I know how to solve equations of $t$, but I'm not sure what $t'$ represents in this equation???? I am investigating the Wilson Cowan neuron population model, and I can follow most of it, but I'm not sure what is meant by $t'$ in the equation for proportion of neurons in the refractory period. Here is a link to the article.
The image below is the equation of interest.
What is $t'$?

Notice the
"$dt'$"
in the integrals.
This means that
$t'$ is the variable of integration.
Any other variable could be used
as long as it is not used
elsewhere in the expression.
For example,
the first integral
could have been written
$\int_{t-r}^t E(z) dz
$.
The only variables
that could not have been used
as the variable of integration
are $t$ and $r$
since they already have a 
specified meaning.
