Hi,
To perform a custom cross validation you can use "cvpartition" to create a custom cross-validation partitioning. You can use the 'Leaveout', 'KFold', or 'Holdout' options, and specify the 'Groups' parameter to ensure that data from the same subject is kept together in the same fold.
You can refer to the following documentation of "cvpartition" for more information: https://www.mathworks.com/help/stats/cvpartition.html
I hope this helps!
