Researchers from Columba University believe they may have found a way of fooling the mind’s eye into seeing things that aren’t real, after manipulating the wiring of a mouse’s brain. By reconfiguring the neuronal connections in the visual cortex, the team not only achieved the remarkable feat of implanting a non-existent image into the animal’s consciousness, but also highlighted the extent of the brain’s plasticity.

Writing in the journal Science, the study authors explain how they set out to examine how neuronal ensembles form. These are “coactive groups of neurons” that fire in unison with one another, and are thought to represent the “building blocks of cortical circuits.” It has previously been suggested that they arise due to Hebbian plasticity, whereby the connections between these neurons – known as synapses – become strengthened.

In an attempt to hijack this process, the researchers injected the mouse with a virus containing light-sensitive proteins that target certain neurons. This enabled them to then control the activity of these neurons using light – a technique known as optogenetics.

When optogenetically activating a collection of neurons, the researchers discovered that these brain cells formed neuronal ensembles, and began to fire as a group whenever one of them became stimulated.

^{Video shows neurons being trained to work together as a result of repeatedly stimulation using light}

To investigate the robustness of these ensembles, the researchers waited until the following day and then once again stimulated a single neuron in the mouse’s visual cortex. Once more, this caused all the neurons in the ensemble to become active, suggesting that the new connections had become hardwired into the rodent’s brain.

Because the neurons targeted were all located in the part of the brain that processes vision, it’s likely that optogenetic stimulation of these cells caused the mouse to see an image of some sort, and that the same vision would have appeared on all subsequent occasions that the same neuronal ensemble was activated. However, the study authors currently have no way of knowing what that image might have looked like, and are in the process of developing a behavioral test to try and learn more about what the mouse “sees” when these neurons fire.

Perhaps more importantly, however, this research reveals just how malleable the brain is, and how readily neurons form new connections and circuits. “I always thought the brain was mostly hard-wired,” said study co-author Rafael Yuste in a statement. “But then I saw the results and said ‘Holy moly, this whole thing is plastic.’ We’re dealing with a plastic computer that’s constantly learning and changing.”