Most diagnostic criteria use the term developmental dyscalculia (DD) to describe moderate to extreme difficulties in fluent numerical calculations that cannot be attributable to sensory difficulties, low IQ or educational deprivation.

Numbers do not seem to be meaningful for dyscalculics—at least, not meaningful in the way that they are for typically developing learners. They do not intuitively grasp the size of a number and its value relative to other numbers.

Common indicators of DD:

• Carrying out simple number comparison and addition tasks by counting, often using fingers, well beyond the age when it is normal
• Finding approximate estimation tasks difficult

Examples:

• To say which is the larger of two playing cards showing 5 and 8, they count all the symbols on each card.
• To place a playing card of 8 in sequence between a 3 and a 9, they count up spaces between the two to identify where the 8 should be placed.
• To count down from 10, they count up from 1 to 10, then 1 to 9, etc.
• To count up from 70 in tens, they say “70, 80, 90, 100, 200, 300…”

There is now widespread evidence that points to a single biological marker in DD: an intraparietal sulcus (IPS) abnormality. The existence of a core deficit in processing numerosities is consistent with recent discoveries about dyscalculic brains:

(i) Reduced activation has been observed in children with dyscalculia during comparison of numerosities, comparison of number symbols, and arithmetic — these children are not using the IPS so much during these tasks.

(ii) Reduced grey matter in dyscalculic learners has been observed in areas known to be involved in basic numerical processing, including the left IPS, the right IPS , and the IPS bilaterally — these learners have not developed these brain areas as much as typical learners (Fig. 1). And

(iii) differences in connectivity among the relevant parietal regions, and between these parietal regions and occipitotemporal regions associated with processing symbolic number form (Fig. 1), are revealed through diffusion tensor imaging tractography — dyscalculic learners have not sufficiently developed the structures needed to coordinate the components needed for calculation.

Kucian and colleagues examined cerebral mechanisms underlying DD. Eighteen children with DD aged 11.2 ± 1.3 years and twenty age-matched typically achieving schoolchildren were investigated using functional magnetic resonance imaging (fMRI) during trials testing approximate and exact mathematical calculation, as well as magnitude comparison.

Children with DD showed greater inter-individual variability and had weaker activation in almost the entire neuronal network for approximate calculation including the intraparietal sulcus, and the middle and inferior frontal gyrus of both hemispheres.

In particular, the left intraparietal sulcus, the left inferior frontal gyrus and the right middle frontal gyrus seem to play crucial roles in correct approximate calculation, since brain activation correlated with accuracy rate in these regions.

Their study showed evidence for a deficient recruitment of neural resources in children with DD when processing analog magnitudes of numbers.

Fig. 1. Structural abnormalities in young dyscalculic brains suggesting the critical role for the IPS. Here, we show areas where the dyscalculic brain is different from that of typically developing controls. Both left and right IPS are implicated, possibly with a greater impairment for left IPS in older learners.

(A) There is a small region of reduced grey-matter density in left IPS in adolescent dyscalculics.

(B) There is right IPS reduced grey-matter density (yellow area) in 9-year-olds.

(C) There is reduced probability of connections from right fusiform gyrus to other parts of the brain, including the parietal lobes.

Fig. 2 The paradigm used during fMRI examination consisted of approximate and exact calculation, approximate and exact control conditions as well as magnitude comparison. Each condition was presented in three blocks of 80 s.

Fig. 3 Brain activation patterns of children with DD (N = 18) and control children (N = 20) during each condition are depicted on the SPM standard brain template.

The activated brain regions shown had been subjected to a FWE or FDR correction with a minimum number of 10 voxels, with one exception in Figure 2C, where the shown cluster comprises only 5 voxels. A, B, C: approximate calculation – approximate control condition.

D, E: exact calculation – exact control condition. F, G: magnitude comparison – rest.