Detecting Abnormal Eye Movements in Patients with Neurodegenerative Diseases – Current Insights

Figures & data

Table 1 Key Terms in Ocular Motor Research, Examinations and Analysis and Their Definitions

Term	Definition
Catch Up Saccades	Rapid eye movements to adjust the eye’s position to refocus following a head movment as part of a faulty vestibulo-ocular reflex.
Convergence	Eye movements where the eyes move inward, towards each other.
Divergence	Eye movements where the eyes move outward, away from each other.
Drift	Slow, involuntary eye movements during attempted fixation.
Electro-oculography	Measuring eye movements by detecting electrical potentials.
Error Rate	The frequency of errors in reaching the target during eye movements.
Fixation	Maintenance of the gaze on a single location.
Hypermetric	Saccades that overshoot the intended target.
Hypometric	Saccades that fall short of the intended target.
Infrared Oculography	Tracking eye movements using infrared light.
Latency	The delay between the initiation and the onset of an eye movement.
Microsaccades	Small, involuntary eye movements during fixation to refresh the image on the retina.
Nystagmus	Involuntary, rhythmic oscillation of the eyes, often indicating underlying issues.
Pursuit	Smooth eye movements to track moving objects.
Pursuit Gain	The ratio of eye velocity to target velocity during pursuit movements.
Pursuit Speed	The speed at which the eye moves to follow a moving target.
Saccade	Rapid, ballistic eye movements to shift gaze from one point to another.
Saccade Accuracy	The precision of the eye in landing on the target during a saccade.
Saccade Peak Velocity	The highest speed reached by the eye during a saccade.
Saccade Speed	The average speed of the eye during a saccade.
Saccadic Bradykinesia	Decrement in the velocity and amplitude of repetitive horizontal saccades, often seen in Parkinson’s disease.
Saccadic Intrusions	Unwanted saccades that disrupt fixation.
Square Wave Jerks	Small, involuntary eye movements away from and back to the fixation point.
Ocular Tremor	Involuntary, rhythmic oscillation, affecting eye movements.
Video-oculography	Tracking eye movements using video imaging.
Vestibulo-ocular reflex	Mechanism that stabilizes vision by producing eye movements that counteract head movements, ensuring clear vision while in motion.

Figure 1 Schematic representation of saccadic pathways. Voluntary saccades arise from frontal eye fields and travel to the basal ganglia (caudate nucleus, substantia nigra) before descending to the superior colliculus and forward onto brainstem eye movement structures. Additional brain regions (egDLPFC) may be recruited for to execute more cognitively complex saccades, such as anti-saccades. Reflexive saccades have a direct pathway from visual cortex (not shown) to the posterior parietal cortex and from here onto the superior colliculus. There is an additional pathway from the posterior parietal cortex to activate frontal eye fields and then travel via the basal ganglia towards the superior colliculus, and reticular formation before entering the midbrain (vertical) and pontine (horizontal) ocular motor plant. dlPFC dorsolateral prefrontal cortex.

Table 2 Quantitative Metrics Acquired from Classes of Eye Movements

Type of Eye Movements	Quantified Parameters
Saccadic Eye Movements	Latency (ms) Velocity (°/sec) Duration (ms) Gain
Anti-Saccades	Latency (ms) Error rate (%) Corrected errors (%)
Smooth Pursuit	Latency (ms) Gain Phase lag Saccadic frequency
Nystagmus	Amplitude (°) Frequency (Hz)

Figure 2 Ocular motor features of neurodegenerative diseases and their associated diagnostic value (red: low diagnostic value, yellow: medium diagnostic value and green: high diagnostic value) based on key publications highlighting ocular motor features of each neurodegenerative disease.