Overlapping saccades and glissades are produced by fatigue in the saccadic eye movement system.

Saccadic eye movements and their neurological control signals change significantly as the human fatigues. Electronic instrumentation with a bandwidth extending from DC to 1 kHz enabled the recording of anomalous looking saccadic eye movements that occurred as the subject's physiological state changed. Fatigue can produce: overlapping saccades in which the high-frequency saccadic bursts should show large pauses; glissades in which the high-frequency bursts should be much shorter than appropriate for the size of the intended saccades; and low-velocity, long-duration, non-Main Sequence saccades in which the motoneuronal bursts should be of lower frequency and longer duration than normal. As few as 30 saccades of 50 deg magnitude or a longer sequence of saccades as small as 10 deg can produce these aberrant eye movements and their concomitant neuronal firing pattern variations. The effects of fatigue could explain some of the variations between and spread within published data for velocity vs amplitude of human saccadic eye movements. Measuring the resistance to eye movement fatigue could become either a common clinical tool for diagnosing specific or general disease states, or a research tool for studying dyslexia or fatigue.