Neurons are multipurpose

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Chapter 13 endnote 5, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is: matter how finely or coarsely you look at brain tissue ​— ​as networks, regions, or individual neurons ​— ​that tissue contributes to more than one category of mental event, such as anger, attention, or even vision or hearing. [...] A single brain cell can be multipurpose, as we discussed in chapters 1 and 2, contributing to multiple psychological states.

I have used the term "domain-general"[1] but the philosopher Michael Anderson has used the more straightforward multiuse.[2]

The brain is a complex system in which a single neuron contributes to multiple mental phenomena. In chapter 4, we learned that neurons in one sensory system, such as vision, carry information for other sensory systems, such as auditory and somatosensory, in a supporting role. (In a sense, all neurons can be thought of as association neurons, depending on what phenomenon you are interested in explaining.) This is often a more surprising realization than it should be. Evolution favors multi-use solutions.

This feature of “multiple use” is not unique to human brains. In monkey brains, neurons in the control network respond to the distinguishing features of different categories at different times (e.g., distinguishing cars at one point, distinguishing dogs at another, up to 24 categories), depending on which other neurons they are firing; increasing the computational power of the cortex.[3] In rat brains, certain neurons in subcortical regions fire during positivity in some moments but during negativity in others, again depending on context.[4][5] Bird brains contain neurons critical for singing that also regulate homeostatic budgeting (they are limbic) and social behavior.[6] These neurons, which are equivalent to neurons in the human default mode network, are contributing to multiple mental phenomena at exactly the same time.

Scientists used to believe that there were neurons in the ventral temporal cortex dedicated to representing faces, objects, plants, bodies, words, and so on, but these neurons instead code for the conceptual similarities among the things that you see.[7] Neurons within the “fusiform face area” located in the right ventral temporal cortex represents the shapes of words and neurons within the “word form area” located in the left ventral temporal cortex represents faces.[8] There is no one-to-one mapping between a neuron and a function even in motor cortex; a single neuron in primary motor cortex connects to many different pools of motor neurons in the spinal cord (which direct physical movements), and widely different neurons contribute to the control of a single body part.[9][10][11][12][13] Motor neurons are involved in representing concepts, in language, and in decision making[14] and even help to regulate the autonomic nervous system, the endrocrine system, and the immune system.[15] The very same neurons code for vastly different information at different times (e.g., first encode info about stimuli and then change to encode info about movements; or first about decision making and then about movements).[16] Whatever your interoceptive network is doing to keep your body-budgets solvent, it is also allowing you to think, to feel, to see and smell, etc.

Progress in science doesn’t always mean designing more precise measurement tools, or having a turf battle over what to name something. It often requires trying to understand what this kind of functional overlap means for how the mind works.

Notes on the Notes

  1. Barrett, Lisa Feldman and Ajay B. Satpute. 2013. "Large-scale brain networks in affective and social neuroscience:  Towards an integrative architecture of the human brain." Current Opinion in Neurobiology 23 (3): 361-372.
  2. Anderson book [full reference to be provided]
  3. 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.
  4. Berridge [full reference to be provided]
  5. citations Anderson p. 32 [full reference to be provided]
  6. Syal, Supriya, and Barbara L. Finlay. 2011. "Thinking outside the cortex: Social motivation in the evolution and development of language." Developmental Science 14 (2): 417-430.
  7. Grill-Spector, Kalanit, and Kevin S. Weiner. 2014. "The functional architecture of the ventral temporal cortex and its role in categorization." Nature Reviews Neuroscience 15 (8): 536-548.
  8. Behrmann Marlene [full reference to be provided]
  9. Graziano et al., 2002a [full reference to be provided]
  10. Graziano et al., 2002b [full reference to be provided]
  11. Schieber, Marc H., and Lyndon S. Hibbard. 1993. "How somatotopic is the motor cortex hand area?" Science; Washington 261 (5120): 489.
  12. Schieber, 2001 [full reference to be provided]
  13. Graziano, 2011 [full reference to be provided]
  14. E.g., Pulvermuller, Friedemann. 2013. "How neurons make meaning: brain mechanisms for embodied and abstract-symbolic semantics." Trends in Cognitive Sciences 17 (9): 458-470.
  15. Dum, Richard P., and Peter L. Strick. 2013. "Transneuronal tracing with neurotropic viruses reveals network macroarchitecture." Current Opinion in Neurobiology 23 (2): 245-249.
  16. Cisek, Paul, and John F. Kalaska. 2010. "Neural mechanisms for interacting with a world full of action choices." Annual Review of Neuroscience 33: 269-298.