The 49-pixel electrode array pictured consists of a 7x7 arrangement of 0.89-mm diameter, flat-topped stainless steel electrode ‘pins’, each surrounded by a 2.36-mm diameter air gap insulator. A flat stainless steel plate coplanar with the electrode pins serves as the return current path. The electrodes are  arranged on a square grid with 2.54 mm interelectrode spacing. The lower photo shows a detail of the electrode array.