Why is Jane imprisoned in the attic in the first place?

The woman in "The Yellow Wallpaper" is suffering from
post-partum depression.  At that time, the treatments for things like depression were
very different from what we have today.  Women were often prescribed bed rest, quiet,
and solitude to help them recover.  Sometimes, women were even asked to eat animal fats
and drink strange (by today's standards) concoctions.  The doctor for the woman in the
story thinks she will get better if she avoids mental simulation like reading or
writing.  Of course, we now know that staying in bed and having no distractions but
thinking are not the most helpful things for depressed patients.  The woman's husband
and her doctor persist in this line of supposed treatment until the woman basically goes
insane.