Neurobiological Basis of Language Learning Difficulties
Authors: Saloni Krishnan, Kate E. Watkins, Dorothy V.M. Bishop
Journal: Trends in Cognitive Sciences (2016)

Individuals with SLI and dyslexia have impaired or immature learning mechanisms; this hampers their extraction of structure in complex learning environments. These learning difficulties are not general or confined to language. Problems are specific to tasks that involve implicitly learning sequential structure or complex cue–outcome relationships. Such learning is thought to depend upon corticostriatal circuits. In language learning studies, the striatum is recruited when adults extract sequential information from auditory-verbal sequences and as they learn complex motor routines relevant for speech. Neuroimaging studies indicate striatal abnormalities in individuals with language disorders. There is a need to probe the integrity of neural learning systems in developmental language disorders using tasks relevant for language learning which place specific demands on the striatum/MTL. In this paper we highlight why there is a need to examine subcortical learning systems in children with language impairment and dyslexia, rather than focusing solely on cortical areas relevant for language. First, behavioural studies find that children with these neurodevelopmental disorders perform less well than peers on procedural learning tasks that depend on corticostriatal learning circuits. Second, fMRI studies in neurotypical adults implicate corticostriatal and hippocampal systems in language learning. Finally, structural and functional abnormalities are seen in the striatum in children with language disorders. Studying corticostriatal networks in developmental language disorders could offer us insights into their neurobiological basis and elucidate possible modes of compensation for intervention. In this paper we highlight why there is a need to examine subcortical learning systems in children with language impairment and dyslexia, rather than focusing solely on cortical areas relevant for language. First, behavioural studies find that children with these neurodevelopmental disorders perform less well than peers on procedural learning tasks that depend on corticostriatal learning circuits. Second, fMRI studies in neurotypical adults implicate corticostriatal and hippocampal systems in language learning. Finally, structural and functional abnormalities are seen in the striatum in children with language disorders. Studying corticostriatal networks in developmental language disorders could offer us insights into their neurobiological basis and elucidate possible modes of compensation for intervention. Specific language impairment (SLI) and developmental dyslexia are categorized as types of specific learning disability (Box 1), but for many years the bulk of research on these disorders has looked at perceptual impairments and problems with specific linguistic components such as phonology and grammar. Here we adopt a different perspective: the idea that children with these disorders have impairments in the basic process of learning language. On this view, the language and literacy difficulties experienced by these children are not byproducts of some other primary deficit; instead, they implicate impairment or immaturity of learning mechanisms that allow the extraction of structure from a rich and varying language environment. We review recent developmental and neurobiological studies to evaluate the contribution of different brain systems in language learning, particularly focusing on how they might be affected in children with language and reading disorders.Box 1Why Use Developmental Dyslexia and SLI as Examples of Language Learning Disorders?SLI is diagnosed when oral language lags behind other areas of development for no apparent reason, and is not the result of known developmental concerns, sensory impairments, or global developmental delay. SLI affects between 3–10% of children entering school [93Tomblin J.B. et al.Prevalence of specific language impairment in kindergarten children.J. Speech Lang. Hear. Res. 1997; 40: 1245-1260Crossref PubMed Google Scholar]. Developmental dyslexia is identified when a child has poor literacy skills despite adequate intelligence and opportunity to learn, and similar prevalence levels are reported for this group [94Bishop D.V. Snowling M.J. Developmental dyslexia and specific language impairment: same or different?.Psychol. Bull. 2004; 130: 858-886Crossref PubMed Scopus (610) Google Scholar]. Although the reading difficulties in dyslexia were initially thought to be in the visual perception domain, it is now fairly well established that most children with dyslexia have problems in the linguistic domain, particularly for categorising sounds, establishing sound–letter mappings, and manipulating sound sequences in their first language. These definitions indicate that both groups have specific difficulties with aspects of language learning. In addition, both