Dyscalculia and the calculating brain

Rapin I.  Dyscalculia and the calculating brain.  Pediatric Neurology August 2016 Volume 61, Pages 11–20.

Abstract

Dyscalculia, like dyslexia, affects some 5% of school-age children but has received much less investigative attention. In two thirds of affected children, dyscalculia is associated with another developmental disorder like dyslexia, attention-deficit disorder, anxiety disorder, visual and spatial disorder, or cultural deprivation. Infants, primates, some birds, and other animals are born with the innate ability, called subitizing, to tell at a glance whether small sets of scattered dots or other items differ by one or more item. This nonverbal approximate number system extends mostly to single digit sets as visual discrimination drops logarithmically to “many” with increasing numerosity (size effect) and crowding (distance effect). Preschoolers need several years and specific teaching to learn verbal names and visual symbols for numbers and school agers to understand their cardinality and ordinality and the invariance of their sequence (arithmetic number line) that enables calculation. This arithmetic linear line differs drastically from the nonlinear approximate number system mental number line that parallels the individual number-tuned neurons in the intraparietal sulcus in monkeys and overlying scalp distribution of discrete functional magnetic resonance imaging activations by number tasks in man. Calculation is a complex skill that activates both visual and spatial and visual and verbal networks. It is less strongly left lateralized than language, with approximate number system activation somewhat more right sided and exact number and arithmetic activation more left sided. Maturation and increasing number skill decrease associated widespread non-numerical brain activations that persist in some individuals with dyscalculia, which has no single, universal neurological cause or underlying mechanism in all affected individuals.

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From the article

For instance, among 378 eight-year-old German-speaking children with a performance intelligence quotient greater than 85, von Aster and Shalev reported a 6% prevalence of dyscalculia, but only 1.8% (a third of those affected) with pure dyscalculia, compared with 4.2% in whom dyscalculia was associated with another prevalent developmental disorder like dyslexia, attention-deficit disorder,9 or anxiety.  Similarly, among 50 11- to 13-year-old Swedish children with developmental dyscalculia, 16 (32%) were considered to have a specific deficit accessing number concepts and the 34 others (68%) more general problems with calculation…

Bilingual persons tend to calculate faster lifelong in their native language than in their second—perhaps now better—language. This testifies to the bonding of numbers to language and also to the drill used to inculcate elementary arithmetic facts. fMRIs in small groups of competent adults illustrate that cognitive resources allocated to problem solving depend on their demands. Fast answers to easy problems like adding single digits and multiplying numbers two to four activated not only the IPS of both hemispheres but also the left angular gyrus, recipient of overlearned verbal material, while deactivating the right. As the subjects had to muster additional cognitive resources and more effortful strategies for multiplying larger numbers (five to nine), for subtraction, and, especially, for division, answers tended to be slower, less accurate, and associated with left angular deactivation and activation of the right, together with widespread mostly bilateral cortical, limbic, and subcortical networks and their connecting white matter tracts…

Among tentative results, MRI morphometry disclosed in 23 seven- to nine-year old dyscalculic children smaller volumes of many gray matter areas of the bihemispheric number network than in well-matched competent controls, and fractional anisotropy revealed especially marked axonal disorganization of posterior callosal interparietal connecting fibers.  In a reaction time and visual event-related potential (ERP) study, seven adolescent girls with severe pure dyscalculia and matched unaffected controls had to report whether a flashed Arabic digit was smaller or larger than five.68 Presence in both groups of the expected distance effect—discrimination easier the more distant the target from five—and normal early automatic ERPs suggested that deficient visual perception of digits or their positions on the number line did not explain the dyscalculia, whereas absence of a right parietal 400-450 poststimulus ERP in the dyscalculic group pointed to inadequate complex processing. Notably, the subjects, who had no other deficit in spatial skills, had severe difficulty identifying fingers in rotated pictures of hands. fMRIs in 18 9- to 12-year-old children with pure dyscalculia were compared with those of 20 matched controls on each of three tasks: choosing which of two answers was the exact or the approximate sum of two previously flashed single digits, or which of two displays of nine or fewer randomly dispersed small drawings of vegetables or fruits was the larger.69 fMRIs of the two groups did not differ when answering exact additions or estimations of number magnitude problems, but did in judging approximate answers to additions.  Although the groups activated the same brain networks, the dyscalculic children's allocation for approximate assessment of number magnitude was weaker, more variable, and less widespread. Lack of fMRI differences from controls in retrieval of exact calculation facts and nonsymbolic magnitude estimation suggested neither was critical to dyscalculia, leading the investigators to postulate a specific deficit in approximate number appraisal…

Physicians' first responsibility when consulted for dyscalculia is to consider whether there is any evidence for a biologically treatable neurological disorder such as epilepsy or a genetic, metabolic, or structural brain disorder that might mandate EEG, brain imaging, or other investigations unrelated to the focus of this article. Physicians concerned with development need to discuss with the parents how far to go in probing for the biologic etiology of the problem, making clear which tests are for the child's or family's benefit and which to answer research questions. Their more pressing responsibility is to probe for the highly prevalent associated developmental problems such as attention-deficit disorder, anxiety, or depression amenable to counseling and pharmacologic interventions, and for unfavorable environmental circumstances to be addressed. Dealing with these nonspecific issues together with cognitive counseling may determine the success or failure of remedial educational interventions…

There is currently a great deal of interest in the potential benefits of noninvasive transcranial magnetic, electrical, and random noise stimulation aimed at altering intrinsic oscillations of underlying brain circuitry in individuals with developmental and psychiatric disorders.  For example, repeated transcranial direct current electrical stimulation of areas identified as relevant to particular cognitive abilities was reported to have long-term beneficial effects in children and adults with learning disabilities.  At this stage this is still preliminary research requiring much more work before it can be recommended ethically in the clinic, irrespective of exploding numbers of lay publications touting beneficial results.