=====Number of neurons in a voxel=====

In describing fMRI, I often try and communicate the degree of information averaging that is taking place in a single voxel. Here is a computation relevant to this:

  - Number of neurons in the human cerebral cortex: 16 billion (60 billion glia)
    * Lent, R., Azevedo, F. A. C., Andrade-Moraes, C. H. and Pinto, A. V. O. (2011), [[http://onlinelibrary.wiley.com/doi/10.1111/j.1460-9568.2011.07923.x/abstract|How many neurons do you have? Some dogmas of quantitative neuroscience under revision]]. European Journal of Neuroscience. doi: 10.1111/j.1460-9568.2011.07923.x
    * Suzana Herculano-Houzel. (2009) [[http://www.frontiersin.org/human_neuroscience/10.3389/neuro.09.031.2009/full|The human brain in numbers: a linearly scaled-up primate brain]]. 
  - Number of 1mm gray-matter, cortical voxels in an anatomical scan: ~684,000
  - Number of 1mm gray-matter voxels in a 3mm isotropic functional voxel: 27

Number of neurons in a functional voxel:

{{:public:other:math_984_99e5cf1f9f79d8ec283cbfb38e07d460.png|}}

<code>{{1.6x10^10}/684000}*27 ≈ 630,000</code>

So, there are roughly 630 thousand neurons in a given functional (cerebral cortical) voxel, and roughly 4 times as many glial cells.

An earlier version of this page had an estimate that was an order of magnitude larger, and derived from a widely cited (but apparently erroneous) rule-of-thumb of the number of cortical neurons.