Tetra - Visual dyslexia video tutorial

Developmental dyslexia is a clinical condition characterized by reduced lexical performance as a function of the age. It is a worldwide problem, found in countries with very different languages and graphical systems. Its prevalence ranges from 1% in Egypt up to 20-25% in Brazil. In general its average prevalence in the world is about 3-5 %. Its aetiologi is not yet clear. However it is very likely the eredofamiliar component plays an important role. Undoubtedly, developmental dyslexia is a neuropsychiatric and logopedic condition. Still, even if the visuoperceptive aspect has been considered less important to date, signs and symptoms as well as a growing body of evidence suggest the occurrence of subtle alterations involving the visual domain in a subgroup of patients. We will name these patients as "visual dyslexics". This finding is predictable, considering that when reading letters have to be correctly perceived and processed before their conversion into phonemes takes place. Clearly, the perception and processing of letters is provided by the visual channel. Probably the reason for why the visual approach has been under-estimated so far, is that during an ordinary ophthalmological and orthoptic examination dyslexic children turn out to be normal. Besides, it is restrictive to believe that just for this reason the patient does not conceal slight visuoperceptive alterations: in fact, they are not detectable as such by using the ordinary clinical approach but could be revealed by specific psychophysical techniques. Really, the neurophtalmological alterations as reported in literature refers both to the oculomotor and sensory domain. A strand of research maintains that the visual dyslexics suffer from instability of motor dominance, so as to make imprecise and unstable the fixation of letters and syllables during their sequential analysis: generally the visual axis of the dominant eye is taken as the reference one and if a minimal and temporary discrepancy takes place (as often happens), the brain ignore the non-dominant eye and considers the fixation of the dominant eye to process the stimulus. On the contrary, if the motor dominance is unstable, the temporary difference between the visual axes direction of the two eyes gives a conflictual perceptive situation. This conflict would lead to unstable fixation, moving to and from around the letter, making reading slow and imprecise. The same conflict can occur also in case of weak sensory dominance. Subjects who show stable sensory dominance have chosen their left or right eye as the reference one; in this case if two different letters fall on the two retinae, the visual system automatically tends to process the stimulus projected onto the retina of the reference eye. In this way time after time the letters' processing is made univocal. Now, if sensory dominance is not stable, when two different letters are up to be processed simultaneously, a stalemate takes place because the visual system is unable to decide which of the two stimuli to be processed. In this case, again, slow and inaccurate reading is expected. So far, the test used for the assessment of the ocular dominance in dyslexic children is the Dunlop test. Indeed, the Dunlop test does not seems to be suitable: in fact it is not practical, as it requires a synoptophore, special slides and a training period for the technician as well as for the patient. In addition, results are argued to be affected by the age of the child. Finally, the Dunlop test is unable to split motor and sensory dominance. A test of dominance practical, fast, easy to use and able to measure separately motor and sensory dominance is therefore desirable. The advanced solution is the domitest M and the domitest S. A second stream of research has highlighted in a subgroup of dyslexics subtle visuoperceptive alterations involving one of the two visual pathways: the magnocellular system. The magnocellular system is in charge of motion perception and of contrast sensitivity at high temporal frequencies and low spatial frequencies. Indeed, contrast sensitivity studies have found a magnocellular impairment in a consistent proportion of dyslexics. As a matter of fact, a class of patients seems to be less sensitive to contrast of gratings of spatial frequency below 1.5 cycles/degree and temporal frequency higher than 10 hertz. If on the one hand the way this impairment affects reading is unclear, on the other hand such a contrast sensitivity deficit can be regarded as an epiphenomenon, a marker of visual dyslexics. It is desirable therefore a test specifically devised to measure contrast sensitivity at the very low spatial frequencies, found to be defective in dyslexic patients. The exam available in the Tetra platform satisfies this need: it measures contrast sensitivity at spatial frequencies of 0.5, 0.75, 1.5 till to 12 cycles/degree. Moreover, it is driven by a staircase-type alghorithm which makes the contrast sensitivity assessment very fast, taking less than two minutes. An interesting subject, also under a rehabilitative point of view, is the finding of increased lateral masking (also known as crowding) in many dyslexic patients. Normally, adjoining letters making up a word tend to influence (or mask) each other, so that their perception is made more difficult compared to the same characters presented separately. The closer are the letters, the stronger is the lateral masking. Beyond a given spatial interval (called critical spacing) the reciprocal masking ceases. Now, in many dyslexics the critical spacing is found to be wider. For this reason adjoining letters that normally do not mask each other, in patients are crowded. As a matter of fact, widening the interval between characters often improves the lexical function in disabled readers. It is showed that abnormal crowding (that is to say wider critical spacing) may depend on abnormal spatial relationship perception. According to this hypothesis, in dyslexics the visual space would be "shrunk" along the horizontal axis and "stretched" along the vertical one. We define this condition "vertical anisotropy". Presumably, as a perceptive consequence of this abnormality, adjoining letters are illusory perceived closer, that is to say the critical spacing turns out to be larger. The estimate of the spatial relationship perception and related anisotropy would be therefore an important tool for correctly evaluate the visuoperceptive impairment, in each single patient. For this purpose a specific psychophysical test aimed at evaluating spatial relationship perception and the related vertical anisotropy has been devised: the eidomorphometry. Finally, in each patient it is desirable to assess how and to what extent the alleged visuomotor and visuosensory alteration affect the lexical task. This is the aim of the reading performance test or REPORT, suitable to measure the reading rate for words and non-words as a function of the "stretching" of the lexical string. A complete analysis is performed and the outcome is presented as indexes and graphs. In conclusion: domitest M, domitest S, Eidomorphometry and REPORT: complemented each other in the TETRA platform are the solution for those who intend to study the lexical function under the visuoperceptive perspective.