The brain's abilities of perception aren't necessarily limited to the five traditionally recognised senses; other stimuli can help it make sense of the world. In a new study published in Current Biology, researchers in Japan found that rats could cope without one of the main senses—sight—when they were given another "sense" to help them navigate.
The blind rats' brains were fed geomagnetic information thanks to a microstimulator implanted into their visual cortex and connected to a digital compass. This neuroprosthesis indicated when the rat's head was facing north or south by emitting an electrical pulse to one of the two respective electrodes. The ability to be able to tell where your body is in space is known as the "allocentric" sense; it's similar to what you might call your sense of direction.
Over email, first author Hiroaki Norimoto from the University of Tokyo explained that the digital compass they used is like the microchip you'd find in your smartphone and that the whole device weighs only 2.5 grams.
The study went as follows: Seeing rats were placed in a maze where they had to turn either left or right to find food, depending on if they came at it from north or south. While at first their chance of choosing the right direction first time was 50 percent, they learned which way to go thanks to visual cues. Rats who had their eyelids sutured didn't learn.
These blind rats were then given the prosthesis. In tests where the sensor was turned on, they found the food with much more success than when it was turned off. This was also the case when the rats only had the sensor turned on at the very start of the maze, so only had their initial orientation to go on.
With the sensors, the blind rats also performed as well as the seeing rats in a more complex maze with five directions.
"Hence, we have demonstrated that a geomagnetic neuroprosthesis can restore the spatial navigation deficits of blind animals," the authors wrote.
"We used complex spatial mazes that cannot be solved without vision," Norimoto added. "Sensor-implanted rats were able to develop normal navigation strategies that otherwise emerge via the visual system."
However, it's not as simple as "replacing" vision. "To be honest, however, we do not know the exact answer to a question 'What did the rat see (feel),'" Norimoto said. "We found that the device worked similarly when the stimulators were implanted in other brain regions such as the whisker area, rather than the visual system." In that scenario, would it be more a sense of touch?
A broader takeaway from the study is that it showed it was possible for an adult mammal brain to adapt to incorporate a new information source it hadn't previously experienced.
That could have implications not only for those whose senses are impaired, but also for biohackers wishing to extend their perception in new ways, and not just geomagnetism.
"If the ethics board permits, I am dreaming that humans can expand their senses through artificial sensors ("supersensory" organs), including geomagnetism, ultraviolet, radioactive rays, humidity, ultrasonic, radio wave, pheromones etc." Norimoto said. "These are technically feasible. Sensing sunlight UV may be important for reducing skin cancer. Also, ultrasonic and radio wave may enable a next-generation form of human-to-human communications."