Sunday, May 21, 2006

Dyslexia and eye tracking problems--correlation or cause? Guest post by Joel Schneider

The following is a guest blog post by Joel Schneider (Clinical psychologist, Illinois State University), a member of IQs Corner Virtual Community of Scholars project. Joel reviewed an article by Hutzler et al. (2006) that investigated whether eye movement tracking was a potential cause of severe reading disorders (dyslexia). The question asked by this study was: "Do People with Dylexia Have Trouble Controlling Their Eyes While Reading?"

Hutzler, F., Kronbichler, M., Jacobs, A.M., & Wimmer, H. (2006). Perhaps correlational but not causal: No effect of dyslexic readers’ magnocellular system on their eye movements during reading. Neuropsychologia, 44, 637–648.

  • During reading, dyslexic readers exhibit more and longer fixations and a higher percentage of regressions than normal readers. It is still a matter of debate, whether these divergent eye movement patterns of dyslexic readers reflect an underlying problem in word processing or whether they are – as the proponents of the magnocellular deficit hypothesis claim – associated with deficient visual perception that is causal for dyslexia. To overcome problems in the empirical linkage of the magnocellular theory with reading, a string processing task is presented that poses similar demands on visual perception (in terms of letter identification) and oculomotor control as reading does. Two experiments revealed no differences in the eye movement patterns of dyslexic and control readers performing this task. Furthermore, no relationship between the functionality of the participants’ magnocellular system assessed by the coherent motion task and string processing were found. The perceptual and oculomotor demands required during string processing were functionally equivalent to those during reading and the presented consonant strings had similar visual characteristics as reading material. Thus, a strong inference can be drawn: Dyslexic readers do not seem to have difficulties with the accurate perception of letters and the control of their eye movements during reading – their reading difficulties therefore cannot be explained in terms of oculomotor and visuo-perceptual problems.
Joel's comments

Previous research has shown that poor readers fixate their eyes on each part of sentences longer, their eyes make more stops as they scan the page, and they look backwards to previously read material more frequently.

Is that why they have trouble reading? Probably not.

A new study of 48 German adolescent boys (24 with dyslexia) used a clever method to test the commonly held belief that visual tracking irregularities cause reading problems. It has been suggested that in dyslexics the flow of visual information is not suppressed properly while the eyes make saccadic movements (the quick movements the eyes naturally make when scanning from side to side). These deficits are said to cause words and letters to appear dance around on the page and sometimes merge together.

The difficulty with testing this hypothesis is that when you observe eye movements while someone is reading, it is hard to know whether their irregular eye movements are causing the reading problems or they are the result of poor reading skills. To get around this problem, a new task was devised to be very similar to reading but to have no requirements to pronounce anything or understand anything. Instead of having the boys read words, they were given strings of consonants like this:
In this “String Processing” task, they had to indicate whether each “word” had a double letter. Thus, the task required the boys to look at each word in a manner similar to reading but no higher-order reading skills were necessary. In another task, the boys also had to read pronounceable pseudowords such as “ZIB VULL CRUF BAF.” Their eye movements were recorded while performing both tasks.

While reading the pseudowords, the dyslexic boys’ eye movements showed the same unusual patterns as found in previous research. However, while performing the String Processing task, their eye movements were indistinguishable from those of the control group. Although it is possible that there might be some children with reading problems due to eye-movement control problems, these results suggest that such problems are not typical. To quote the title of the article, the relationship between eye movement irregularities and reading problems is “perhaps correlational but not causal.”

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