|Rat head direction cell responses in zero-gravity parabolic flight
|Year of Publication
|Taube, J., R. Stackman, J. Calton, and C. M. Oman
|MVL Report Number
Astronauts working in zerogravity(0-G) often experience visual reorientation illusions (VRIs).For example, when floating upside down, they commonly misperceivethe spacecraft floor as a ceiling and have a reversed sense of direction.Previous studies have identified a population of neurons in the rat’sbrain that discharge as a function of the rat’s head direction (HD) ina gravitationally horizontal plane and is dependent on an intactvestibular system. Our goal was to characterize HD cell dischargeunder conditions of acute weightlessness. Seven HD cells in theanterior dorsal thalamus were monitored from rats aboard an aircraftin 0-G parabolic flight. Unrestrained rats locomoted in a clear plexiglasrectangular chamber that had wire mesh covering the floor,ceiling, and one wall. The chamber and surrounding visual environmentwere relatively up-down symmetrical. Each HD cell was recordedacross forty 20-s episodes of 0-G. All HD cells maintained asignificant direction-specific discharge when the rat was on the chamberfloor during the 0-G and also during the hypergravity pull-outperiods. Three of five cells also showed direction-specific responseson the wall in 1-G. In contrast, direction-specific discharge wasusually not maintained when the rat locomoted on the vertical wall orceiling in 0-G. The loss of direction-specific firing was accompaniedby an overall increase in background firing. However, while the ratwas on the ceiling, some cells showed occasional bursts of firing whenthe rat’s head was oriented in directions that were flipped relative tothe long axis of symmetry of the chamber compared with the cell’spreferred firing direction on the floor. This finding is consistent withwhat might be expected if the rat had experienced a VRI. Theseresponses indicate that rats maintain a normal allocentric frame ofreference in 0-G and 1-G when on the floor, but may lose their senseof directional heading when placed on a wall or ceiling during acuteexposures to 0-G.