Elon Musk’s Neuralink used a brain implant to enable a monkey to see something that wasn’t physically there, according to an engineer, as it moves toward its goal of helping blind people see.

The device, called Blindsight, stimulated areas of a monkey’s brain associated with vision, Neuralink engineer Joseph O’Doherty said Friday at a conference. At least two-thirds of the time, the monkey moved its eyes toward something researchers were trying to trick the brain into visualizing.

The results were the first Neuralink has publicized about tests of Blindsight, a brain chip that mimics the function of an eye. This is a closely watched frontier for brain device development, a scientific field that’s testing the boundaries of how technology can be used to potentially treat intractable conditions.