Letter- and Digit-Position Dyslexia

 

Is the impairment in letter- and digit-position dyslexia located at the input stage or the output stage?

The study described in the previous post showed that children with letter position dyslexia, which causes letters to be transposed when words are read, read multidigit numbers correctly and without transposing digits. By contrast, one adult participant in the study who also had letter position dyslexia made errors when reading both numbers and words.

The study described in this post was conducted with this participant and another woman, a doctoral student in mathematics, who had letter position dyslexia when reading words as well as difficulties reading numbers. The purpose of the study was to identify the source of the two participants’ difficulties in reading words and numbers by comparing the patterns of errors they made when reading words and numbers.

The two participants were asked to read migratable words aloud. These are words such as from, which can be mistakenly read as form when the order of the letters is transposed. The participants also read aloud multidigit numbers that did not contain the digits 0 or 1 and in which none of the other digits was repeated. The digits 0 and 1 were excluded because fewer digit-migration errors occur when people read numbers containing these digits. The numbers and words were presented under time constraints.

Both participants made many letter- or digit-migration and transposition errors when reading migratable words and multidigit numbers. Members of a control group who performed the same task made errors on only 0.1% of the words and 4% of the numbers.

Where was the impairment in the two participants located: at the input stage, involving the visual analysis of the stimulus, or at the output stage, involving the production of the spoken response?

Recall that when we read a single word aloud, we first process it through orthographic visual analysis, which is the input stage, and determine whether it is represented in the orthographic lexicon containing the written forms of familiar words. If the word is present in the orthographic lexicon, it activates its meaning in the semantic lexicon. At this stage, the phonological lexicon is also activated. This lexicon contains the spoken forms of words, namely, the sequence of sounds required to say or produce the word. The output stage also includes holding the word in the phonological loop until it is spoken. An impairment at the output stage means that the word has been read correctly and that the child makes an error only when attempting to say the word that was correctly identified.

To determine whether the impairment was located at the input or output stage, the researchers gave the two participants spoken-production tasks involving words and multidigit numbers that did not include written visual input. They also administered silent-reading tasks involving words and multidigit numbers that did not require spoken production. In this way, the researchers could separate input from output and identify the location of the impairment. We can use the same approach when conducting a diagnostic assessment with a child.

Word- or multidigit-number production tasks without written visual input include, for example, word-repetition and number-repetition tasks. The experimenter, or the psychologist conducting the assessment, says a word or a multidigit number, and the child is asked to repeat it. The two participants performed the word- and multidigit-number repetition tasks without errors.

Word-reading tasks without spoken production can include tasks in which the child reads a word silently and points to the corresponding object. For example, the child might read post and choose between a picture of a letter and a picture a planter.

A lexical-decision task, in which the child reads a letter string and decides whether it is a real word, also does not require spoken production. The same is true of a task in which the child reads a word silently and is asked to define it. In this case, if the child reads post but defines pots, we know that the word was read with a letter transposition.

Another such task requires the child to read two words, for example form, form or form, from and decide whether they are identical or different.

The latter three tasks were administered to the two participants, who made many errors on them.

A task involving the reading of multidigit numbers without spoken production might require the child to read two multidigit numbers silently, such as 23456 and 23546, and decide whether they are identical.

Another task involves identifying digit sequences. In this task, the child is asked to decide whether the digits in a displayed number form a sequence that increases by one at each step, such as 23456, or do not form such a sequence, such as 23546. Both participants had greater difficulty than the control group on these two tasks.

The participants’ performance patterns, namely, good performance on production tasks without written input and poor performance on reading tasks without spoken production, suggest that their difficulties reading words and multidigit numbers were located at the input stage.

Thus, people who have letter position dyslexia in word reading together with digit position dyslexia in number reading probably have an impairment at the visual-orthographic input stage, assuming that it is possible to generalize from an extremely detailed study of only two people.

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.