Children with developmental dyscalculia have more difficulty with subtraction than addition. Why? And what is unique in their brain activity?

Rosenberg‐Lee, M., Ashkenazi, S., Chen, T., Young, C. B., Geary, D. C., & Menon, V. (2015). Brain hyper‐connectivity and operation‐specific deficits during arithmetic problem solving in children with developmental dyscalculia.Developmental science, 18(3), 351-372.‏http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4320038/

 In this research done with 7-9 year old children, the authors compared addition and subtraction abilities of children with developmental dyscalculia (DD) and typically developing children (TD).

Children diagnosed as DD scored at or below the 25^th percentile on the Numerical Operations subtest of the Wechsler Individual Achievement Test – Second Edition; WIAT-II.  Children diagnosed as TD scored at or above the 75^th percentile on this test.  Children in both groups had a FSIQ of 80 or above, and scored at or above the 25^th percentile  on  the Word Reading subtest of the WIAT-II.  Sixteen  DD and 20 TA children participated in the study.

The fMRI experiment consisted of addition and subtraction problems which were either simple or complex.   Each calculation trial lasted five seconds.  In the Complex addition task, participants were presented with an equation involving two addends and asked to indicate, via a button box, whether the answer shown was correct or incorrect (e.g. ‘3 + 4 = 8’). The first operand ranged from 2 to 9, the second from 2 to 5 The Simple addition task was identical except that one of the operands was always ‘1’ (e.g. ‘3 + 1 = 4’). In the Complex subtraction task, the first operand ranged from 3 to 14 and the second operand from 2 to 5. In the Simple subtraction task, the first operand ranged from 2 to 14 and the second operand was always ‘1’.

Here I'll focus on a few findings that are of interest for me, and not on all findings of this study.

·         DD children solved addition tasks with the same level of accuracy as TD children, but were slower.  DD children were significantly deficient with the subtraction tasks, in comparison with the TD children. Children with DD failed to respond in the allotted time in a large proportion of trials during the subtraction task. However, for trials in which they made a response, accuracy in the DD participants was relatively high at 75.4%, suggesting that DD participants were actively engaged in the task but were unable to solve many of the problems with the same fluency as their TD peers.

·         Timed trials exacerbate the difficulties children with DD have when solving subtraction problems consistent with their difficulties on timed number fact and story problems. The latter are typically due to use of slower and more effortful counting strategies to solve the problems, as contrasted with direct retrieval of the answer in children without mathematical difficulties. This pattern may be exacerbated with subtraction because, unlike addition, subtraction problems are not commutative (e.g. 4 − 3 ≠ 3 − 4), which makes memorization of answers more difficult and thus results in less fluent problem solving for all students.

·         Children with DD engage multiple fronto-parietal circuits differently from TD children. Children with DD may require greater engagement of these circuits, even while achieving only weaker levels of performance. Alternatively, greater engagement of these circuits may result in the activation of problem-irrelevant information that in turn disrupts problem solving. The latter view is consistent with behavioral studies that show the intrusion of problem-irrelevant information into working memory when children with DD attempt to retrieve arithmetic answers from long-term memory. 

·         Hyper-connectivity, rather than gross under-activation, is the primary neural source of problem solving difficulties in children with DD.  DD children showed hyper-activation on both addition and subtraction problems in multiple frontal, parietal and visual areas. Children with DD showed especially high levels of hyper-activation in parietal cortex for both correctly and incorrectly solved subtraction problems.

·         There is a network of brain regions that show aberrant responses during arithmetic problem solving.  Arithmetic deficits in DD are unlikely to be localized to a single brain region.  Rather, both localized processing deficits in multiple brain areas as well as the coordination between multiple brain circuits are impaired in DD. These conclusions are consistent with the proposal that most neurodevelopmental disorders and learning disabilities arise from diffuse disruptions and aberrant connectivity between regions rather than focal lesions.