What happens to the prisoner's mind?
He begins to go insane, imagining what it is will be like to be a prisoner and envisioning the cruel deaths that may be his fate.
Part of his tumultuous conflict is deciding whether death would be more welcome than being treated like a doomed laboratory mouse.
The setting of the tale enhances the protagonist's conflicted state of mind. The pit is dark, reminding the reader of both the tomb and of hell. The prisoner's grasp on reality becomes ever more tenuous as his senses go wild in the darkness and he imagines increasingly worse fates that may befall him.
The prisoner becomes increasingly insane. Every time he wakes some new and more horrific fate has befallen him. Every time he sees light in his prison, something that should symbolize hope, he has to endure some new and even more frightening torture. Light comes to symbolize dispair rather than hope. This entire story is essentially a psychoanalysis of a torture victim. It relates how torturers break down the mind of a victim by giving him false hope and illusions of power by allowing him attempts at escape.