Saccades are the rapid, simultaneous movements of both eyes used to shift the line of sight quickly from one point of interest to another. These movements are among the fastest the human body can produce, operating primarily to ensure that new visual information is precisely centered on the fovea, the small region of the retina responsible for sharp, detailed central vision. The efficiency of this process is fundamental to how people perceive and interact with the visual world. Testing these movements provides a window into the integrity of the brain circuits that control them and the important diagnostic information derived from those tests.
The Physiology and Characteristics of Saccades
The primary biological purpose of a saccade is foveation, the act of bringing an object of interest onto the fovea for clear viewing. To accomplish this, a complex neural network, spanning the brainstem to the cerebral cortex, must coordinate the necessary muscle contractions and relaxations of the eyes. Analyzing the characteristics of these movements offers objective data on the health and functionality of this extensive neurological circuitry.
Researchers and clinicians focus on three measurable characteristics that define the quality of the saccade. Latency is the time delay between when a visual target moves and when the eyes begin to move toward it. This measurement reflects the time needed for the brain to process the visual signal, plan the new movement, and issue the command.
Velocity indicates how fast the eye travels to the new target position, often reaching hundreds of degrees per second. The speed of the eye movement is directly related to the amplitude, or distance, of the saccade, with greater distances requiring faster speeds. This speed is generated by a burst of activity in the brainstem’s premotor neurons.
Accuracy determines how closely the eye lands on the intended target. Deviations can result in either an undershoot, known as hypometria, or an overshoot, known as hypermetria.
Methodologies for Measuring Eye Movement
Measuring the fast and precise nature of saccades requires specialized technology to capture the movements with high temporal and spatial resolution. The patient is typically seated in front of a screen and asked to follow a small visual target that appears and disappears at different locations, prompting a series of rapid eye movements. The resulting raw data is captured using techniques such as Video-Oculography and Electro-Oculography.
Video-Oculography (VOG)
VOG is the most common modern method, using high-speed infrared cameras to capture video images of the eye. This system tracks features like the position of the pupil and the corneal reflection of an infrared light source to determine gaze direction. VOG uses algorithms to calculate the exact position of the eye frame-by-frame, providing a precise, non-invasive measurement of horizontal, vertical, and torsional movements.
Electro-Oculography (EOG)
EOG is an older, lower-cost technique that detects changes in the eye’s natural electrical field. Electrodes placed on the skin around the eye detect changes in the corneo-retinal potential as the eye moves, corresponding to the eye’s position. While VOG offers higher spatial accuracy and is the current standard, EOG can be advantageous when optical tracking is difficult, such as when a patient has a narrow eye cleft or is wearing certain types of lenses.
Both methods require the patient to perform calibration steps, such as looking at a series of dots, to ensure the recorded data accurately maps the eye position to the target location.
Clinical Insights Derived from Saccade Testing
Analyzing deviations in saccade characteristics provides neurologists with objective evidence of dysfunction in specific brain regions. This testing has become a valuable tool for tracking neuropsychological dysfunction in various neurological diseases.
A slowing of saccadic velocity often indicates damage to the brainstem or basal ganglia, which contain the motor pathways responsible for generating movement speed. When the burst neurons in the brainstem are compromised, the eye cannot achieve the necessary acceleration, a finding associated with conditions like progressive supranuclear palsy.
Issues with increased latency or inaccuracy typically point to problems in the cortical and subcortical areas responsible for planning and fine-tuning the movement. A prolonged latency suggests a delay in the cognitive processing or planning phase, often linked to disorders affecting the basal ganglia or cortical areas.
Saccadic inaccuracy, such as consistent overshooting (hypermetria) or undershooting (hypometria), is a strong indicator of cerebellar dysfunction. The cerebellum acts as a calibrator for movement, and its vermis is particularly involved in regulating the size and precision of the eye movement. Saccade testing provides a non-invasive, objective means to assess the integrity of the brainstem, cerebellum, and cortical circuits, offering diagnostic support in the evaluation of movement disorders and demyelinating diseases.