Questions about Physionet EEG Motor Movement/Imagery Dataset

[dolce]

Hello all:

My group and I are working on a project with Physionet EEG Motor Movement/Imagery Dataset: https://physionet.org/content/eegmmidb/1.0.0/. However, the dataset doesn't mention the information of reference electrode and ground electrode.
Does anyone know what montage was used to collect the data? Was it referential electrodes or bipolar electrodes?
Please share your experience or suggestion if you are also using the same dataset.

Thanks,
Yixuan Lyu