Answer
${\bf 1.08}\;\rm \mu A$
Work Step by Step
We can model this problem as follows.
Let's assume that the brain is a perfect sphere of radius 8 cm, so we need to measure the magnetic field strength of a loop current at a distance of 8 cm from its center where the point is on the loop's axis.
We know that the magnitude of the magnetic field at some point on the axis of a current loop is given by
$$B=\dfrac{\mu_0 I R^2}{2(z^2+R^2)^{3/2}} $$
At the center of the loop where $z=R$, so
$$B=\dfrac{\mu_0 I R^2}{2(R^2+R^2)^{3/2}} =\dfrac{\mu_0 I R^2}{2 (2)^{3/2}R^3} $$
$$B= \dfrac{\mu_0 I }{ (2)^{5/2}R } $$
We need to find the current, so
$$I=\dfrac{ (2)^{5/2}R B}{\mu_0}$$
Plug the known;
$$I=\dfrac{ (2)^{5/2}(0.08)(3\times 10^{-12})}{(4\pi\times 10^{-7})}$$
$$I=\color{red}{\bf 1.08}\;\rm \mu A$$
