If you look at the size of a matrix
X = rand(3,4)size(X)
You will see that it is of size 3×4. However, when you look at the size of X(:,1,:)
it is not a matrix but a three-way array of size 5x1x61.
Thus, we can easily see that it is in fact a 5×61 matrix, but MATLAB sees it as a three-way array where one of the ways/modes is of dimension 1.
So in order to be able to plot this actual matrix held in a three-way array, the three-way array must be converted to matrix.
The function squeeeze will do this (type help squeeze)