disorders have a strong genetic basis, particularly for cases where nonword repetition deficits are noted [94Bishop D.V. Snowling M.J. Developmental dyslexia and specific language impairment: same or different?.Psychol. Bull. 2004; 130: 858-886Crossref PubMed Scopus (610) Google Scholar].There is a great deal of overlap between dyslexia and SLI. At a cognitive level, both groups show core deficits in phonology, particularly on tasks that tap into the ability to understand the sound structure of words (phonological awareness) and those that involve nonword repetition (motor control and phonological memory). Oral language problems place children at risk for literacy problems, both in reading comprehension and accuracy [95Catts H.W. The relationship between speech-language impairments and reading disabilities.J. Speech Lang. Hear. Res. 1993; 36: 948-958Crossref Google Scholar]. On the other hand, children at risk of dyslexia often experience early language difficulties, even before they start to read [96Nash H.M. et al.Preschool language profiles of children at family risk of dyslexia: continuities with specific language impairment.J. Child Psychol. Psychiatry. 2013; 54: 958-968Crossref PubMed Scopus (0) Google Scholar]. However, these disorders are not identical or even points on a continuum of phonological ability. Bishop and Snowling [94Bishop D.V. Snowling M.J. Developmental dyslexia and specific language impairment: same or different?.Psychol. Bull. 2004; 130: 858-886Crossref PubMed Scopus (610) Google Scholar] have argued that these disorders are best characterised using a 2D model of phonological and non-phonological language skills. While children with dyslexia have typical non-phonological skills but impaired phonological ability, children with SLI are more likely to present with impairments of both phonological and non-phonological skills (for example, semantic and syntactic difficulties). Ramus and colleagues [74Ramus F. et al.Phonological deficits in specific language impairment and developmental dyslexia: towards a multidimensional model.Brain. 2013; 136: 630-645Crossref PubMed Scopus (95) Google Scholar] have provided empirical support for this theory, but posit further dimensions with respect to phonological skills, separating out phonological representations and access.Language problems are of significant concern in both SLI and dyslexia because poor language learning has been linked to negative consequences for academic achievement, self-esteem, social and emotional development, and employment [97Conti Ramsden G. et al.Adolescents with a history of specific language impairment (SLI): strengths and difficulties in social, emotional and behavioral functioning.Res. Dev. Disabil. 2013; 34: 4161-4169Crossref PubMed Scopus (0) Google Scholar]. Understanding why language learning is affected, and the specific mechanisms that impair their learning, could allow us to design optimal means of compensating for these difficulties. SLI is diagnosed when oral language lags behind other areas of development for no apparent reason, and is not the result of known developmental concerns, sensory impairments, or global developmental delay. SLI affects between 3–10% of children entering school [93Tomblin J.B. et al.Prevalence of specific language impairment in kindergarten children.J. Speech Lang. Hear. Res. 1997; 40: 1245-1260Crossref PubMed Google Scholar]. Developmental dyslexia is identified when a child has poor literacy skills despite adequate intelligence and opportunity to learn, and similar prevalence levels are reported for this group [94Bishop D.V. Snowling M.J. Developmental dyslexia and specific language impairment: same or different?.Psychol. Bull. 2004; 130: 858-886Crossref PubMed Scopus (610) Google Scholar]. Although the reading difficulties in dyslexia were initially thought to be in the visual perception domain, it is now fairly well established that most children with dyslexia have problems in the linguistic domain, particularly for categorising sounds, establishing sound–letter mappings, and manipulating sound sequences in their first language. These definitions indicate that both groups have specific difficulties with aspects of language learning. In addition, both disorders have a strong genetic basis, particularly for cases where nonword repetition deficits are noted [94Bishop D.V. Snowling M.J. Developmental dyslexia and specific language impairment: same or different?.Psychol. Bull. 2004; 130: 858-886Crossref PubMed Scopus (610) Google Scholar]. There is a great deal of overlap between dyslexia and SLI. At a cognitive level, both groups show core deficits in phonology, particularly on tasks that tap into the ability to understand the sound structure of words (phonological awareness) and those that involve nonword repetition (motor control and phonological memory). Oral language problems place children at risk for literacy problems, both in reading comprehension and accuracy [95Catts H.W. The relationship between speech-language impairments and reading disabilities.J. Speech Lang. Hear. Res. 1993; 36: 948-958Crossref Google Scholar]. On the other hand, children at risk of dyslexia often experience early language difficulties, even before they start to read [96Nash H.M. et al.Preschool language profiles of children at family risk of dyslexia: continuities with specific language impairment.J. Child Psychol. Psychiatry. 