There are now three distinct methods to measure visual imagery objectively, without relying on someone's description of the vividness. Here is a video in which

There are now three distinct methods to measure visual imagery objectively, without relying on someone's description of the vividness. Here is a video in which prof Joel Pearson describes them: https://youtu.be/tA_4HNaKsS0 https://youtu.be/tA_4HNaKsS0 , IIRC Joel Pearson was involved in developing each of the methods, so I'm sure you can find his publications on the subject as well.