Is the Visual Analyzer Orthography-Specific?

 

Is the Visual Analyzer Orthography-Specific? Reading Words and Numbers in People with Letter Position Dyslexia

In this study, Professor Naama Friedmann, Dr. Dror Dotan, and Einav Rahamim of Tel Aviv University compared word reading and number reading in people with letter position dyslexia. This comparison can teach us about differences between the types of visual processing required for reading words and multidigit numbers.

Prof. Naama Friedman

Letter position dyslexia, or LPD, apparently results from an impairment in the visual analysis system. This system has three functions, each of which may be impaired independently:

A. Letter identification. When this function is impaired, a person may be unable to identify individual letters at all or may confuse visually similar letters when reading.

B. Binding letters to words: assigning each letter to the word to which it belongs. When this function is impaired, letters may “migrate” from one word to another. For example, a person may read "the yellow melon" as "the mellow melon".

C. Encoding the positions of letters within a word, or encoding their positions relative to the first and last letters of the word. When this function is impaired, letter transpositions occur during reading. For example, a child may read the word "form" as "from" or the word "causal" as "casual". This is the function that is impaired in letter position dyslexia, the condition examined in this study.

The participants were 11 people with letter position dyslexia. Ten were children between the ages of 9 and 15, and one was 29 years old. All but one of the participants had no brain injury, and their dyslexia was therefore developmental. It is not clear to me why the one participant with brain injury was included in the study. The participants were selected because they made letter transposition errors in more than 10% of the words they read, but did not make other types of reading errors. It should be noted that seven of the children also had attention disorders, and six attended special education programs. Each participant with letter position dyslexia was matched with a participant from the control group.

Following the initial screening, the researchers assessed the participants using five tasks containing migratable words, namely words in which transposing letters produces another existing word. In four of the five tasks, the participants with letter position dyslexia made an average of 25%–33% errors, compared with an average of 2%–4% in the control group. These differences were statistically significant.

The four tasks were reading individual words; deciding whether two words, such as from-form or causal-casual, were the same or different; defining words; and making lexical decisions. In the word-definition task, for example, a participant silently read the word "calm" and was asked to define it. If the participant said, “It is a sea creature that you eat,” the researchers concluded that the word had been read with a letter transposition as the word for “clam.” In the lexical-decision task, the participants were asked whether a letter string such as "baet" was a real word. If a participant answered “yes,” the researchers inferred that a letter transposition had occurred during reading.

In the fifth task, which involved reading migratable words within a text, the participants with letter position dyslexia made an average of approximately 12% errors, compared with approximately 2% in the control group. This difference was also statistically significant.

The researchers then examined the difference between reading words and reading numbers. They asked the participants to read four- and five-letter migratable words and four- and five-digit numbers. The group with letter position dyslexia read the words with an average letter transposition error rate of 25%, compared with an average of 2% in the control group. This difference was statistically significant. In contrast, when reading multidigit numbers, all but one of the participants with letter position dyslexia performed well and did not differ from the control group, whose digit transposition error rate was 2%.

In two additional tasks comparing the processing of letters and numbers, the participants were asked to determine whether pairs of words were the same or different, for example form-from or from-from. They were also asked whether pairs of four-, five-, or six-digit numbers, such as 63784–63784 or 63874–63784, were the same or different.

In the word task, the group with letter position dyslexia made errors on 31% of the trials in which the two words were different compared with approximately 3% in the control group. This difference was statistically significant. In contrast, in the number-comparison task, all but two of the participants with letter position dyslexia performed well and did not differ from the control group, which made errors on 4% of the trials.

These findings indicate that the impairment in the visual analysis system found in participants with letter position dyslexia is specific to word reading and does not affect the reading of multidigit numbers. In other words, the component of the visual analysis system responsible for identifying the positions of letters within words is specific to orthography.

In my opinion, this provides further evidence that the tests we use to assess visual processing are not related to reading. The visual processing required for reading is probably orthographic in nature.

Friedmann, N., Dotan, D., & Rahamim, E. (2010). Is the visual analyzer orthographic-specific? Reading words and numbers in letter position dyslexia. Cortex, 46(8), 982–1004.