The first test measured the ability of the groups to distinguish colors. The control group scored the best, the former LSD users fell in the middle, and the HPPD group scored the worst. The results were statistically significant with a P < 0.001, which theoretically means that there was less than 1 in 1000 chance that the measured differences were caused by random chance. [Abstract] The second test measured the ability of each group to distinguish light flicker. A light was rapidly strobed on and off. Above a certain frequency subjects perceived the flashing light to be on continuously. Again, the control group performed best, the former LSD users fell in the middle, and the HPPD subjects scored the worst. The results were again statistically significant (P<0.001). [Abstract] These experiments seem to show that even HPPD-free LSD users may be experiencing long-term changes in their vision. It is not known why these changes are so distracting to people with HPPD while not being bothersome to other LSD users. It is very important to note, however, that not much research has been done in this area and these results have not been confirmed by other research.