What is it that we can do and most animal species can't manage? Language, tool-making and tool-usage, music-making and music perception, social cooperation (gjoint attentionh) and pictorial depth perception (seeing the 3D depth in a 2D picture, painting, drawing or photograph) come to mind.
Are these all unrelated talents, or is there a commonality that allows certain brains to handle these kinds of processes, and other brains to fail? I suggest that there is a common cognitive mechanism: these and the other uniquely human talents are examples of "triadic thinking" - having three simultaneous processes in mind at the same time. It sounds difficult, but it is something we all do virtually all of the time. In general, the animal brain can handle two items - associations between X and Y, but "conditional associations" (X in relation to Y in light of Z) are already beyond the cognitive capacities of our furry friends. For us, it's easy - and easily demonstrated in the realm of harmony perception and pictorial depth perception. Discussions of these talents at the perceptual level (harmony and pictorial depth perception) can be found here:
Empirical Musicology Review, 2006, Empirical Musicology Review, 2007, Music Perception, 2007, Spatial Vision, 2007, Empirical Studies of the Arts, 2008, American Scientist, 2008, Music Perception, 2009
and the full cognitive story has been published in: Harmony, Perspective and Triadic Cognition (Cambridge University Press, New York, 2012).
Why is it that only the human brain can do three-way associations? Some of the answer comes from 19th Century neuroanatomy. With the exception of a small sliver of neocortex in the superior temporal sulcus of the chimpanzee brain, only the human brain has significant expanses of trimodal association neocortex. These are cortical areas at which sensory information from the visual, auditory and touch modalities converge on the same neurons. And it is there that the algorithms for triadic processing first evolved. Brodmann Areas 39 and 40 in posterior cortex are trimodal and portions of prefrontal cortex are also. Prefrontal cortex is where "conditional associations" are formed. See Terry Deacon's, The Symbolic Species, for a discussion of the brain localization of conditional associations and see Michael Tomasello's, The Cultural Origins of Human Cognition, for a slightly different perspective on "triadic cognition".
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