We know what a "leaf" is, and we know what "wet" means. But combining these concepts together into a "wet leaf" yields a new and possibly more specific idea. Similarly, a "brown leaf" is qualitatively different than any old leaf. Our ability to flexibly and dynamically combine concepts enables us to represent and communicate an enormous set of ideas from a relatively small number of constituents. The question of what neural systems might support conceptual combination has been a focus of research for Amy Price at Penn. Combinatorial semantics is an especially timely topic as there are ongoing debates about the anatomical systems most strongly involved in semantic memory more generally (angular gyrus? anterior temporal lobes? ventral visual regions?), as well as the nature of the information being represented (to what degree do concepts rely on sensorimotor cortices?).
In a new paper out this week in the Journal of Neuroscience (Price et al., 2015), Amy presents data from both fMRI and patients with neurodegenerative disease suggesting that the angular gyrus plays an important role in conceptual combination. Amy designed a clever task in which participants read word pairs that varied in how easily they could be combined into a single concept. For example, you could imagine that "turnip rock" is difficult to combine, whereas a "wet rock" is easier. Amy used all adjective-noun pairs, but still found a considerable amount of variability (for example a "plaid apple" combines less easily than a "plaid jacket"). This "ease of combination" was initially quantified using subject ratings, but Amy found that lexical co-occurrence statistics for these word pairs strongly correlate with their degree of combination, and thus co-occurrence measures were used in all analyses.
These findings are in good agreement with previous work emphasizing an important role for the angular gyrus in semantic representation (Binder & Desai 2011; Bonner et al. 2013).
Binder JR, Desai RH (2011) The neurobiology of semantic memory. Trends in Cognitive Sciences 15:527-536. doi:10.1016/j.tics.2011.10.001
Price AR, Bonner MF, Peelle JE, Grossman M (2015) Converging evidence for the neuroanatomic basis of combinatorial semantics in the angular gyrus. Journal of Neuroscience 35:3276–3284. http://www.jneurosci.org./content/35/7/3276.short (PDF)