Hearing Voices

 is a Fallout 3 quest that is added to the game with the Point Lookout add-on.

Return to Desmond
After speaking with Nadine and getting the key to unlock Jackson's sacred praying grounds beneath the Wrecked Seatub, return to Desmond and tell him the news. Of course, Desmond chastises you for not interrogating the Tribal's leader first. Make your way to the Seatub and enter the caverns.

Finding Jackson
The path to finding Jackson is easy enough, follow the torches to his inner sanctum. Although there are Mirelurks in the caves, you can sneak your way through without fighting them. Upon entering the final room, you find Jackson kneeling before a table speaking to what appears to be a holographic talking brain. Let Jackson and the Brain finish, then speak with the Brain. It does not reveal who he might be straight away but explains he is using the local tribal cult as means to destroy a signal jamming device that Desmond is using to restrict his broadcasting signal. Although he is rather disappointed that the tribals take his instructions as destroying his ancestral mansion instead of the jamming device.

Return to Desmond Again
Report back to Desmond in the Calvert Mansion with the information you obtained from Jackson and Professor Calvert in the cave. This starts the Thought Control quest.

Reward
300 XP