function [xdot,xinit,option] = alpha( t, x, flag)
% Alpha: second order response

global stim
switch flag
case ''
	s = interp1(stim(:,1),stim(:,2),t);	% linear interpolation
	a = 1;
	xdot = [ s - a*x(1);
		x(1) - a*x(2)];
case 'init'
	xdot = [stim(1,1) stim(end,1)];	% time span
	xinit = [0;0];		% initial condition
	option = odeset('MaxStep',0.1);	% option: not to skip the stimulus
end