Down sampling an EEG signal
Actually I am not quite sure if you should/need to downsample it with conventional methods. Did you consider using CNN (with stride larger than 1) first and then feed segment of the output to LSTM? The former step gives you some downsampling effect but it is probably better than any conventional preprocessing methods since those layers will be trainable and the whole model can be trained end-to-end (assuming that you have sufficient data).