Scientists have genetically engineered neurons in mice to give them hallucinations. The research could help identify why certain people experience hallucinations, associated with several mental illnesses, and inform new treatments.

Two genes were inserted into the neuron cells of the mice’s visual cortex, the part of the brain that processes visual information. One coded for a light-sensitive protein that would activate the neuron when exposed to the specific wavelength of the trigger light. The other gene coded for green florescent protein that emits green light when the neuron was activated.

The top of the mice’s skulls were carefully removed to display their visual cortex, which was then protected by a glass screen. The mice were then shown horizontal lines on a screen, and the neurons that responded to the horizontal lines emitted green light. This enabled researchers to identify specific the neurons that responded to the horizontal lines. The test was repeated but the mice were shown vertical lines on the screen instead, enabling the identification of the different neurons that responded to the vertical lines.

The scientists trained the mice to show two distinct behaviours when they saw a horizontal or a vertical line on the screen. A hologram was then projected onto the mice’s visual cortex that only targeted the neurons associated with seeing the horizontal line. Despite the mice not being shown any horizontal line on the screen, they performed the behaviour associated with seeing the horizontal lines. The mice performed the behaviours associated with seeing vertical lines when the corresponding neurons were activated. Therefore, the mice believed they were seeing the lines on the screen and were hallucinating.

The researchers were surprised that so few neurons were required to form a convincing hallucination, which could explain why some humans are quite susceptible to them.