A comparative fMRI study by Peeters et al. (2009) provided evidence that a specific sector of left inferior parietal lobule is devoted to tool use in humans, but not in monkeys. We propose that this area represents the neural substrate of the human capacity to understand tool use by using causal reasoning.

Cohen Kadosh & Walsh (CK&W) discuss the limitations of the behavioral, imaging, and single-cell studies related to number representation in human parietal cortex. The limitations of the imaging studies are grossly underestimated, particularly those using adaptation paradigms, and the problem of establishing a link between single-cell studies and imaging is not even addressed. Monkey functional magnetic resonance imaging (fMRI), however, provides a solution to these problems.

Single-cell studies of monkey posterior parietal cortex (PPC) have revealed the extensive neuronal representations of three-dimensional subject motion and three-dimensional layout of the environment. I propose that navigational planning integrates this PPC information, including gravity signals, with horizontal-plane based information provided by the hippocampal formation, modified in primates by expansion of the ventral stream.