The first part of this study is concerned with the perceptibility of moving stimuli in the presence of pursuit eye movements. The visibility of various sine-wave gratings (0.2-12 c/deg) was investigated by the simultaneous measurement of eye movements and contrast thresholds under several different viewing conditions. The term "grating" indicates that the visual stimulus was composed of regularly alternating (vertical) light and dark bars. The "spatial frequency" (c/deg) of which refers to the density of the bars (cycles) per unit of visual angle. "Threshold contrast" (Cm,,) or "contrast sensitivity" (CS = 1/Cmi,). is defined as the minimal difference in relative illumination of the bars necessary to detect the pattern rather than an uniform ulliminated field. The first three experiments were designed to produce a - more or less - controlled amount of relative motion between the visual stimulus (grating) and the retina.