Ed Yong investigates work being done to create one:
While [UC-Berkeley's Jack] Gallant decodes what we see, Moran Cerf from the California Institute of Technology is decoding what we think about. He uses tiny electrodes to measure the activity of individual neurons in the hippocampus, a part of the brain involved in creating memories. In this way, he can identify neurons that fire in response to specific concepts – say, Marilyn Monroe or Yoda. Cerf’s work is a lot like Gallant’s – he effectively creates a dictionary that links concepts to patterns of neural activity. "You think about something and because we learned what your brain looks like when you think about that thing, we can make inferences," he says.
But both techniques share similar limitations. To compile the dictionaries, people need to look at a huge number of videos or concepts. To truly visualise a person’s thoughts, Cerf says, "That person would need to look at all the concepts in the world, one by one. People don’t want to sit there for hours or days so that I can learn about their brain."
The Dish 