Incognito: The Secret Lives of the Brain

by David Eagleman

One of the earliest examples of this framework came from the neuroscientist Donald MacKay, who in 1956 proposed that the visual cortex is fundamentally a machine whose job is to generate a model of the world.46 He suggested that the primary visual cortex constructs an internal model that allows it to anticipate the data streaming up from the retina (see the appendix for an anatomical guide). The cortex sends its predictions to the thalamus, which reports on the difference between what comes in through the eyes and what was already anticipated. The thalamus sends back to the cortex only that difference information—that is, the bit that wasn’t predicted away. This unpredicted information adjusts the internal model so there will be less of a mismatch in the future. In this way, the brain refines its model of the world by paying attention to its mistakes. MacKay pointed out that this model is consistent with the anatomical fact that there are ten times as many fibers projecting from the primary visual cortex back to the visual thalamus as there are going the other direction—just what you’d expect if detailed expectations were sent from the cortex to the thalamus and the forward-moving information represented only a small signal carrying the difference. What all this tells us is that perception reflects the active comparison of sensory inputs with internal predictions. And this gives us a way to understand a bigger concept: awareness of your surroundings occurs only when sensor...