Your perceptions of the outside world arise through brain activity. Scientists in China have managed to reverse-engineer this process, using brain activity to guess what people are looking at. Their algorithm, which analyses functional MRI brain scans collected while volunteers gaze at digits and letters, is able to furnish uncannily clear depictions of the original images. It has been termed a mind-reading algorithm; a more accurate, though less catchy, description would be a "reconstruction of visual field" algorithm. The algorithm, called the Deep Generative Multiview Model, was highlighted this month by MIT Technology Review as an emerging technology to watch. What is true for the visual cortex is also true for our auditory systems: if you hear a song, the auditory part of your brain whirrs into action. Scientists in the US have developed a programme that can turn the associated firing of neurons back into real sounds. These technologies are turning thoughts into pictures and sounds. In short, science is coming remarkably close to being able to access what is inside our heads. If such algorithms were to find their way into advertising, we may find ourselves digitally stalked not only by images of hotels and consumer goods that we once clicked on, but also by pictures we glanced at or by songs that we streamed. This requires access to brain signals, but who would bet against such a future? Millions of people, by wearing fitness bands, sign up to having their physiological signals charted round the clock.