Double blind means neither the subject nor the assessor knows whether that subject had the control (placebo or such) treatment or the studied treatment. In HBOT, the subject would have to be put in the HBOT chamber with the pressure increased but the oxygen level left at a normal ratio for the control group. (even this would not be true double blind) Then, the subjects would be studied for any changes before the data is collated by whether or not the data is for a control group subject or a studied treatment group subject using random identifiers. This removes any bias or other influence that may skew the results.

The studies so far have focused on MRI images that show an increase in capillary growth in those who have had HBOT. They extrapolate this to say that capillary growth signifies neurogenesis or such.

What is not discussed is whether these 40 to 80 sessions have an impact on healing due to the need to lay relaxed for these scheduled period of times. Meditation claims to have similar effects on capillary growth. Or whether there is a placebo effect.