Primate brains

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Chapter 12 endnote 11 & 14, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

[note 11] ...a human brain is almost five times as large as a macaque brain. [...] Most of the evolutionary changes have occurred in the cortical areas that have many neurons for processing prediction errors.


[note 14] Chimpanzees, our genetically closest cousins, have larger brains than macaques do, with more of the wiring necessary for integrating sensory information. A human brain is still three times as large as a chimp brain, though, with more of this critical wiring.

The human brain makes meaning of events in the world as they relate to the needs of the body. As the primate brain evolved, it expanded most in regions that integrate prediction errors from vision, hearing, interoception, and other sensory systems, including parts of the interoceptive network (specifically, nodes within the default mode network, the dorsal extent of the salience network) and the control network. The expansion was primarily due to more connections between the neurons, and (I speculate based on anatomical studies [1]) mainly for neurons that are necessary for multimodal learning and predictions across multiple time scales.

Jared Diamond called humans the “third chimpanzee,” along with the common chimpanzee and the bonobo. And he may be correct. The human brain appears to be chimp-sized visual, auditory, somatosensory and motor networks connected to expanded interoceptive and control networks, each of which contains some limbic tissue (i.e., association tissue has expanded[2][3][1]). Instead of the “triune brain,” we have a “souped-up” ape brain, rewired at the front, that more efficiently integrates, compresses and summarizes information across all your sensory systems. This is the same machinery from chapters 4 through 6 that helps you build goal-based concepts.

In the sensory networks (similar in humans and apes), there are more neurons, they are smaller in size and less connected to one another. In multimodal association cortex (in the frontal, temporal, and parietal lobes), where human cortex has expanded relative to apes, there are fewer neurons, they are much larger in size, and they have many more connections. Expansion has occurred primarily in cortical layers 2 and 3,[1] where prediction errors are processed, and where the neurons that control prediction errors (sometimes called precision units) reside.

Some of these larger, better connected neurons are in the default mode and salience components of the interoceptive network and they compute shared, multimodal features (i.e., goal based abstractions) across concepts. Some are in the control network, and they assist in categorization.[4] (When compared to humans, macaques do not have a fully formed control network;[5] ditto for the dorsal extent of the salience network.[6]) Some are in the language network (that spatially overlaps with the default mode and control networks). This is interesting, because it might explain why humans are capable of the things other primates can’t do: increased abstraction and social coordination, as well as mental time travel, that is required for developing social reality and full-blown culture. For more discussion on this point, see compressing prediction errors in the cerebral cortex.

One intriguing possibility is that the evolutionary changes in the primate brain follow the conceptual cascade: less change within the sensory networks that represent sensory features and greater change in networks with multimodal association neurons that represent multi-sensory features (i.e., summaries of the co-occurring sensory details). This is roughly a back-to-front gradient. Similar back-to-front gradients have been proposed for various brain functions.[7] For example, the brain controls and executes physical actions with representations that are selected, maintained, and inhibited in multisensory (i.e., abstract) neurons that cascade to the neurons that represent finer-grained details. [8][9][10]

As far as evolutionary changes, the front of the brain is where a lot of where the action is. The largest differences between chimps and humans happen to be the regions that:

  1. Myelinate late (consistent with the fact that much of human-chimp brain size differences occur as human brain growth after birth)[11]
  2. Have more long range connections that are important for giving us the kind of mind we have[11]
  3. Tend to have the greatest individual variation in humans (so early experience matters!)[11]

Also, these are the regions that contain the brain’s rich club hubs, the brain’s backbone for communication that is responsible for synchronizing brain activity, and possibly consciousness.[12][13]


Notes on the Notes

  1. 1.0 1.1 1.2 Finlay, Barbara L., and Ryutaro Uchiyama. 2015. “Developmental Mechanisms Channeling Cortical Evolution.” Trends in Neurosciences 38 (2): 69–76.
  2. Preuss, Todd M. 2011. "The human brain: rewired and running hot." Annals of the New York Academy of Sciences 1225 (S1): E182-E191. (table 3)
  3. Sherwood, Chet C., Amy L. Bauernfeind, Serena Bianchi, Mary Ann Raghanti, and Patrick R. Hof. 2012. "Human brain evolution writ large and small." In Progress in Brain Research, Volume 195: Evolution of the Primate Brain From Neuron to Behavior, edited by Michel A. Hofman and Dean Falk, 237-254. New York: Elsevier.
  4. E.g., Rigotti, Mattia, Omri Barak, Melissa R. Warden, Xiao-Jing Wang, Nathaniel D. Daw, Earl K. Miller, and Stefano Fusi. 2013. "The importance of mixed selectivity in complex cognitive tasks." Nature 497 (7451): 585-590.
  5. Mantini, Dante and others. 2013. "Evolutionarily novel functional networks in the human brain?" The Journal of Neuroscience, 33(8):3259 –3275.
  6. Touroutoglou, A., E. Bliss-Moreau, J. Zhang, D. Mantini, W. Vanduffel, B. Dickerson, and L. F. Barrett. 2016. “A Ventral Salience Network in the Macaque Brain.” Neuroimage 132: 190–197.
  7. Badre, David. 2008. "Cognitive control, hierarchy, and the rostro-caudal organization of the frontal lobes." Trends in Cognitive Science 12 (5): 193-200.
  8. Fuster, Joaquı́n M. 1997. "Network memory." Trends in Neurosciences 20 (10): 451-459.
  9. Fuster, Joaquı́n M. 2001. "The prefrontal cortex—an update: time is of the essence." Neuron 30 (2): 319-333.
  10. Fuster, Joaquın M. 2004. "Upper processing stages of the perception–action cycle." Trends in Cognitive Sciences 8 (4): 143-145.
  11. 11.0 11.1 11.2 Buckner, Randy L. and Fenna M. Krienen. 2013. "The evolution of distributed association networks in the human brain." Trends in Cognitive Sciences, 17(12): 648-665,
  12. Van den Heuvel, Martijn P., and Olaf Sporns. 2013. “An Anatomical Substrate for Integration Among Functional Networks in Human Cortex.” Journal of Neuroscience 33 (36): 14489–14500.
  13. Chanes, Lorena, and Lisa Feldman Barrett. 2016. “Redefining the Role of Limbic Areas in Cortical Processing.” Trends in Cognitive Sciences 20 (2): 96–106.