The gradient of luminosity between open/enclosed arms, and not the absolute level of Lux, predicts the behaviour of rats in the plus maze.

The effect of the gradient of luminosity between the open and the enclosed arms (O/E(DeltaLux)) of the elevated plus maze (EPM), upon the level of fear/anxiety in rats submitted to the trial 1/trial 2 paradigm was investigated. Male Wistar rats were assigned to freely explore either of three EPM configuration, with the enclosed arm walls constructed with either translucent glass (O/E(DeltaLux)=11), opaque glass (O/E(DeltaLux)=96) or wood (O/E(DeltaLux)=141), for 2 consecutive days (trial 1/trial 2). Independently of the EPM configuration, rats exhibited increased fear during trial 2 relative to trial 1, thus indicating that the level of O/E(DeltaLux), at least in the range used here, is not a determinant variable for the establishment of increased anxiety induced by prior maze experience. The groups tested under 11 and 141 O/E(DeltaLux) were those who exhibited the low and higher level of open arm avoidance, respectively. There was also an increased open arms avoidance over trial 1 in rats tested under 11 and 96 O/E(DeltaLux), only. These results suggest that the enclosed arm preference of rats during trial 1 EPM procedure may be changed by the level of O/E(DeltaLux) of the test. The present results are discussed with respect to the controversy regarding the role of luminosity on EPM performance.