2013; 54: 958-968Crossref PubMed Scopus (0) Google Scholar]. However, these disorders are not identical or even points on a continuum of phonological ability. Bishop and Snowling [94Bishop D.V. Snowling M.J. Developmental dyslexia and specific language impairment: same or different?.Psychol. Bull. 2004; 130: 858-886Crossref PubMed Scopus (610) Google Scholar] have argued that these disorders are best characterised using a 2D model of phonological and non-phonological language skills. While children with dyslexia have typical non-phonological skills but impaired phonological ability, children with SLI are more likely to present with impairments of both phonological and non-phonological skills (for example, semantic and syntactic difficulties). Ramus and colleagues [74Ramus F. et al.Phonological deficits in specific language impairment and developmental dyslexia: towards a multidimensional model.Brain. 2013; 136: 630-645Crossref PubMed Scopus (95) Google Scholar] have provided empirical support for this theory, but posit further dimensions with respect to phonological skills, separating out phonological representations and access. Language problems are of significant concern in both SLI and dyslexia because poor language learning has been linked to negative consequences for academic achievement, self-esteem, social and emotional development, and employment [97Conti Ramsden G. et al.Adolescents with a history of specific language impairment (SLI): strengths and difficulties in social, emotional and behavioral functioning.Res. Dev. Disabil. 2013; 34: 4161-4169Crossref PubMed Scopus (0) Google Scholar]. Understanding why language learning is affected, and the specific mechanisms that impair their learning, could allow us to design optimal means of compensating for these difficulties. Children with developmental language disorders struggle to learn new words [1Kan P.F. Windsor J. Word learning in children with primary language impairment: a meta-analysis.J. Speech Lang. Hear. Res. 2010; 53: 739Crossref PubMed Scopus (0) Google Scholar] and syntactic constructions [2Hsu H.J. Bishop D.V. Training understanding of reversible sentences: a study comparing language-impaired children with age-matched and grammar-matched controls.Peer J. 2014; 2: e656Crossref PubMed Google Scholar]. Is this a linguistic problem, or do they exhibit difficulties with learning new information more generally? Learning is not a unitary phenomenon. Neuropsychological studies have suggested functional and neurological distinctions between different types of learning (Box 2). Ullman and Pierpont [3Ullman M.T. Pierpont E.I. Specific language impairment is not specific to language: the procedural deficit hypothesis.Cortex. 2005; 41: 399-433Abstract Full Text PDF PubMed Google Scholar] were the first to suggest that the procedural learning system, which is involved in implicit learning, was impaired in individuals with SLI. They proposed that procedural impairments could account for poor learning of grammatical rules, such as the past tense inflection of regular verbs (also see [4Pinker S. Ullman M.T. The past and future of the past tense.Trends Cogn. Sci. 2002; 6: 456-463Abstract Full Text Full Text PDF PubMed Scopus (388) Google Scholar]). The postulated impairments in procedural learning were not, however, specific to language; they would have broader effects, with deficits predicted in the acquisition of any skill involving sequences – irrespective of whether the sequences were sensorimotor or abstract. By contrast, declarative learning systems, which support the sort of idiosyncratic mapping required to learn new vocabulary or inflection of irregular verb forms, were argued to be relatively intact.Box 2Procedural/Declarative Distinctions in MemoryInitial distinctions between learning systems were based on studies of a noted patient, Henry Molaison, who became densely amnesic after bilateral hippocampectomy [98Scoville W.B. Milner B. Loss of recent memory after bilateral hippocampal lesions.J. Neurol. Neurosurg. Psychiatry. 1957; 20: 11-21Crossref PubMed Google Scholar]. While he could not acquire new information such as facts and events, he was able to learn new visuomotor skills in rotary pursuit, bimanual tracking, and tapping tasks [99Corkin S. Acquisition of motor skill after bilateral medial temporal-lobe excision.Neuropsychologia. 1968; 6: 255-265Crossref Google Scholar]. This led to a distinction being made between memory systems – ones that encoded explicit knowledge about facts and events, termed 'declarative' learning systems, and 'procedural' systems, which encoded skill-based information that is implicitly expressed through performance [100Corkin S. What's new with the amnesic patient H.M?.Nat. Rev. Neurosci. 