We introduce an image annotation approach for the analysis of volumetric electron microscopic imagery of brain tissue. The core task is to identify and link tubular objects (neuronal fibers) in images taken from consecutive ultrathin sections of brain tissue. In our approach an individual 'flies' through the 3D data at a high speed and maintains eye gaze focus on a single neuronal fiber, aided by navigation with a handheld gamepad controller. The continuous foveation on a fiber of interest constitutes an intuitive means to define a trace that is seamlessly recorded with a desktop eyetracker and transformed into precise 3D coordinates of the annotated fiber (skeleton tracing). In a participant experiment we validate the approach by demonstrating a tracing accuracy of about the respective radiuses of the traced fibers with browsing speeds of up to 40 brain sections per second.