In this seven second video, the brain activity of a fish is registered in real time. (Direct link to video)
The brain belongs to a zebrafish, and the neuronal activity is the response to a visual stimulus that resembles food for this species.
Gizmodo describes the flashes as "thought forming in the brain."
But still it's exciting to see mental activity visualized in this way.
Photo above by Rene Spaargaren from Geekosystem