Fig. 3: Surgery 1: Implantation of chamber and headpost.

a Chamber and headpost were held by a custom implantation holder that was attached to a micromanipulator. Note the cross-shaped markers on the side of the holder, used for alignment to the interaural axis, prior to the surgery (red arrow). A wedge-shaped guide pointing downwards in the center of the holder was used for mediolateral alignment to the central skull suture. b Following skull preparation, the aligned chamber and headpost were cemented onto the skull. c Once the cement had hardened, chamber and headpost were released from the holder. d The chamber was closed with a 3D-printed nylon lid for protection. Inset shows photograph of the implant at the end of the first surgery. e Near-isometric projection, (f) top view and (g) side view of the 3D segmentation from a CT scan after the first surgery in Monkey D. Radio-opaque cement, metal parts and reference wires show the highest contrast and are colored in green. Bone is shown in semi-transparent gray. The fitted MRI-based template brain is shown in red. Note that the cement layer in the center of the chamber is very thin and therefore not visible everywhere in the segmented data.