Capítulo XVIII
TRAUMATISMOS, ENVENENAMIENTOS Y ALGUNAS OTRAS CONSECUENCIAS DE CAUSA EXTERNA
Humans, unlike other primates, have evolved an outstanding ability to manipulate complex tools (Johnson-Frey, 2004). Although non-human primates can use simple tools to reach for food or objects that are otherwise out of reach, non-human primates are still unable to perform complex tool-use that requires an understanding of the causal relationship between the tool‟s mechanical properties and the goal to achieve with it (Ambrose, 2001). For instance non-human primates are unable to combine the usage of two tools (e.g., merge two sticks together) in order to reach an object far in extra personal space (Frey, 2007). Anthropological research has suggested that the moment that the hand became free from being a limb primarily engaged in locomotion, manipulable skills could emerge (Berthelet & Chavaillon, 1993). Hands became an interface to
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interact with the external world and its subsequent morphological changes played a critical role in human evolution. The emergence of the opposable thumb signalled a critical step in the development of skilled manipulation abilities and is considered a physical adaptation that contributed to the survival and evolution of the human species by allowing control of external objects (Marzke & Marzke, 2000; John Russell Napier & Tuttle, 1993; Young, 2003). This fundamental step in human evolution is thought to have caused not only morphological and functional changes of the hand but also of the brain. Indeed, from the neuro-structural point of view, the close anatomical relationship between language and praxis, both being lateralised in the left hemisphere, has been considered a sign of an evolutionary step in which the neural substrate subserving skilled actions (e.g., tool-use) provided the underpinnings for the development of language and thought. Thus, the evolution of the hand and the emergence of tool-use have been considered key turning points in human evolution (Berthelet & Chavaillon, 1993; Marzke & Marzke, 2000; John Russell Napier & Tuttle, 1993; Young, 2003).
The extraordinary human manual dexterity and ability to manipulate complex tools rely on a left lateralised network of areas that comprises temporal, parietal and frontal cortices. In this thesis, I provided compelling evidence that representations for hands and tools extensively overlap in the left LOTC and left aIPSv. Moreover, functional connectivity analyses showed selective connectivity between the left LOTC (but not neighbouring regions such as EBA and LO) and the left aIPSv hand/tool regions. These results therefore strengthen the notion that representations for hands and tools are uniquely linked, and that this association manifests itself neurally in both visual and motor areas in the human brain. These results give rise to the interesting question of whether or not a similar coupling between hand and tool representations can be observed in homologous brain areas of non-human primates that proficiently manipulate simple tools (e.g., sticks to reach for food). Evidence for differential functional hand-tool representations between human and nonhuman primate brains would further address the pivotal role of the hand‟s morphological transformations through evolution.
Although primate tool-use is not as dexterous as human complex tool-use (Frey, 2007) and might lack much of the conceptual reasoning that permits for understanding of the abstract relationships between tools and action-goals (Ambrose, 2001), monkeys use simple tools such as rakes or pliers proficiently after training (Umilta, et al., 2008). In a recent fMRI study, Peeters and colleagues (2009) investigated functional homologues between monkeys (before and after tool-use training) and humans during observation of actions performed either with the hand or with a tool.
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Overall, their results revealed a general correspondence across species in line with the findings described in this thesis for hand-tool selective representations in the left LOTC and left aIPSv. Indeed, the authors reported a region in the lateral occipitotemporal cortex (homologues of the inferior temporal TEO in monkey) and an area within the intraparietal sulcus which responded during observation of both hand and tool actions in humans and monkeys (the authors also reported evidence for activation in rostra IPL unique in human for observation of tool actions). However, a possible confound in fully interpreting the outcomes of this comparative study (Peeters, et al., 2009) can be found in the videos used in the experimental paradigm. That is, in the videos showing tool actions, the hand manipulating the tool was partially visible. Therefore, this confound does not allow us to conclude with 100% certainty that TEO and the intraparietal sulcus in monkeys respond to both hand and tool actions. Indeed, it might be that the response found during observation of tool actions in these regions was partially driven by the presence of the hand on the screen.
In light of the findings reported in this thesis, future studies will be needed to determine the structural and functional homologues of hand and tool representations between humans and non- human primates. The remaining open questions are the following: (1) Do hand/tool areas reported in the human left LOTC and left aIPS also exist in the non-human primate brain? And if so, do they show a similar functional profile? (2) Does extensive tool-use training modulate the functional profile of putative hand-preferring (and tool-preferring) areas in the non-human primate brain? Comparative studies that address these questions will help to further describe quantitative and qualitative differences (Peeters, et al., 2009) and similarities (Kriegeskorte, et al., 2008) between the human and non-human primate visual system. Moreover, comparative studies on the hand-tool link and its neural correlate will further test the proposition advanced in this thesis that specialisation of such a neural substrate has an evolutionary basis.