The parietal lobe has a unique central location in the brain, and it is involved in higher cognitive functions. Investigating its functions and connectivity is essential to understand its role in uniquely human abilities. Two recent works have put emphasis on the importance of the parietal lobe for tool use.
Catani and colleagues investigated the intralobar parietal connectivity in human and monkey brains, using diffusion imaging tractography. In general, the patterns of white matter connectivity are similar in both species, although with some differences for areas that are distinct in humans. The larger tract connects the superior parietal lobule (SPL) to the angular and supramarginal gyri of the inferior parietal lobule, within the IPS. The authors suggest it might act to mediate the interaction between the two lobules during object manipulation, and to coordinate both dorsal and ventral visuospatial networks. The second and third larger tracts link the postcentral gyrus to the inferior parietal lobule and to the SPL, respectively. These might transmit tactile and proprioceptive information on the body orientation relatively to an object for guiding motor actions and grasp. The connection between the postcentral and the angular gyri was only observed in humans, leading the authors to highlight its role in specific cognitive functions. Particularly, its connections to SPL are key for tool use, mathematical thinking, and language and communication.
Kastner and coworkers reviewed the organization and function of the dorsal pathway of the visual system of monkey and human brains, focusing on the areas of the posterior parietal cortex within and adjacent to the intraparietal sulcus (IPS). Monkeys and humans have diverged in the functional contributions of the IPS since their last common ancestor, as some functionally-defined areas that are located within the IPS in monkeys have been partially relocated outside this sulcus in humans. The authors suggest this relocation might be due to expansion for accommodation of human-specific abilities, such as tool use. They hypothesize that humans might have developed a derived and advanced tool network from the modification of the macaque circuit for object manipulation. First, the human dorsal vision pathway must provide object shape information regardless of size and viewpoint, facilitating object recognition and mental manipulation. Second, object information is integrated with cognitive information such as working memory, allowing maintaining the information over a period of time. Finally, humans have areas that respond specifically to tool use, some of which also integrate frontal and temporal networks involved in action recognition and semantic knowledge related to tools and actions, respectively.
Both studies point at the parietal lobes and visuospatial integration as key elements for human cognitive capacity, as suggested by evidence in paleoneurology, evolutionary neuroanatomy, and cognitive archaeology.