LD Blog

In the 60s and 70s, after extensive investigations about the relationship between visual perception and Learning Disabilities, most of those working on educating students with Learning Disabilities abandoned the hypothesis that reading performance was affected by visual-perceptual processes. The diagnostic tests of visual perception had weak psychometric characteristics, alternative explanations (especially problems with phonemic awareness) fit the problem more closely, and research showed that training perceptual processes produced little or no benefits.

In the 90s, however, a different hypothesis about the relationship between visual perception and reading problems arose. In this version, the theory is that among people with dyslexia the magnocellular or large-cell system of the brain, which processes fast-moving objects, interacts poorly with the parvocellular (small-cell) system that processes patterns and colors. The evidence for this hypothesis comes from several lines of studies, including one that compares how well individuals with and without reading disability detect movement in visual stimuli on laboratory tasks. Another line of evidence comes from fMRI studies of differences in brain activity when individuals with and without reading disability view moving and still images. Still another line of evidence comes from the interpretation of studies of eye movements during reading; when people read, their eyes move systematically across lines of text, fixing on a bite of text and then jumping to the next bite. Individuals with reading disabilities have fewer fixations and are more likely to look back at previous text than their peers who do not have reading problems. According to the magnocellular hypothesis, this is the result of the faulty interaction between the systems because the magnocelluar system controls eye movements.

F. Hutzler, M. Kronbichler, A. M. Jacobs, and H. Wimmer devised a clever means of testing this hypothesis and reported about it in Neuropsychologia. Here’s the abstract:

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.

Hutzler et al.’s conclusion—their “strong inference”— pretty much says what needs to be said here.

Link to the article by Hutzler et al. and a flash of the electrons to Liz Ditz for pointing me to an analysis by Joel Schneider that led me to the article.