CASE REPORT Sensorimotor Recovery Following Spaceflight May Be Due to Frequent Square-Wave Saccadic Intrusions Millard Reschke, Jeffrey T. Somers, R. John Leigh, Jody M. Krnavek, Ludmila Kornilova, Inessa Kozlovskaya, Jacob J. Bloomberg, and William H. Paloski RESCHKE M, SOMERS JT, LEIGH RJ, KRNAVEK JM, KORNILOVA L, KOZLOVSKAYA I, BLOOMBERG JJ, PALOSKI WH. Sensorimotor recovery following spaceflight may be due to frequent square-wave saccadic intrusions. Aviat Space Environ Med 2004; 75:700 – 4. Square-wave jerks (SWJs) are small, involuntary saccades that disrupt steady fixation. We report the case of an astronaut (approximately 140 d on orbit) who showed frequent SWJs, especially postflight, but who showed no impairment of vision or decrement of postflight performance. These data support the view that SWJs do not impair vision because they are paired movements, consisting of a small saccade away from the fixation position followed, within 200 ms, by a corrective saccade that brings the eye back on target. Since many returning astronauts show a decrement of dynamic visual function during postflight locomotion, it seems possible that frequent SWJs improved this astronaut’s visual func- tion by providing postsaccadic enhancement of visual fixation, which aided postflight performance. Certainly, frequent SWJs did not impair performance in this astronaut, who had no other neurological disorder. Keywords: adaptation, gaze-holding, astronaut, long duration, dynamic visual acuity, dynamic posture. F OR OPTIMAL VISION of details that comprise the environment, images must be brought to the foveal region of the retina, where photoreceptor density is greatest. Furthermore, such images should be held fairly steadily in the foveal region and image speed should be less than 4°  s -1 for clear vision of objects with higher spatial frequencies, such as printed text (5). Thus, abnormal eye movements may degrade vision if they move the image of the object of regard away from the fovea, or if they cause excessive drift of images across the retina. Voluntary shifts in the direction of gaze are achieved by rapid eye movements called saccades. We do not appear to see during saccades, mainly because of the high speed of images across the retina, but also because the visual system suppresses and masks the blurred image due to the eye movement (4,15). Thus, vision remains clear and stable despite making several sac- cades every second. During attempted fixation of a stationary target (such as during threading a needle), most individuals can suppress saccades (18). However, some individuals show small saccades, known as square-wave jerks (SWJs), which intrude on steady fix- ation (see inset Fig. 1). SWJs consist of pairs of small, conjugate saccades, typically 0.5 to 2.0° in size, which take the eye away from the fixation position and then return it after a period of about 200 ms. Although SWJs are prominent in certain neurological disorders (11), for example cer- ebellar diseases (especially Friedreich’s ataxia) and Par- kinsonian disorders (typically, progressive supranu- clear palsy), they are also encountered in some healthy subjects (9,16,17) who do not usually complain of any visual symptoms. In this paper, we report an astronaut who had been aboard a space station for several months and who was noted in the course of a voluntary study of eye movements to have frequent SWJs, but without detectable effects on vision or functional performance, either preflight or postflight. CASE HISTORY Prior to the flight, a 47-yr-old veteran astronaut passed an Air Force Class I physical, and volunteered to participate in a series of experiments designed to eval- uate compensatory eye movements, dynamic visual function, and postural performance. To be classified as a veteran, an astronaut must have a minimum of one prior spaceflight. For the purposes of this case history, the performance of the astronaut was compared with only long-duration veteran flyers aboard the Mir space station. All testing was performed in accordance with From the Neurosciences Laboratories, NASA-Johnson Space Cen- ter, Houston, TX (M. Reschke, J. J. Bloomberg, W. H. Paloski); Wyle Laboratories, Houston, TX (J. T. Somers, J. M. Krnavek); the Depart- ments of Neurology, Biomedical Engineering, and Neurosciences, Department of Veterans Affairs Medical Center and University Hos- pitals, Case Western Reserve University, Cleveland, OH (R. J. Leigh); and the Institute of Biomedical Problems, Ministry of Health of Rus- sian Federation, Moscow, Russia (L. Kornilova, I. Kozlovskaya). This manuscript was received for review in March 2004. It was accepted for publication in June 2004. Address reprint requests to: Dr. Millard Reschke, NASA-Johnson Space Center, 2101 NASA Parkway, Mail Code SK3, Houston, TX 77058; millard.f.reschke@nasa.gov. Reprint & Copyright © by Aerospace Medical Association, Alexan- dria, VA. 700 Aviation, Space, and Environmental Medicine • Vol. 75, No. 8 • August 2004