Just as a global posiÃÂtionÃÂing sysÃÂtem (GPS) helps find your locaÃÂtion, the brain has an interÃÂnal sysÃÂtem for helpÃÂing deterÃÂmine the body's locaÃÂtion as it moves through its surroundings.
A new study from researchers at PrinceÃÂton UniÃÂverÃÂsity proÃÂvides eviÃÂdence for how the brain perÃÂforms this feat. The study, pubÃÂlished in the jourÃÂnal Nature, indiÃÂcates that cerÃÂtain position-tracking neuÃÂrons -- called grid cells -- ramp their activÃÂity up and down by workÃÂing together in a colÃÂlecÃÂtive way to deterÃÂmine locaÃÂtion, rather than each cell actÃÂing on its own as was proÃÂposed by a comÃÂpetÃÂing theory.
Grid cells are neuÃÂrons that become elecÃÂtriÃÂcally active, or fire, as aniÃÂmals travel in an enviÃÂronÃÂment. First disÃÂcovÃÂered in the mid-2000s, each cell fires when the body moves to speÃÂcific locaÃÂtions, for examÃÂple in a room. AmazÃÂingly, these locaÃÂtions are arranged in a hexagÃÂoÃÂnal patÃÂtern like spaces on a ChiÃÂnese checker board.
Together, the grid cells form a repÃÂreÃÂsenÃÂtaÃÂtion of space, said David Tank, Princeton's Henry L. HillÃÂman ProÃÂfesÃÂsor in MolÃÂeÃÂcÃÂuÃÂlar BiolÃÂogy and leader of the study. Our research focused on the mechÃÂaÃÂnisms at work in the neural sysÃÂtem that forms these hexagÃÂoÃÂnal patÃÂterns, he said. The first author on the paper was gradÃÂuÃÂate stuÃÂdent Cristina DomÃÂnisoru, who conÃÂducted the experÃÂiÃÂments together with postÃÂdocÃÂtoral researcher Amina Kinkhabwala.
DomÃÂnisoru meaÃÂsured the elecÃÂtriÃÂcal sigÃÂnals inside indiÃÂvidÃÂual grid cells in mouse brains while the aniÃÂmals traÃÂversed a computer-generated virÃÂtual enviÃÂronÃÂment, develÃÂoped preÃÂviÃÂously in the Tank lab. The aniÃÂmals moved on a mouse-sized treadÃÂmill while watchÃÂing a video screen in a set-up that is simÃÂiÃÂlar to video-game virÃÂtual realÃÂity sysÃÂtems used by humans.
She found that the cell's elecÃÂtriÃÂcal activÃÂity, meaÃÂsured as the difÃÂferÃÂence in voltÃÂage between the inside and outÃÂside of the cell, started low and then ramped up, growÃÂing larger as the mouse reached each point on the hexagÃÂoÃÂnal grid and then falling off as the mouse moved away from that point.
This rampÃÂing patÃÂtern corÃÂreÃÂsponded with a proÃÂposed mechÃÂaÃÂnism of neural comÃÂpuÃÂtaÃÂtion called an attracÃÂtor netÃÂwork. The brain is made up of vast numÃÂbers of neuÃÂrons conÃÂnected together into netÃÂworks, and the attracÃÂtor netÃÂwork is a theÃÂoÃÂretÃÂiÃÂcal model of how patÃÂterns of conÃÂnected neuÃÂrons can give rise to brain activÃÂity by colÃÂlecÃÂtively workÃÂing together. The attracÃÂtor netÃÂwork theÃÂory was first proÃÂposed 30 years ago by John HopÃÂfield, Princeton's Howard A. Prior ProÃÂfesÃÂsor in the Life SciÃÂences, Emeritus.
The team found that their meaÃÂsureÃÂments of grid cell activÃÂity corÃÂreÃÂsponded with the attracÃÂtor netÃÂwork model but not a comÃÂpetÃÂing theÃÂory, the oscilÃÂlaÃÂtory interÃÂferÃÂence model. This comÃÂpetÃÂing theÃÂory proÃÂposed that grid cells use rhythÃÂmic activÃÂity patÃÂterns, or oscilÃÂlaÃÂtions, which can be thought of as many fast clocks tickÃÂing in synÃÂchrony, to calÃÂcuÃÂlate where aniÃÂmals are located. Although the PrinceÃÂton researchers detected rhythÃÂmic activÃÂity inside most neuÃÂrons, the activÃÂity patÃÂterns did not appear to parÃÂticÃÂiÃÂpate in posiÃÂtion calculations.
