The Phonological Loop A Key Innovation in Human Evolution

Abstract

The phonological loop-here referred to as a specialized auditory-vocal sensorimotor circuit connecting posterior temporal areas with the inferior parietal lobe (Brodmann's areas 40 and 39) and the ventrolateral prefrontal cortex (Broca's region, Brodmann's areas 44 and 45)-is proposed to have been a fundamental element associated with the origin of vocal language in human evolution. This circuit derives from auditory prefrontal networks that preexist in the nonhuman primate but that acquired an unprecedented development in the human brain. The phonological loop overlaps and possibly coevolved with a more ancient circuit involved in hand manipulation and gesture coding ( the parieto-frontal mirror neuron network) and is complementary to a "ventral" auditory circuit connecting the anterior temporal region with anterior Broca's area (area 45) via the extreme capsule. The development of the phonological loop produced a significant increase in short-term memory capacity for voluntary vocalizations, which facilitated learning of complex utterances that allowed the establishment of stronger social bonds and facilitated the communication of increasingly complex messages, eventually entailing external meaning and generating a syntactically ordered language. Furthermore, this circuit served as a template for the generation of more elaborate networks supporting these new modes of communication. Finally, the increase in short-term memory capacities associated with the elaboration of such networks facilitated the generation and processing of recursive linguistic structures based on long-distance dependencies between linguistic elements and on syntactic movement of phrase constituents. Thus, the acquisition of a functional phonological circuit can be considered as a key innovation that made possible a series of subsequent changes in human evolution leading to the complex and recursion-based language of modern humans.