关键词: 发展性阅读障碍, 视听时间敏感性, 脑神经机制

Abstract: Developmental dyslexia refer to a pattern of specific learning difficulty characterized by problems with accurate or fluent word recognition, and it can’t be explained by the patients’ intelligence and inadequate education. As for the cognitive mechanism of dyslexia, there still no clear conclusion in current researches. Of them, phonological deficit theory is the most accepted by researchers. Phonological deficit theory considers that dyslexia is the result of impaired auditory modality. However, reading is a process not only needs auditory modality, but also visual modality. It requires learning the binding of graphemes and phonemes automatically. Nowadays, more and more researchers found that reading impairment is the reflect of audiovisual integration deficit not limited in single modality. The most typically theory base on cross-modality processing deficit is graphemes-phonemes binding theory. It raises that dyslexia readers couldn’t integrate visual inputs and auditory inputs into unit, then result in reading difficulties. Graphemes-phonemes binding deficit as one of the most important performance of dyslexia, its potential mechanism maybe deficit in more basic audiovisual cognitive process. A series of research found that dyslexia readers have audiovisual temporal sensitivity deficit during audiovisual processing. Audiovisual temporal sensitivity means the sensing capability one perceive the onset time between visual stimulus and auditory stimulus. Some researchers put forward that because dyslexia readers have difficulty in judging timing about audiovisual information, so that they have ambiguities in the pair relationship between graphemes and phonemes and then unable to construct text normally. Audiovisual temporal sensitivity can be explored through three types of task: the first one is simultaneity judgment task, which require subjects to judge if the audiovisual stimulus is happen simultaneously; the second one is temporal-order judgment task, which needs subjects to judge the order between auditory stimulus and visual stimulus; the last one is audiovisual integration task, which asks participants report their perceived fusion stimulus under synchronous or asynchronous audiovisual stimulus. Timing window is the main index of audiovisual temporal sensitivity, it means the time span between two different onset stimulus. Many studies using these tasks to explore audiovisual temporal sensitivity evidenced that compare with typical readers, people with developmental dyslexia show deficits in audiovisual temporal sensitivity, and their timing window is wider than typical readers. Adequate binding between graphemes and phonemes needs happen in a narrow timing window, and broaden audiovisual timing window will result in such binding construct unsuccessfully, and finally result in dyslexia. Some intervention studies found that the timing window has plasticity in healthy people and the span of timing window can narrow after training. Therefore, training audiovisual temporal sensitivity of dyslexia readers and narrowing their timing window may have great potentials in improving dyslexia symptom. Some neuropsychological studies found that the brain region working in the audiovisual timing processing located in posterior Superior Temporal Sulcus(pSTS), which receive signals from visual cortex and auditory cortex and integrate these signals. Compare with typical readers, the activation of pSTS in dyslexia readers is abnormal, it maybe the neuro mechanism of audiovisual temporal sensitivity deficit. Also, some Electroencephalogram studies found that Mismatch Negativity(MMN) and Late Discriminatory Negativity(LDN) evoke abnormally in dyslexia readers and it relate to their audiovisual temporal sensitivity deficit. In conclusion, many studies have evidenced that compare with typical readers, people with developmental dyslexia show deficits in audiovisual temporal sensitivity not only in the behavior level, but also in the neuropsychological level. However, most of such studies were conducted under the context of alphabetic language. There still small amount of research about such domain in Chinese culture. Future researches need improve the experimental design, enlarge the study about audiovisual temporal sensitivity in the context of Chinese, and develop interventions based on temporal sensitivity, so that provide references about the core mechanism and clinical treatment for dyslexia.

Key words: developmental dyslexia, audiovisual temporal sensitivity, brain mechanism