The Courier's brain is removed by the Think Tank of the Big Empty and subsequently forgotten about. Soon after, Dr. Mobius takes the opportunity to seize the brain and secretly take it away to his stronghold, the Forbidden Zone. The brain enjoys the peace and quiet of its new home, along with the relative safety and cleanliness, when compared to the wasteland.
The brain, if left in The Sink and not integrated back in the Courier, will act like any of the other appliances. These actions are limited to talking to itself which include mathematics, claiming it doesn't want to be a Robobrain but wants some "treads", and humming the music of Dead Money's "Begin Again" song which sometimes plays in the mysterious broadcast intended as The Sink's atmosphere music.
The Courier's brain can be implanted into the Courier's skull after the end of Old World Blues by using the "Swap Bodyparts" option on the Sink's Auto-Doc, this gives the Big Brained perk.
Like the spine and heart, the brain can be taken out and put back in at any time.
If the Courier is of low Intelligence, a dialogue can be opened questioning whether the Courier's other organs can speak, a topic which annoys the brain. Further dialogue also reveals that the tank the brain floats in has been saturated with ground Mentats, thus explaining the improved Intelligence.
The brain is also one of the most versatile characters in the dialogue exchanges it has with the player character. Upon the discussion of reasons why the brain should go back into the PC head, the dialogue can be different depending on the Courier's Karma.
Once the Courier's brain has been "flushed" to the Sink or taken into the Courier's inventory, the empty tank shuts down. However, the tank is still treated as an NPC and can 'Detect' even when crouching.
pcps3xbox360 After progressing through the dialog with the Courier's brain the option to say "So, what, you'd rather just stay here? Never leaving that tank of... whatever that stuff is?" appears, however, if you choose it you will get stuck in a never ending speech loop. This also occurs on PC as well, specifically at the moment when the brain begins reminiscing about past feats by Courier. In the observed case, loop occurred between brain recalling Jason Bright`s departure and activation of Helios One. It seems that there are missing dialogue and sound files for the Courier's brain other than the two references toward Jason Bright and Helios One. [verified]
pc Workaround: to get out of dialogue loop, in console type player.cam (close all menus). This will exit the current dialogue with the brain tank, and allow players to walk away or enter dialogue again, but the loop will occur once more if same response is selected, so choose wisely. [verified]
The trigger to this bug is the line about Jason Bright. If the Courier does not complete Come Fly With Me prior to speaking with the brain, this bug will not occur.
This bug is still presented in PC v140525. There are quite a few talk-route to get past it, both straight-until-complete, and stop-and-talk-again. Should players fall into the route that lead to the bug, use the player.cam method above, then choose the line to negotiate with the brain. That is if reload the Auto save is not an option.
Reloading the game from a previous save is the best way to fix this on console.