You mention that your father worked as a test and aerobatic pilot: the military aviation medics in the UK became very concerned, as long ago as the 1950s, about the effects of negative "g" on the brain, and around that time imposed limits (I don't have details) on exposure to negative "g" for RAF (and probably RN) aircrew.
The RAF Institute of Aviation Medicine, which did the work, is long gone. Its successor is the RAF Centre of Aviation Medicine (a much smaller outfit), located at RAF Henlow, in Bedfordshire.
I have not tried to track down any papers, but something may be available through The National Archives at Kew - possibly worth trying to follow up if negative "g" featured prominently in your father's flying. (My background is pilot, not medic, so definitely not an expert in this field)