Navigating a complex environment requires an egocentric representation: a neural signature of how you and your body relate to objects in your visual field. Part of the difficulty in exploring space for those without sight is the challenge of understanding how object positions relate to each other (allocentric representation) and to oneself (egocentric representation). Understanding how the brain encodes this activity is therefore a key step forward in understanding the neurobiology of vision, leading to possible treatments for blindness.
In a recent publication in the journal Current Biology, Norimoto and Ikegaya demonstrate that stimulation of the primary visual cortex (V1) relating to geomagnetic compass position allows rats to navigate mazes faster than blind rats and at the same rate as sighted rats. Head-mounted bilateral microstimulators along with digital geomagnetic compasses were used on the rats to give the animals real time feedback on their head directions. Further, blind rats which received this V1 geomagnetic stimulation were able to form an internal memory of the relative position of turns in a maze, so that when the stimulator was off, they were as good as sighted rats at navigating the learned maze.
In the experiments, the researchers trained rats to solve mazes for food rewards. The time taken to solve mazes, trial number, and turns into incorrect maze arms were used to assess how the rats navigated the mazes. In a first experiment, rats were required to run a T-shaped maze to find a sucrose reward. Both blind rats and blind rats with the sensor implant (but the sensor turned off) performed the same: around 50% success at finding the bait even after substantial training. However, with the sensor turned on, implanted rats rapidly learned to navigate the maze. If the sensor was again turned off, performance of these rats again dropped to chance.
In a follow-up experiment, the researchers used a complicated maze with multiple start points and measured how the rats learned to navigate the space. Sighted rats learned quickly, while blind rats made significantly more errors than sighted rats. Blind rats with the V1 prosthesis began the initial session performing like the blind rats, but rapidly approached training levels of the sighted rats. In fact, when the sensors were turned off, blind rats with the visual cortical prosthesis performed no different than sighted rats, and both performed better than blind rats.
Taken together, these data suggest that geomagnetic-based stimulation of primary visual cortex can be used to help animals navigate through space and can allow the creation of an internal representation of their egocentric position in relation to objects around it in space. This representation persists, even after stimulation ceases. Further development will be needed to test performance on more complicated tasks, focusing the goal on one day being able to restore sight to the blind through visual prostheses.
- Norimoto H, Ikegaya Y. (2015) Visual cortical prosthesis with a geomagnetic compass restores spatial navigation in blind rats. Current Biology. In Press. doi: 10.1016/j.cub.2015.02.063