I was kindly directed to the attached article from 1985 about the background of the Mona Lisa.
It doesn't discuss what the bridge might symbolize, but it does integrate the painting's visual elements with the themes of Leonardo's notebooks (which include accrued learning as a garment per the below, and patience as a garment as cited by Smith).
"I am fully aware that the fact of my not being a lettered man may cause certain arrogant persons to think that they may with reason censure me, alleging that I am a man without letters. Foolish folk! Do they not know that I may retort by saying, as
did Marius to the Roman patricians: 'They who themselves go adorned in the labour of others will not permit me my own?' They will say that, because of my lack of book learning, I cannot properly express what I desire to expound upon. Do they know that my subjects
are based on experience rather than the words of others? And experience has been the mistress of those who wrote well. And so, as mistress, I will acknowledge her and, in every case, I will give her as evidence." [from Leonardo's notebooks]
I found this passage to be evocative of the idea that Leonardo may be presenting a highly complex and multi-planar map of sorts in the Mona Lisa, using the technique Smith describes as "comparazione," which includes transdisciplinary mappings of nature
and natural history, and human biology, but also culture itself, and the history of art/science/technology as a "flow" which is comparable to those in nature. All of these are thus integrated, and centered as if gravitationally on the sitter (hence perforce
the viewer) as "Experience," Leonardo's highest value in both science and art.
Experience in the painting is a complex and hybrid agent, related but not identical to the garment (as hands are to the work), functioning as peer, subject, artist, advocate, teacher, companion, and portrait. Moreover it is an interactive personification
which we cannot but identify with, and indeed, communicate with in present immediate time. This fundamentally interactive gaze is, I would further hypothesize, a powerful thesis regarding what we would now call cognitive neuroscience, including questions
of intersubjective intelligence and what Francisco Varela calls "the embodied mind."
Best wishes and regards to all,