Electrode device for monitoring and/or stimulating activity in a subject

An electrode device is disclosed that is removably implantable at least partially in a bone or other tissue. The electrode device includes a head and a shaft connected to the head. The shaft has a shaft body extending distally from the head in an axial direction of the shaft, and a conductive element including a conductive surface at a distal end of the shaft. A plurality of discrete anchor elements can project from an outer surface of the shaft body in a transverse direction of the shaft. A conductive wire can be permanently fixed to a proximal end surface of the conductive element, the end surface being located in or adjacent the head. The head can have a convex outer surface and a concave inner surface. An electrode array and a reamer tool is also disclosed.