To have sum apply to all elements of the array rather than across a particular dimension, you can use the 'all' flag. (Alternates that are common in old code: sum(sum(X)) or sum(X(:)) also work for a matrix X).
a=rand(10);
i = 4;
N = 5;
cols = (N+1)/2:N;
sum(a(i,cols),'all')
