中華心理衛生協會

Purpose: According to 1970s' dyslexic literature, there are at least two subgroups of dyslexia, auditory-linguistic and visuo-spatial. The former has difficulties with speech articulation, poor sound blending, naming and labeling. The later has predominant visuospatial problems, such as visual discrimination, and visual memory (Ingram, Mason, & Blackburn, 1970; Mattis, French, & Rappin, 1975; Pirozzolo, 1979). This concludes that not only the phonological deficits, but also the visuo-spatial deficits are the cause of many children's reading failures. On the other hand, the best way of examining visuospatial cognition is to study the subject's performance in a mental rotation task. In these experiments in mental imagery, the subjects were presented with two-dimensional drawings that depicted in very different orientations, and with variable angular disparities. The subjects had to determine whether the two-dimensional drawings depicted the same or different object. Shepard and colleagues, as well as other researchers, have produced results indicating that mental manipulation of images involves a sequence of mental transformation similar to those that would be carried out in manipulating real object (Shepard and Metzler, 1971). The position emission tomography (PET) experiments have confirmed the role of the parietal cortex during mental rotation (Harris et al., 2000). While rotation is taking place, its accuracy and speed may critically depend on eye movement, as well as attention control. The saccade and attention control are clearly controlled by the parietal cortex (Stein, 1992). And neuropsychological examinations have led to the conclusion that injury to and/or dysfunction of the inferior parietal region often produces either acquired or developmental dyslexia (e.g., Paulesu et al., 1996). If the dysfunction and/or damage to the inferior parietal cortex are manifested in developmental dyslexia, it is possible that the dyslexic children's mental rotation would be deficient owing to their possible parietal cortical dysfunction. Recent research results have indicated impaired mental rotation ability in dyslexic children (Karádi, Kovács, Szepesi, Szabó, & Kállai, 2001). Though there are many differences between Chinese and Western characters, the application of visuo-spatial ability may be the same through reading. For example, readers have to search for clues among words and sentences back and forth. In the experiments reported here, we examined the performance of dyslexic children in a letter mental rotation task in order to study their allocentric spatial cognition. We hypothesized that Chinese dyslexic students would have worse performance on the mental rotation task. Methods: This study designed the mental rotation tasks based on Shepard and Metzler's paradigm in 1971. All stimuli were presented on screen. Subjects had to judge as soon as possible if the rotated stimuli were positive or mirrored ”R”. The rotation angle ranged from 0 to 360 degree, with an average 15 degree change. Total subjects were 18 dyslexic and 24 normal students, aged from 8 to 12 years old. Results: Excluding the intelligence influence, the results showed that the accuracy of dyslexic students is lower than the normal, but there's not significantly different between the reaction time. Conclusions: While reading, readers have to make effective controls of eyeballs in order to find out sufficient information or clues. The parietal cortex is responsible for the saccade and attention control (Stein, 1992). For this reason, this study concludes Chinese dyslexic students have the visuo-spatial defect so that they have difficulties in reading comprehension, and it's consistent with the hypothesis of the parietal cortex dysfunction of dyslexic student. As our experiment proves a disturbance in mental rotation in the case of dyslexics, there is, therefore, indirect evidence of a dysfunction of the parietal cortex during the performance of the mental rotation task. This paper increases additional evidence in the research because a few experiments had confirmed the parietal dysfunction behaviourally. Leonard et al. (1993) used magnetic resonance imaging (MRI) to demonstrate various structural anomalies in dyslexic children's brains. The anomalies consist of parietal shift in the right hemisphere, atypical Sylvian fissure and multiple Heschl's gyri. The right parietal shift represented deviation in ratio between the size of the right temporal gyrus and the size of the parietal lobe in the planum temporale. The right hemisphere had a significant shift of planar tissue from the temporal to the parietal bank in the planum temporale. On the basis of these morphological observations Leonard et al. (1993) put forward the hypothesis that dyslexic patients may show exceptional allocentric visuo-spatial ability owing to the right parietal shift. This hypothesis is, apparently, surported by our observations that, among the dyslexics, the proportion of children with difficulties in an allocentric mental rotation task is higher than among normally reading children. Results in much recent research oriented towards dyslexia show that the core deficit of developmental dyslexia is the phonological disability of the conscious process of word identification (Shaywitz, 1998). However, it seems correct to regard dyslexia as a form of learning disability which is also accompanied by other non-linguistic (vestibular-spatial dysfunction) deficits.