Do you know how much more you can get done when you're not constantly looking for places to urinate? It's quite a lot, I can tell you.
The Courier's brain is the isolated, conscious, and interactive brain of the Courier.
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 voice of the brain always sounds male even if the Courier is female. This is explained by the fact that a female voice modulator is apparently hard to come by in Big MT.
The brain has some small scarring due to Benny shooting the Courier in the head, an event the brain does not remember fondly.
If your Courier is a male with the Confirmed Bachelor perk or a female with the Cherchez La Femme, you can hit on your brain. It is disgusted by the insinuation. This also seems to work if a male character has the Lady Killer perk, and logically if a female character has the Black Widow perk.
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 himself which include mathematics, claiming he 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 player the Big Brained perk.
Like the Spine and Heart, the brain can be taken out and put back in at anytime.
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 he 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 player's Karma.
"Oh, yes, energy weapons. I'd almost forgotten how much fun those are."
"Nice to see you back. I trust the adventuring is going well."
"Are you... Are you coming on to me?! Sweet Lord, I don't even have the words for how repugnantly wrong that is!"
"Would you like a cookie?"
"Barring the unlikely event that I evolve some sort of rocket propelled locomotion, I will be here."
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 Player's Brain other than the two references toward Jason Bright and Helios One.
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 player to walk away or enter dialogue again, but the loop will occur once more if same response is selected, so choose wisely.