Cedric Boeckx has a new paper out in the Journal of Neurolinguistics, “A conjecture about the neural basis of recursion in light of descent with modification”. The central thesis is summarised as follows: “I argue that the expansion of the parietal region associated with the globularization of the neurocranium in our species contributed to the transformation of the connection between Broca’s and Wernicke’s region via Geschwind’s territory, and enabled the pairing of evolutionary ancient networks that together became capable of constructing and processing not just sequences, but sequences of sequences”.
Boeckx writes that recursion “rests firmly on primate cognition and neural circuitry”. More precisely, the paper claims that “the neuroanatomical reconfiguration of the parietal lobe brought about by globularization extended the Broca-Wernicke connection (to be precise, its dorsal dimension) into “Geschwind’s territory”, forming a fronto-parieto-temporal circuit that provides the basis for richer representational capacities, viz. recursive capacities”.
Here, he discusses and agrees with the Friederici-inspired story that the development of the fronto-temporal dorsal stream somehow brought about a shift from single-instance concatenation operations to hierarchically organised sequences. The additional detail Boeckx provides is to claim that the expansion of the parietal lobe – a consequence of a more globularised braincase – served to bring about the strengthening of dorsal stream connections. This in turn resulted in “the pairing of two evolutionary ancient networks (one fronto-parietal, the other fronto-temporal), both of which build and process sequences”. This idea has been expressed in Berwick and Chomsky’s recent book Why Only Us, and who were themselves re-articulating the findings of primatologists from the past couple of years. However, this story says nothing about how the brain actually implements what Boeckx refers to as “the nature of the computation (and algorithm) involved”.
Boeckx goes on to claim that this pairing of two streams (each capable of finite-state computations) “could have the effect of boosting computational possibilities. Instead of operating on one-dimensional sequences, one now operates on two-dimensional, ‘tree’ representations”. Boeckx believes that if the fronto-parietal dorsal stream were to be “combined (integrated) with another sequencing machine, sequences of sequences would naturally emerge as a result”. But this appears to be something of a magic step: Pairing two finite-state machines does not produce a higher-order device, and pairing a sequence with another sequence does not necessarily produce “sequences of sequences”. While it may be true that syntactic labeling plus a form of “spell-out” provides the human-specific features of language, and that both labeling and spell-out can be loosely (but only partly) attributed to the hierarchically organised fronto-parietal structures and the fronto-temporal loop connections to externalisation, the jump from single-instance concatenation to recursion likely required more than the pairing of two sequence processors.
Still, the paper is filled with attractive ideas, such as the following: “Perhaps the fact that the parietal lobe has long been associated with numerosity … may help us understand why syntactic structures exhibit a spontaneous ‘logicality’ … intimately related to quantification, processed by the fronto-parietal network”. There is almost certainly something to this, and as Boeckx is well aware the field could certainly do with more of these sorts of multidisciplinary gestures.
There are some very concrete proposals about the neurocomputational properties of brain waves with respect to language, for instance here, here, here and here. There does, however, seem to be much confusion concerning this oscillatory approach to language. Goucha, Zaccarella and Friederici (2016), for instance, make the following claim:
“Alternative mechanisms based on brain oscillations have been proposed as a crucial element for the emergence of language (Murphy, 2015b) … However, those mechanisms seem to already be in place in other species. For example, despite the crucial brain expansion that took place in primates and especially humans compared to other mammals, the rhythmical hierarchy of oscillations is mainly kept unchanged (Buzsáki et al., 2013). As Friederici and Singer (2015) pinpoint, the basic neural mechanisms behind cognition through the hierarchical embedding of oscillations are transversal across animals.”
However, while the hierarchy of brain rhythms themselves may be preserved, it is crucially their cross-frequency coupling relations which are human-specific – a major topic for future research.
Goucha, T., Zaccarella, E., & Friederici, A.D. 2016. A revival of the homo loquens as a builder of labeled structures: neurocognitive considerations. Ms. Max Planck Institute for Human Cognitive and Brain Sciences, Germany.