2002; 3: 153-160Crossref PubMed Scopus (0) Google Scholar].Ever since, the acquisition of declarative memory has been shown to depend upon heavily interconnected MTL structures, including the hippocampus, entorhinal, perirhinal, and parahippocampal cortices [101Squire L.R. Zola-Morgan S. The medial temporal lobe memory system.Science. 1991; 253: 1380-1386Crossref PubMed Google Scholar]. These regions are well adapted for rapid learning, and are particularly relevant when learning occurs over a single exposure, when associations between stimuli must be made over space or time, when acquired information must be used flexibly, and when learning depends on the awareness of what is being learned (reviewed in [102Squire L.R. Dede A.J.O. Conscious and unconscious memory systems.Cold Spring Harb. Perspect. Biol. 2015; 7: a021667Crossref PubMed Scopus (3) Google Scholar]). An umbrella term of 'non-declarative' learning is used to describe learning that can be accomplished without the MTL system, and this includes procedural learning of skills and habits, perceptual learning and priming, and classical conditioning [102Squire L.R. Dede A.J.O. Conscious and unconscious memory systems.Cold Spring Harb. Perspect. Biol. 2015; 7: a021667Crossref PubMed Scopus (3) Google Scholar]. Procedural learning is specifically used to describe a gradual trial-and-error learning system that leads to the formation of habits, which uses the reward related circuitry through the striatum. Knowledge acquired through this system is rigid and not verbalisable [102Squire L.R. Dede A.J.O. Conscious and unconscious memory systems.Cold Spring Harb. Perspect. Biol. 2015; 7: a021667Crossref PubMed Scopus (3) Google Scholar].In some situations, both 'declarative' and 'procedural' systems can achieve learning, for example, tasks learned via memorisation can also be learned using a gradual trial-and-error strategy. The nature of the interactions between these two systems differ based on the information to be learned and the stage of learning [103Shohamy D. et al.Basal ganglia and dopamine contributions to probabilistic category learning.Neurosci. Biobehav. Rev. 2008; 32: 219-236Crossref PubMed Scopus (125) Google Scholar]. Making optimal transitions between procedural and declarative memory could explain improvements in performance. When solving arithmetic problems, children must switch from a procedure-based counting strategy to a memory-based problem-solving one. In an imaging study [104Qin S. et al.Hippocampal–neocortical functional reorganization underlies children's cognitive development.Nat. Neurosci. 2014; 17: 1263-1269Crossref PubMed Scopus (0) Google Scholar], increased hippocampal–neocortical connectivity was longitudinally associated with a switch to memory-based strategies. With respect to language, more-efficient proceduralisation could accelerate language learning because speech actions could be chunked into sequences earlier in time. While there is no direct evidence for this, a recent study offers some indirect insight. Humans with a mutation of the FOXP2 gene present with speech/language disorders and have difficulties learning and performing sequences of orofacial movements [105Watkins K.E. et al.Behavioural analysis of an inherited speech and language disorder: comparison with acquired aphasia.Brain. 2002; 125: 452-464Crossref PubMed Google Scholar]. Mice carrying the humanised version of FOXP2 could more readily use procedural strategies in a cue-enriched T-maze task than could wild mice [106Schreiweis C. et al.Humanized Foxp2 accelerates learning by enhancing transitions from declarative to procedural performance.Proc. Natl. Acad. Sci. 2014; 111: 14253-14258Crossref PubMed Scopus (0) Google Scholar]. Initial distinctions between learning systems were based on studies of a noted patient, Henry Molaison, who became densely amnesic after bilateral hippocampectomy [98Scoville W.B. Milner B. Loss of recent memory after bilateral hippocampal lesions.J. Neurol. Neurosurg. Psychiatry. 1957; 20: 11-21Crossref PubMed Google Scholar]. While he could not acquire new information such as facts and events, he was able to learn new visuomotor skills in rotary pursuit, bimanual tracking, and tapping tasks [99Corkin S. Acquisition of motor skill after bilateral medial temporal-lobe excision.Neuropsychologia. 1968; 6: 255-265Crossref Google Scholar]. This led to a distinction being made between memory systems – ones that encoded explicit knowledge about facts and events, termed 'declarative' learning systems, and 'procedural' systems, which encoded skill-based information that is implicitly expressed through performance [100Corkin S. What's new with the amnesic patient H.M?.Nat. Rev. Neurosci. 