The brain differentiates human and non-human grammars: Functional localization and structural connectivity

TLDR

The human language faculty is grounded in the ability to process hierarchically structured sequences, a capacity that exceeds processing of simple transitional probabilities observed in non-human primates. The study demonstrates that processing of hierarchical versus local transition sequences is supported by distinct brain areas. Local transition processing is subserved by the left frontal operculum, while hierarchical dependencies are computed by Broca's area, and tractography reveals distinct structural connectivity signatures that support segregation of these two regions in the left inferior frontal cortex.

Abstract

The human language faculty has been claimed to be grounded in the ability to process hierarchically structured sequences. This human ability goes beyond the capacity to process sequences with simple transitional probabilities of adjacent elements observable in non-human primates. Here we show that the processing of these two sequence types is supported by different areas in the human brain. Processing of local transitions is subserved by the left frontal operculum, a region that is phylogenetically older than Broca's area, which specifically holds responsible the computation of hierarchical dependencies. Tractography data revealing differential structural connectivity signatures for these two brain areas provide additional evidence for a segregation of two areas in the left inferior frontal cortex.