In new court documents obtained by TMZ ... Mavis' lawyer, who was court-appointed after Jay filed a conservatorship petition, says her memory is in such decline, Mavis struggles to recognize her husband and recall her own birth date.

The 77-year-old philanthropist's memory issues are confirmed by a neurologist overseeing her care, which started after a 2018 car accident.

According to the docs, Mavis is struggling with "advanced dementia," which is causing her to experience quite a bit of disorientation. Per the court doc, Mavis thinks often about her late parents -- who passed more than 20 years ago.

The lawyer describes Jay as a loving husband, who is trying to make sure Mavis will be financially protected, in the event he passes away before her.

The lawyer explains ... "Estate planning is something that most everyone needs, but by the time you need it, if you don't have it, it is too late to get it. This Petition is designed to give Mavis that protection with the protection of Court supervision."

The legal counsel notes Mavis, despite her clear "cognitive impairment," agrees with the conservatorship plan -- as she trusts Jay completely after 44 years of marriage.

While Mavis' lawyer supports Jay being appointed her conservator, he notes he has not spoken with her next of kin -- her brother, Rikki Nicholson -- who reportedly lives next door to the couple.