Simulation of spaceflight with whole-body head-down tilt: influence on intraocular pressure and retinocortical processing.

Cephalad fluid shifts occur in the microgravity environment of spaceflight. Whole-body head-down tilt was used to simulate the influence of these fluid shifts upon intraocular pressure (IOP) and the bioelectrical activity of neural elements in the retinocortical pathway. Noninvasive techniques were used to monitor IOP, pattern reversal electroretinograms (prERGs), and pattern reversal visual evoked cortical potentials (prVEPs) when subjects were oriented either upright or in a head-down position (6 degrees or 90 degrees). The results indicate that there is a significant elevation in IOP when an individual is oriented in a head-down position. Significant alterations of neurophysiological processing in the retinocortical pathway also occur when individuals are oriented in a head-down position.