code: def _cross_validation_for_one_voxel(clf, vid, num_folds, subject_data, labels):
      
      # no shuffling in cv
      skf = model_selection.StratifiedKFold(n_splits=num_folds,
                                            shuffle=False)
      scores = model_selection.cross_val_score(clf, subject_data,
                                               y=labels,
                                               cv=skf, n_jobs=1)
      logger.debug(
          'cross validation for voxel %d is done' %
          vid
      )
      return (vid, scores.mean())
docs: Score classifier on data using cross validation.