Developmental dyscalculia (DD) refers to a learning disorder which specifically affects the ability to acquire school-level arithmetic skills. It occurs when a child has a substantially lower mathematical ability compared to what can be expected for the person’s chronological age (1), measured intelligence (2) and age-appropriate education (3). The prevalence is 3% to 6%. It is often comorbid with other learning difficulties (e.g. dyslexia; ADHD).

There is a lot of heterogeneity within DD. This maybe due to variation across studies in criteria used to identify mathematical difficulties (e.g. discrepancy criteria vs. percentile cut-off points). This may include individuals whose math deficits stem from exogeneous sources (e.g. poor teaching quality) rather than those whose math deficits result from a learning disorder. Another reason is that math skills are heterogeneous and vulnerable to disruption from a wide range of endogenous and exogenous sources, leading to heterogeneity in DD.

Primary dyscalculia refers to math deficits that stem from an impaired ability to acquire those skills. Secondary dyscalculia (i.e. pseudo-dyscalculia) refers to math deficits caused by external factors (e.g. poor teaching quality). Children with the most severe math deficits have cognitive deficits in very basic number processing (i.e. number sense) while children with moderate impairments do not. Primary versus secondary dyscalculia may thus be a distinction in DD severity.

DD is characterized by a poor retrieval of arithmetic facts from memory and the perseverant use of immature calculation strategies (e.g. counting on a hand). However, there may also be impairments in a neurobiological system for processing numerical magnitudes. This neurobiological impairment may give rise to the difficulties associated with DD.

Children with DD fail to develop fluent fact-retrieval mechanisms and they know fewer arithmetic facts. Due to the impaired fact retrieval in DD, they use immature or inefficient problem-solving strategies. However, despite these characteristics, DD is difficult to identify as math difficulties may emerge at varying stages of the educational process (e.g. different grades).

The skills required for successful mathematical performance change over the course of development. Some children may have a specific deficit at an early learning stage and this may disrupt the acquisition of later skills. This disruption may occur because foundational skills are not present or because inefficient or immature procedural skills (e.g. manner of calculation) result in extra effort to carry out simple calculations. This extra effort may make it more difficult for the children to follow and learn more complex procedural knowledge.

The number sense refers to a cognitive mechanism that supports the representation and processing of numerical magnitudes. There may be a deficit in the number sense in DD. Numerical magnitude information is not activated automatically in DD children. There may thus be a reduced automatic activation of semantic numerical representations in DD. There appear to be very elementary deficits in basic number processing in DD.

The distance effect refers to the increase in reaction times and errors as the distance between numbers decreases when comparing numbers. The mental number line is measured using this comparison and the distance effect is believed to give some indication of deficits in this mental number line. The distance effect decreases in size over the course of development (i.e. increase in precision of number sense) but children with DD have larger distance effects than typically developing children. Children with DD may thus have a less-refined, immature representation of numerical magnitude compared to peers.

Primary DD which includes impaired arithmetic skills associated with a deficient numerical magnitude processing may thus be due to a deficit in the number sense. However, this only occurs in the most severely impaired DD children (i.e. 10%). Primary DD may thus be associated with most severe arithmetic deficits and attributed to a congenital impairment of the ability to represent and process numerical magnitude information.

The cause of DD may also be deficits in domain-general cognitive mechanisms (e.g. working memory; attention; visual-spatial processing). Children with mathematical difficulties underperform on tests of working memory. However, these findings may be due to selection criteria of the studies. Individuals with both working memory problems and arithmetic deficits most likely suffer from secondary DD. Primary DD appears to be independent of working memory impairments.

The access deficit hypothesis states that DD is the result of a specific impairment in symbolic but not non-symbolic numerical comparison.

The intraparietal sulcus is a key brain region involved in the processing of numerical magnitude representation. It may be that children with DD have atypical activation patterns in the intraparietal sulcus. There is reduced modulation of the right IPS in DD children during a non-symbolic numerical comparison task. Children with DD have atypical brain activation during basic numerical magnitude processing and show reduced activation of the intraparietal sulcus during mental arithmetic. There may be some overlap between the role of the intraparietal sulcus in numerical magnitude processing and spatial working memory. There may also be some structural anomalies in the intraparietal sulcus in children with DD (e.g. less grey matter).

The number race is a treatment designed to improve the precision of the numerical magnitude representations in children with DD. Children need to select the larger of two arrays of dots. programme makes the task more difficult or easier depending on performance. Graphogame is a treatment which requires people to compare sets of objects. It requires children to link sets of objects with umber symbols. Both treatments improve number-comparison performance but it does not generalize to counting and arithmetic.