how to explain the "search volume" in spm?

最近，被review问及了一个问题，如下：
It was unclear to me the nature of the correction cited for the correlation of CESD(抑郁量表) with ICA signal. Was this voxelwise or clusterwise? What was the search volume?

关于第二问我很费解，What was the search volume. 这个要我回答什么呢？ 关于search volume，见附件。如何正确的回答他这个问题，search volume 的官方解释到底是什么，我查了很久，没找到书面的解释。请高人多多指点。