Data has his head open several times, for example here:
Given the time of creation of Star Trek - Next Generation series, I don't assume this would be done with CGI, also his head doesn't seem to be thicker due to the second "layer". There are also several occasions, where is Data moving and speaking with his head open, so I would also exclude possibility, that opened head is a replica.
So, I naturally wonder, how is this made?
Thank you, live long and prosper.