2002; 3: 153-160Crossref PubMed Scopus (0) Google Scholar]. Ever since, the acquisition of declarative memory has been shown to depend upon heavily interconnected MTL structures, including the hippocampus, entorhinal, perirhinal, and parahippocampal cortices [101Squire L.R. Zola-Morgan S. The medial temporal lobe memory system.Science. 1991; 253: 1380-1386Crossref PubMed Google Scholar]. These regions are well adapted for rapid learning, and are particularly relevant when learning occurs over a single exposure, when associations between stimuli must be made over space or time, when acquired information must be used flexibly, and when learning depends on the awareness of what is being learned (reviewed in [102Squire L.R. Dede A.J.O. Conscious and unconscious memory systems.Cold Spring Harb. Perspect. Biol. 2015; 7: a021667Crossref PubMed Scopus (3) Google Scholar]). An umbrella term of 'non-declarative' learning is used to describe learning that can be accomplished without the MTL system, and this includes procedural learning of skills and habits, perceptual learning and priming, and classical conditioning [102Squire L.R. Dede A.J.O. Conscious and unconscious memory systems.Cold Spring Harb. Perspect. Biol. 2015; 7: a021667Crossref PubMed Scopus (3) Google Scholar]. Procedural learning is specifically used to describe a gradual trial-and-error learning system that leads to the formation of habits, which uses the reward related circuitry through the striatum. Knowledge acquired through this system is rigid and not verbalisable [102Squire L.R. Dede A.J.O. Conscious and unconscious memory systems.Cold Spring Harb. Perspect. Biol. 2015; 7: a021667Crossref PubMed Scopus (3) Google Scholar]. In some situations, both 'declarative' and 'procedural' systems can achieve learning, for example, tasks learned via memorisation can also be learned using a gradual trial-and-error strategy. The nature of the interactions between these two systems differ based on the information to be learned and the stage of learning [103Shohamy D. et al.Basal ganglia and dopamine contributions to probabilistic category learning.Neurosci. Biobehav. Rev. 2008; 32: 219-236Crossref PubMed Scopus (125) Google Scholar]. Making optimal transitions between procedural and declarative memory could explain improvements in performance. When solving arithmetic problems, children must switch from a procedure-based counting strategy to a memory-based problem-solving one. In an imaging study [104Qin S. et al.Hippocampal–neocortical functional reorganization underlies children's cognitive development.Nat. Neurosci. 2014; 17: 1263-1269Crossref PubMed Scopus (0) Google Scholar], increased hippocampal–neocortical connectivity was longitudinally associated with a switch to memory-based strategies. With respect to language, more-efficient proceduralisation could accelerate language learning because speech actions could be chunked into sequences earlier in time. While there is no direct evidence for this, a recent study offers some indirect insight. Humans with a mutation of the FOXP2 gene present with speech/language disorders and have difficulties learning and performing sequences of orofacial movements [105Watkins K.E. et al.Behavioural analysis of an inherited speech and language disorder: comparison with acquired aphasia.Brain. 2002; 125: 452-464Crossref PubMed Google Scholar]. Mice carrying the humanised version of FOXP2 could more readily use procedural strategies in a cue-enriched T-maze task than could wild mice [106Schreiweis C. et al.Humanized Foxp2 accelerates learning by enhancing transitions from declarative to procedural performance.Proc. Natl. Acad. Sci. 2014; 111: 14253-14258Crossref PubMed Scopus (0) Google Scholar]. The procedural deficit hypothesis inspired a series of studies examining the non-linguistic procedural learning abilities of children with SLI, typically using a serial reaction time (SRT) paradigm (see Glossary). A meta-analysis of eight studies using an SRT paradigm with children with SLI and age-matched controls revealed small but significant effects of language impairment on this task [5Lum J.A.G. et al.Procedural learning deficits in specific language impairment (SLI): a meta-analysis of serial reaction time task performance.Cortex. 2014; 51: 1-10Crossref PubMed Scopus (0) Google Scholar], of the order of 0.33 of a standard deviation. In studies with younger participants, larger effect sizes were found. In SLI, learning was impaired more when sequences were long and complex [6Gabriel A. et al.Procedural learning in specific language impairment: effects of sequence complexity.J. Int. Neuropsychol. Soc. 2013; 19: 264-271Crossref PubMed Scopus (0) Google Scholar]. Similar problems with learning implicit sequences in the SRT task are also seen in younger typically developing children matched on grammatical ability, suggesting that implicit sequence learning in SLI may be immature rather following an atypical developmental trajectory [7Hsu H.J. Bishop D.V. Sequence-specific procedural learning deficits in children with specific language impairment.Dev. Sci. 2014; 17: 352-365Crossref PubMed Scopus (0) Google Scholar]. The learning abilities of individuals with dyslexia have also been examined using SRT measures, motivated by broader theories suggesting that the automatisation of learning is impaired in this disorder [8Nicolson R.I. Fawcett A.J. Procedural learning difficulties: reuniting the developmental disorders?.Trends Neurosci. 