Dyscalcula and Number-processing skills in adults with dyslexia

Overall Summary of Scientific Study and Results
The present study investigated basic numerical skills and arithmetic in adults with developmental dyslexia. Participants performed exact and approximate calculation, basic numerical tasks (e.g., counting; symbolic number comparison; spatial-numerical association of response codes, SNARC), and visuospatial tasks (mental rotation and visual search tasks).

The group with dyslexia showed a marginal impairment in counting compared to age- and IQ-matched controls, and they were impaired in exact addition, in particular with respect to speed. They were also significantly slower in multiplication. In basic number processing, however, there was no significant difference in performance between those with dyslexia and controls. Both groups performed similarly on subtraction and approximate addition tasks.

These findings indicate that basic number processing in adults with dyslexia is intact. Their difficulties are restricted to the verbal code and are not associated with deficits in nonverbal magnitude representation, visual Arabic number form, or spatial cognition.

The Study details
It has long been recognised that language is a uniquely human ability. More recently it has been proposed that humans have an innate capacity to perceive numerosity, sometimes called the “number sense”.

The role of language in the development of human number representations (and hence mathematics) is debated. According to Dehaene's triple code model, three codes underpin our ability to process numbers and hence become numerate: a verbal code (linked to the language system); an analogue magnitude representation (underlying approximate calculation); and a visual code (linked to the Arabic number form).

Neuropsychological patients
Research on neuropsychological patients provides evidence for a double dissociation between language and number-processing systems, but this evidence does not imply that these systems develop independently.

From a developmental perspective, some theorists propose that language is essential for the development of numerical competencies, and there is evidence that the structure of the language system in which one grows up shapes the development of numerical concepts. Others, however, argue that numerical competence can develop independently of language.

Although less studied than reading difficulties, problems of mathematical development provide one way of understanding the relationships between language and number skills. Moreover, reading difficulties (RD) are often accompanied by problems with number work: Estimates of the overlap between reading difficulties and mathematical difficulties (MD) range from 2.3% to 40%.

Within the framework of the triple code model proposed by Dehaene, the most likely candidate for explaining the overlap between reading and mathematical difficulties is the verbal code. According to Dehaene, the verbal code is used most strongly for counting, for addition, and in multiplication tables, while approximate calculation and comparison as well as parity decision are supported more by the nonverbal codes.

Number-processing deficits in dyslexia
A number of clinical studies have documented the mathematical difficulties experienced by people with dyslexia.

Reviewing this literature, Simmons and Singleton concluded that the main difficulty is in recalling number facts. Thus, several studies report children with dyslexia to be slow at calculating or verifying sums.

Problems in multiplication and subtraction are common. Simmons and Singleton proposed that the mathematical difficulties observed in dyslexia might be related to phonological-processing deficits (that also cause reading and spelling problems).

Consistent with this, a number of studies have reported that phonological-processing abilities predict arithmetic impairment.

Contrary to this view, Landerl, Bevan, and Butterworth argue that learning to read and learning arithmetic are independent processes and that “fact retrieval is not, in essence, a verbally mediated process”.

The basis for their assertion came from a study of 8-9-year-old children with reading and/or arithmetic difficulties in which they classified children into three groups:

• (a) children with dyslexia who did not have arithmetical difficulties (dyslexia-only);
  
• (b) children with mathematical difficulties who did not have dyslexia (MD-only);
  
• (c) children with dyslexia and MD.
  

NB: In tests of digit number naming, children with dyslexia-only performed at the same level as controls.

Children classified as having MD-only, however, had longer response latencies—indeed, even longer than those of the children with dyslexia and MD. They went on to argue that children with pure dyslexia do not experience number-processing deficits.

This conclusion needs to be treated with caution. First, the cut-offs used to define mathematical difficulties (3 standard deviations below the mean) and dyslexia (below the 25th percentile) were different, and the criterion used to define dyslexia was relatively lenient.

Second, it is not clear whether the dyslexia group had impairments in phonological processing. These issues limit the ability to generalise on the findings, and therefore the results may not be applied to all people with dyslexia.