请问灰质的0.2阈值如何设置

各位老师好：
我在老师实验室的文章Wang, Z.Q., et al., Spatial Patterns of Intrinsic Brain Activity in Mild Cognitive Impairment and Alzheimer's Disease: A Resting-State Functional MRI Study. HUMAN BRAIN MAPPING, 2011. 32(10): p. 1720-1740.中读到“The global mean ALFF value was calculated for each participant within a group GM mask obtained by selecting a threshold of 0.2 on the mean GM map of all 54 subjects.”即本文取出的灰质是先取组上平均，后卡的0.2的阈值。
我现在使用rest的计算器取出来被试样本的组上平均，请问0.2的阈值应该在哪里设置？以及如何保存呢？谢谢。