function pcn = colorednoisepdf( x, misi, epsilon, tau )
% returns the ISI pdf for PIF with colored noise drive
% x: the input ISI
% misis: the mean isi
% epsilon: a parameter
% tau: the correlation time of the noise
    gamma1 = x/tau+exp(-x/tau)-1.0;
    gamma2 = 1.0-exp(-x/tau);
    pcn=exp(-(x-misi).^2./(4.0*epsilon*tau.^2.*gamma1)).*(((misi-x).*gamma2+2*gamma1*tau).^2./(2*gamma1*tau^2)-epsilon*(gamma2.^2-2*gamma1.*exp(-x/tau))) ./ (2*tau*sqrt(4*pi*epsilon*gamma1.^3));
end