the doctors studying her did something interesting – they performed a visual evoked potential (VEP) on her while she was exhibiting a personality that was blind and again while she was exhibiting a personality that could see. What a rare opportunity to compare the two states. The VEP essentially is a test in which a flash of light is given to the patient while electrodes record the response from her visual cortex. There is typically a delay of about 100 ms. If this is significantly slow or absent that could indicate a lesion in the visual pathway. ... They found that the VEP was present and normal while she expressed a personality that could see, but was absent when she had a personality with persistent psychogenic blindness. That is a rather incredible result, indicating that there is some process in her brain that is actually suppressing her visual system. To be clear, there is no conscious way to do this (again, at least not known, but I guess this could be the way in which she is very neuroatypical). So it seems that her psychogenic blindness was [due] to a reversible inhibition of her visual pathway, in a way that would block the VEP.
[This was in contrast to]
a 2001 study of 72 subjects with psychogenic blindness found that every one had normal VEPs. VEPs are still used to assess these patients – a normal VEP does suggest a nonorganic cause of blindness,
however it is recognized that an abnormal VEP does not rule out a psychogenic cause.