PROJECT SUMMARY/ABSTRACT A central question in vision research is how we perceive a stable world despite continuous retinal motion from eye movements. Understanding these mechanisms is critical to improving diagnosis and treatment of patients suffering from blurred or jumping vision, mislocalization of objects, abnormal tilt and slant perception, loss of balance and falls. Current models of perceptual stability during eye movements ignore the fact that the eye ro- tates around three axes. One important mechanism uses the efference copy of the command that moves the eyes to discount the retinal motion or displacement caused by eye movements. Research has focused on the horizontal and vertical dimensions of motion, overlooking the important contribution from the third (torsional) dimension, in which the eyes rotate around the line of sight, as happens, e.g., every time we tilt our head towards the shoul- der. Torsional eye position directly contributes to our perception of tilt (clockwise or counterclockwise) and of slant (orientation in the sagittal plane). The main reason for the gap in knowledge about torsion has been the technological challenge of reliably measuring torsion. We overcame this barrier by developing a new method for measuring torsional eye movements noninvasively. This proposal aims to study the perceptual effects and brain mechanisms of the efference copy for torsional eye movements. During the mentored phase, the candidate will use transcranial magnetic stimulation (TMS) combined with measurements of perception of upright, and tor- sional eye movement recordings to determine the role of particular cortical circuits in taking into account static torsion that occurs during static head tilts. During the independent phase, the candidate will study how percep- tion is altered around the time of quick torsional eye movements and during torsional vestibular ocular reflex. This will enable us to determine if the brain uses signals in three or two dimensions to compensate for the retinal motion induced by eye movements. These experiments will give us a unique opportunity to provide critical new evidence to modify and expand current models of perceptual stability. Studying torsion, we will be able to dis- criminate between computational models and brain areas that only account for the two dimensions of the retinal image versus those that account for all three dimensions of motion. Ultimately, understanding these mechanisms will provide new diagnostic and therapeutic avenues for people with unsteady vision, spatial disorientation, and falls.