2007; 30: 135-141Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. A meta-analysis of nine studies that used SRT paradigms with individuals with dyslexia revealed a moderate effect of having dyslexia (0.45 of a standard deviation) [9Lum J.A.G. et al.Procedural learning is impaired in dyslexia: evidence from a meta-analysis of serial reaction time studies.Res. Dev. Disabil. 2013; 34: 3460-3476Crossref PubMed Scopus (38) Google Scholar]. This meta-analysis also indicated that age and sequence type influenced the likelihood of finding a difference between dyslexic and control groups. SRT paradigms emphasise the motor aspects of procedural learning. However, these groups also show learning impairments in non-motor paradigms. Both children with dyslexia [10Pavlidou E.V. et al.Do children with developmental dyslexia have impairments in implicit learning?.Dyslexia. 2010; 16: 143-161Crossref PubMed Scopus (0) Google Scholar] and adults with SLI [11Grunow H. et al.The effects of variation on learning word order rules by adults with and without language-based learning disabilities.J. Commun. Disord. 2006; 39: 158-170Crossref PubMed Scopus (0) Google Scholar, 12Plante E. et al.Sensitivity to word order cues by normal and language/learning disabled adults.J. Commun. Disord. 2002; 35: 453-462Crossref PubMed Scopus (0) Google Scholar] appear to have difficulty extracting structure from novel sequences in artificial grammar learning (AGL) paradigms. These difficulties in making judgements about grammaticality are not related to problems holding information in mind because deficits are present even when children with dyslexia can accurately recall a training sequence from memory [10Pavlidou E.V. et al.Do children with developmental dyslexia have impairments in implicit learning?.Dyslexia. 2010; 16: 143-161Crossref PubMed Scopus (0) Google Scholar, 13Pavlidou E.V. Williams J.M. Implicit learning and reading: insights from typical children and children with developmental dyslexia using the artificial grammar learning (AGL) paradigm.Res. Dev. Disabil. 2014; 35: 1457-1472Crossref PubMed Scopus (0) Google Scholar]. Other studies have found that children with SLI perform worse than typically developing peers at extracting regularities from speech streams in statistical learning paradigms [14Hsu H.J. et al.Impaired statistical learning of non-adjacent dependencies in adolescents with specific language impairment.Front. Psychol. 2014; 5: 175Crossref PubMed Scopus (24) Google Scholar], although they can extract relevant information when exposure is doubled [15Evans J. et al.Statistical learning in children with specific language impairment.J. Speech Lang. Hear. Res. 2009; 52: 321-335Crossref PubMed Scopus (177) Google Scholar]. Adults with dyslexia also find extracting regularities in these statistical learning tests difficult, and their performance correlates with their reading ability [16Gabay Y. et al.Impaired statistical learning in developmental dyslexia.J. Speech Lang. Hear. Res. 2015; 58: 934-945Crossref PubMed Scopus (0) Google Scholar]. Although these tasks stretch the definition of procedural learning provided by Ullman [3Ullman M.T. Pierpont E.I. Specific language impairment is not specific to language: the procedural deficit hypothesis.Cortex. 2005; 41: 399-433Abstract Full Text PDF PubMed Google Scholar], there is some indication that the same implicit learning processes are involved (Box 3).Box 3The Basal Ganglia and Their Role in LearningThe basal ganglia are a group of highly interconnected subcortical nuclei, including the striatum (caudate nucleus, putamen, and nucleus accumbens), globus pallidus, substantia niagra, and subthalamic nucleus. The basal ganglia interact with the cortex in parallel processing loops where regions of the cortex project to the striatum, which in turn project to the pallidum and then to the thalamus, and then on to frontal cortex. Such loops through the basal ganglia have traditionally been associated with motor control, particularly the selection and suppression of actions. However, the function of these loops is complementary to the cortical areas that project to the loop. The ventral striatum (nucleus accumbens) receives input from the orbitofrontal cortex, temporal cortex, amygdala, and hippocampus, and is thought to gate behaviour in response to emotional or motivational stimuli. Particularly relevant for learning are loops through the basal ganglia that take on sensorimotor, executive or associative, and limbic functions.The basal ganglia have been implicated in learning gained through practice, such as the formation of skill memories or habits