学 : 有効視野の役割を中心に

Recent reviews of the role of eye movement during text reading indicate that an analysis of saccade length and fixation duration would be a useful tool for understanding cognitive processing during reading. The effective visual field is the region from which we can obtain useful information during each eye fixation and it is known to be relatively small. In that sense, this small region is called window of consciousness during reading. Many of the printed words and characters are seen during a fixation only in the sense that the reader knows that some wordlike object is in a given location. We take the details extracted from several fixations and integrates them into a perception that the detail from a wide area is seen on each fixation. The size of the effective field of view was estimated using the moving window technique in which a kind of field restriction technique was used. The moving window technique, as compared with static window technique, involves highly sophisticated real-time feedback, and provide the most definitive information about the size of the effective visual field size during reading. We found that kanji-based text has a wider effective visual field size, i. e., 5 to 6 characters, necessary for reading than hirakana-only text. The wider span is consistent with the longer saccade length for kanji-based text. We also studied detection of isolated kanji and hirakana using spatial filtering techniques, i.e., Gaussian, Laplacian-Gaussian and low-pass, and found that the space constant required for detection was larger for hirakana than for kanji. Moreover, the critical spatial frequency bandwidth required for hirakana was found to be lower (2 cycles/character) than for kanji (4_8 c/c). The results show that kanji and hirakana are detected in different ways in the early stages of visual processing. It is assumed that native Japanese has a character-dependent dual strategy for computing the saccade length and fixation duration to the next word in the parafovea during reading. As respects reading direction, there is some physiological ground to believe that a horizontal text reading may be better than vertical text reading : visual acuity falls off rapidly in the vertical direction than in the horizontal direction. However, the fact that no directional advantage of horizontal over vertical reading suggests that this physiological ground may have a negligible effect on reading Japanese texts. Other related issues such as asymmetry of effective visual field, visual search guidance, and linguistic aspects of kanji and hirakana are discussed in terms of cognitive psychophysics of reading.