function isivec = isis( spikes )
% returns a single list of isis computed from all trials in spikes
%
% isivec = isis( spikes )
%
% Arguments:
%   spikes: a cell array of vectors of spike times in seconds
%   isivec: a column vector with all the interspike intervalls
%
% Returns:
%   isivec: a column vector with all the interspike intervalls

    isivec = [];
    for k = 1:length(spikes)
        difftimes = diff( spikes{k} );
        % difftimes(:) ensures a column vector
        % regardless of the type of vector in spikes{k}
        isivec = [ isivec; difftimes(:) ];
    end
end