(A) Principal components analysis biplots using all 60 electrodes. Each marker is a recording. Colors indicate batches: Batch 1 (purple), Batch 2 (red), Batch 3(orange), Batch 4 (light green), Batch 5 (grey), Batch 6 (blue), Batch 7 (pink), and Batch 8 (dark green). The large black circle is the dense core of recordings showing weak spike activity. Classes A, B and C are also labeled. (B) PCA biplots using the 47 electrodes following deletion of biased ones. Some regions are associated with specific cultures identified by culture numbers and batch colors.