Over the past two decades, researchers using functional magnetic resonance imaging (fMRI), which tracks the iron in the hemoglobin supplying oxygen to neurons, have been building up increasingly detailed maps and inventories of the mammalian cortex. Thanks to huge advances in machine-learning artificial intelligence—computer algorithms that are able to sort through enormous amounts of information and use statistical methods to make classifications and predictions—fMRI scans can now be used to identify everything from depressive thoughts to the nuanced feelings of envy and schadenfreude. Other algorithms have been able to accurately piece together reconstructions of movie clips watched by subjects, just by analyzing their brain scans; or have detected, in probing the brain activity of swing voters in the US presidential election, responding to photographs and videos of presidential candidates, which candidates provoked anxiety or even disgust, and which elicited positive responses or feelings of empathy

In just the last few years, neuroscience researchers have progressed from decoding images and emotions as they play across the cortex to sounds, words, phrases, and even language. In 2023, in a remarkable demonstration of this emerging technology, a woman called Ann Johnson, who had been paralyzed for 18 years by a brain-stem stroke, was able to speak again through the insertion of a grid of 253 electrodes onto the surface of her brain, which translated her neuronal signals into sentences, in real time, at a rate of 78 words per minute (just about half the speed of standard conversation). The research team at the University of California, led by neurosurgeon Edward Chang, had combined this brain-computer interface with an animated avatar of Johnson’s head, which spoke in her own voice, as reconstructed from a recording of a 15-minute toast she had given at her wedding. Just as the avatar’s mouth spoke Johnson’s words as she thought them, so its expressions were similarly influenced by the nuances of her brain activity, which turned her thoughts about facial gestures into displays of emotion—from smiles to pursed lips and frowns...