Not surprisingly, most research on the neurobiology of language focuses on language alone. However, new biological functions commonly make use of previously-existing mechanisms. Thus language likely relies on prior neurobiological substrates, whether or not these have become further specialized for language. The declarative/procedural (DP) model posits that language learning, storage, and use depend heavily on declarative and procedural memory. After all, most if not all of language must be learned, and these are arguably the two most important learning and memory systems in the brain. Crucially, both systems are well-studied at many levels in both humans and animals, leading to numerous independent predictions about language.
Our work on the DP model is strongly theoretical as well as empirical. The initial proposal of the theory in 1997 has been refined in subsequent theory papers, including extensions to second language, individual differences, and both neurodevelopmental disorders and adult-onset disorders. Much of our work on neurodevelopmental disorders is related to the Procedural Deficit Hypothesis, as well as the Declarative Memory Compensation Hypothesis and the Compensation Underdiagnosis Hypothesis, which are based on the DP model. We have also extended the model to music and math.
Initial paper, which proposed the declarative/procedural model, with supporting empirical evidence:
Ullman, M. T., Corkin, S., Coppola, M., Hickok, G., Growdon, J. H., Koroshetz, W. J., and Pinker, S. (1997). A Neural Dissociation within Language: Evidence that the mental dictionary is part of declarative memory, and that grammatical rules are processed by the procedural system. Journal of Cognitive Neuroscience, 9(2), 266-276.
Ullman, M. T., and Pierpont, E. I. (2005). Specific language impairment is not specific to language: The procedural deficit hypothesis. Cortex, 41(3). 399-433. (special issue on "The neurobiology of developmental disorders", edited by D. Bishop, M. Eckert and C. Leonard)
Ullman, M.T. (2006). Is Broca's area part of a basal ganglia thalamocortical circuit? Cortex, 42, 480-485. (special issue on "Integrative models of Broca's and the ventral premotor cortex", edited by R. Schubotz and C. Fiebach)
Ullman, M.T. (2013). The role of declarative and procedural memory in disorders of language. Linguistic Variation. 13(2), 133-154. [This paper is an updated version of Ullman, 2008, in Handbook of the Neuroscience of Language.]
Ullman, M.T., Lum, J.A.G., & Conti-Ramsden, G.M. (2014). Domain specificity in language development. In P. Brooks & V. Kempe (Eds.), Encyclopedia of Language Development (pp. 163-166). Sage Publications, Los Angeles.
Ullman, M.T. (2015). The declarative/procedural model: A neurobiologically motivated theory of first and second language. In B. VanPatten & J. Williams (Eds.), Theories in second language acquisition: An introduction (2nd ed.). New York: Routledge. pp. 135-158.
Ullman, M. T. (2016). The declarative/procedural model: A neurobiological model of language learning, knowledge and use. In G. Hickok & S. A. Small (Eds.), The Neurobiology of Language. Elsevier. pp. 953-68.