Vaesen lists nine cognitive capacities crucial to tool use in humans and discusses to what extent they are also present in the great apes. One of these capacities is causal reasoning, which, as stressed by Vaesen, does not simply involve noticing the covariance between a cause and an effect, but also allows subjects to infer a mechanism relating the two. The author concludes that current evidence suggests that this capacity is present in apes only to a modest degree. He does not indicate, however, any possible anatomical basis for this cognitive difference between apes and humans.

Recently, Peeters et al. (Reference Peeters, Simone, Nelissen, Fabbri-Destro, Vanduffel, Rizzolatti and Orban2009) examined the neural basis of tool use in human and non-human primates (rhesus monkeys). In a comparative fMRI study, they scanned human volunteers and untrained monkeys, as well as monkeys trained to use tools, while they observed hand actions and actions performed using tools. In both species, presentation of an action activated occipito-temporal, intraparietal, and ventral premotor cortex bilaterally. In humans, however, the observation of an action performed with tools yielded an additional, specific activation of a rostral sector of the left inferior parietal lobule, referred to as the anterior supramarginal gyrus (aSMG) tool use region. They proposed that this region, unique to humans, underlies a specific way of understanding tool actions based on the appreciation of the causal relationship between the intended use of the tool and the result obtained by using it.

That monkey parietal cortex contains only the biological hand-action observation areas need not imply that these areas cannot be modified by the training to use tools, as has been documented by Iriki et al. (Reference Iriki, Tanaka and Iwamura1996). However, the fact that monkeys learned to use simple tools does not necessarily imply an understanding of the abstract relationship between tools and the goal that can be achieved by using them. The tool used, for example, the rake, might simply become, with training, a prolongation of the arm, as shown by the response properties of neurons recorded in the medial wall of the intraparietal sulcus.

The human aSMG region was discovered by an interaction analysis subtracting out static shape differences between the tool-use and hand-action videos. This suggests that the human area uses differences in kinematics to distinguish tool actions from biological actions. This links nicely with another species difference that has been discovered by Orban et al. (Reference Orban, Claeys, Nelissen, Smans, Sunaert, Todd, Wardak, Durand and Vanduffel2006): The human parietal cortex is much more sensitive to visual motion than is its monkey counterpart. Some of these motion-sensitive areas, such as dorsal intraparietal sulcus anterior (DIPSA), are very close to the tool area, providing, possibly, an anatomical link with aSMG. Thus, human parietal cortex is not simply more sensitive to three-dimensional form from motion (Vanduffel et al. Reference Vanduffel, Fize, Peuskens, Denys, Sunaert, Todd and Orban2002), providing more sophisticated higher-order visual analysis capacities for guiding tool action; this cortex is also more sensitive to lower-order motion, providing the kinematics signals for the aSMG region.

Two further implications of the aSMG discovery are relevant to the present discussion. First, as commented upon by Peeters et al. (Reference Peeters, Simone, Nelissen, Fabbri-Destro, Vanduffel, Rizzolatti and Orban2009), the grouping of tool-related neurons in the aSMG might dramatically increase the computational power of this neuronal population. Interestingly, Vaesen discusses how causal reasoning, which we propose to be implemented in the aSMG, may be instrumental in the development of technology by increasing the cost-effectiveness of individual learning strategies. As mentioned above, the grouping of tool-related neurons is lacking in monkeys. However, these neurons might be scattered throughout the biological hand-action observation circuit and, therefore, remained unnoticed in the MR scanner. To what extent the grouping may already be present in great apes remains a topic for further experimentation. The existence, however, of such an embryonic grouping, if present, could explain some of the rudimentary cognitive abilities related to tool use in apes.

Second, the human aSMG area corresponds to regions where MR responses have been measured during pantomiming and imagining tool use (see Lewis Reference Lewis2006 for review). Hence, the human aSMG region is involved in both the observation of tool actions and their planning. It is, therefore, conceivable that it houses neurons with mirror-neuron–like properties (Rizzolatti & Craighero Reference Rizzolatti and Craighero2004) that allow for both tool use and tool-use understanding. This may support tool imitation and learning by imitation. A word of caution is of course needed, as the presence of mirror neurons has yet to be demonstrated in aSMG, and their presence is a necessary but not sufficient condition for imitation to develop.

In conclusion, with the proviso that Peeters et al. (Reference Peeters, Simone, Nelissen, Fabbri-Destro, Vanduffel, Rizzolatti and Orban2009) studied rhesus monkeys and not great apes, their findings are in striking agreement with the review of Vaesen and provide a neuronal basis for species differences in eye-hand coordination and in causal reasoning related to tool use.