MENTAL ROTATON OF LETTERS IN DYSLEXICS AND IMPLICATIONS FOR DIAGNOSIS AND EDUCATIONAL TREATMENT

A classical mental rotation task (Cooper & Shepard, 1975) was performed with dyslexics and non-dyslexics. Group differences were found for mean reaction time but there was no difference in mental rotation effects between the groups. The results were interpreted within the framework of the Functional Coordination Deficit model (Lachmann, 2002). It was concluded that dyslexics rotate the letters mentally just as good as non-dyslexics but have a problem to finally decide, whether the letter is normal or mirrored. This problem occurred because of a failure in suppression of symmetry in the representation of graphemes. Dyslexics represent letter like objects. Therefore, they fail to discriminate between orientational alternatives and for the same reason fail to establish unambiguous relation between grapheme and phoneme representations. According to the International Classification of Diseases, developmental dyslexia is a specific disability in learning to read and to spell adequately despite of at least normal intelligence, adequate instruction and socio-cultural opportunities. Furthermore, sensory defects in vision and hearing should be excluded as to be responsible for the problems in reading and writing. One of the major symptoms of dyslexia are reversal errors, that is, the children confuse letters which are horizontally or vertically symmetrical to each other or rotated (e.g., p / q; b / d; p / d). These errors are normal in beginning readers but become very rare after practice. In dyslexics, however, they still occur even after years of reading experience. Orton (1925, 1928) postulated that these problems reflect the cardinal symptom of developmental dyslexia. He assumed that these confusion errors (strephosymbolia) are caused by a faulty development of cerebral dominance and interhemispheric communication. Later on, phonological skills were considered as to be more relevant to explain dyslexia (e.g., Bradley & Bryant, 1978; Liberman et al., 1971). In this context, it was often argued that Orton believed that dyslexics have a perceptual deficit and actually see letters in a wrong orientation (Vellutino, 1977). However, as argued by others (Corballis & Beale, 1993; Lachmann, 2002; Lachmann & Geyer, 2003), Orton assumed reversals to be the result of labeling problems in memory and not of problems in visual perception. There are a couple of alternative models which try to connect the original ideas of Orton with more recent neurological theories (Corballis & Beale, 1993) and experimental results (Lachmann & Geyer, 2003; Brendler & Lachmann, 2001). One is the multicausal Functional Coordination Deficit (FCD) model (Lachmann, 2002). Within this model, reversal errors are explained as a result of a failure in suppressing symmetry generalization in reading. Symmetry generalization, as a result of evolutionary and individual development, is assumed to be learned as an infant to warrant behavioral advantages, such as object constancy. In reading, however, this mechanism may be a hindrance, because graphemes are visual symbols, and as such they have to have a non-ambiguous relation to the respective phonological information they represent (Friederici & Lachmann, 2002). In this context, it is argued that learning to read is comprised of learning to treat graphemes as symbols instead of objects. This skill is achieved very early, during the first stage of reading acquisition. A failure in complete suppression of symmetrical information in the representation of visual symbols during reading produces ambiguous relations between visual and phonological information and thus disturbs the functional coordination and may cause problems in learning to read. In this respect, reversal errors do not reflect a visual perceptual deficit (Vellutino, 1977) or an incomplete hemispheric dominance (Orton, 1925), and they do not reflect the one and only cardinal symptom of dyslexia. Instead, the FCD assumes that this kind of problem occurs in a subgroup but not in all dyslexics (Lachmann, 2002; Lachmann & Geyer, 2003). This assumption indicates that reversals are not necessarily the result of problems in the processing of spatial information per se, neither for objects nor for symbols. They are rather an effect of a disability to represent symbols, such as graphemes, strictly view-point dependent. In particular, problems occur when different orientations have to be represented and performed distinctly to be connected unambiguously with distinct phoneme categories. This hypothesis was tested using a classical mental rotation task (Lachmann, Rusiak, Brendler & Jaskowski, in preparation). In this task a rotation has to be performed mentally in order to decide whether a letter is of normal or mirrored orientation. It is predicted that dyslexics rotate the visible letter as good as others in their imagination, but have a problem to finally decide upon the orientation of this letter. They know that the letter is, for instance, a „R”, but they have problems to match this activated phoneme to the corresponding grapheme orientation. This will result in increased response latencies and error rates because of increased decision time but not in differences in the mental rotation. If the problem is more related to spatial operations, however, there should be an increase in the operation time with rotational angle. The experiment will be described in more detail in later sections after we reviewed some studies of mental rotation.