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Knowing there are 100,000,000,000 neurons in an average human brain, and 7,000 dendrites in each neuron, and neurons are connected to each other by dendrites and axon terminals, how many synapses are there in an average human brain? (I don't know how many axon terminals per neuron there are on average.)

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The human neocortex contains approximately 1.5x10^14 synapses, connecting its 19–23 billion neurons.

Source: Pakkenberg, B., Pelvig, D., Marner, L., Bundgaard, M. J., Gundersen, H. J. G., Nyengaard, J. R., & Regeur, L. (2003). Aging and the human neocortex. Experimental Gerontology, 38(1-2), 95-99. doi:10.1016/S0531-5565(02)00151-1

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    $\begingroup$ For the less mathematically inclined, $1.5 \times 10^{14} = 100,000,000,000,000$, or 150 trillion. $\endgroup$ – Eoin Aug 11 '14 at 11:04
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    $\begingroup$ 150 000 000 000 000 instead :) $\endgroup$ – mounaim Aug 12 '14 at 11:56
  • $\begingroup$ @Dylan the average number of human brain neurons is 100 billion :) not 19-23. This is what I'm reading everywhere human-memory.net/brain_neurons.html for exemple $\endgroup$ – mounaim Aug 12 '14 at 12:08
  • $\begingroup$ The numbers in Pakkenberg et al apply to human neocortex, presumably ignoring subcortical structures (plus cerebellum, for instance), which would inflate the numbers somewhat. Also, if you have a solid reference from a research article using stereology, please feel free to include it. The numbers bandied around are usually just orders of magnitude (i.e. accurate to within a multiple of 10). $\endgroup$ – Dylan Richard Muir Aug 12 '14 at 13:35
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    $\begingroup$ This 2009 paper estimates the human brain as having 85 billion neurons. Interesting TED talk about it, too. $\endgroup$ – Chelonian Aug 30 '14 at 5:15
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The neocortex is just sight and hearing. You really need to include the entire nervous system's number of nerves to truly appreciate the processing power of the human neural network. How many nerves are in the human body? Even an estimate of 10^11 (100 billion) neurons times an average of 7000 dendrites or 7.0 * 10^4 connections is 7.0 * 10^15 (7,000,000,000,000,000) or 7 quintillion neural connections. Each neuron operates once every 10 ms or 100 times per second, or 700 * 10^15 or 700 quintillion operations per second.

Furthermore, the chemical connection between neurons are effectively "analog", or floating-point, so the correct term isn't "bits per second" but FLOPS. Collectively, my estimate above based on common google searches is 700 exaFLOP per second. For comparison, new the supercomputer being installed at Argonne Labs operates at 1 exaFLOPS and comes with no software.

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Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    $\begingroup$ Welcome. We ask answers to be backed up by credible sources, preferably journal papers. $\endgroup$ – AliceD Mar 25 at 19:41
  • $\begingroup$ There are multiple inaccuracies in this answer. The neocortex is not just "sight and hearing", but also taste, touch, smell, motor control, movement planning, speech, and many many more. Neurons don't perform an "operation" 100 times per second, with any meaningful comparison to a CPU's operations. Direct use of FLOPS to measure synaptic computation is also tenuous. Please back up your statements with references. $\endgroup$ – Dylan Richard Muir Mar 27 at 10:22

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