Can Catch-Up Eye Movements Mask Alcohol-Related Loss of Smooth Tracking?

Yes, in a laboratory tracking task, compensating eye movements can conceal part of an alcohol-related smooth-pursuit deficit. A 16-person study found reduced smooth tracking alongside larger or more frequent catch-up saccades. Those rapid corrective movements recovered much of the lost tracking distance, but did not restore the same smooth stabilization of the target image.

The distinction is relevant to DUI evidence: observing that eyes generally keep up with a target does not establish that every component of visual tracking is unchanged. Equally, the study does not validate a roadside impairment diagnosis from a particular eye movement.

Two alcohol doses and repeated baseline comparisons

Tyson and colleagues studied eight women and eight men, averaging approximately 26 years of age. Participants completed lower- and higher-dose visits in counterbalanced order, with target alcohol values of 0.02% and 0.06%. The participant and the experimenter conducting ocular testing were unaware of the dose assigned for that day.

Before dosing, participants completed baseline runs. After dosing, researchers repeatedly measured responses to target motion with randomized direction, speed and timing. This unpredictability reduced the ability to anticipate the target’s movement.

The comparison was not a conventional alcohol-versus-alcohol-free placebo trial: both visits involved alcohol, with pre-dose baselines and overlapping measured exposure ranges used to examine dose effects and possible expectation influences.

The alcohol percentages came from breath testing

The study used an Alco-Sensor IV breathalyzer, while expressing results in BAC terminology. It did not report paired venous whole-blood assays establishing each percentage. Serum/plasma, urine and biomarkers were not interchangeable exposure measures in this experiment.

Data were grouped into 0.01-percentage-point alcohol bins. A result reported at 0.015% therefore refers to a bin centered at that value and the associated group analysis. It is not evidence of a sharp universal biological threshold at precisely that concentration.

Smooth pursuit and saccades did different work

Smooth pursuit moves the eyes continuously with a moving object. A catch-up saccade is a rapid corrective movement that repositions the eyes when they fall behind. The researchers analyzed those components separately rather than judging tracking only by total distance covered.

Initial pursuit acceleration, steady-state pursuit gain and visual direction precision showed significant changes in the bin centered at approximately 0.015%. At increasing alcohol levels, reduced pursuit was accompanied by compensating changes in saccade amplitude and rate. Saccadic movement dynamics became detectably altered at higher levels than some pursuit measures.

Combined pursuit and saccadic displacement could remain close to the target’s displacement through approximately 0.055% in this experiment. But a series of corrective jumps is not identical to continuously stabilizing an image on the retina.

What the experiment did not prove about visual acuity

The authors reasoned that residual image slip during less-smooth tracking could degrade dynamic visual acuity. That is a physiological interpretation supported by the tracking pattern, not a directly measured on-road reading failure, missed pedestrian or crash outcome.

The findings also did not show uniform deterioration across every ocular measure. Pupillary light-response dynamics and pupil size showed no consistent significant dose-related effects across the tested range. Eccentric gaze holding showed an overall dose-related trend, but individual alcohol-bin comparisons did not reach significance under the specified corrected tests.

Those null findings matter. A normal-looking pupil response cannot exclude the pursuit changes observed in this task, while a pursuit finding cannot be relabeled as a proven pupil or gaze-holding deficit.

Limitations affect individual interpretation

The cohort was small, young, healthy and selected for relatively low habitual drinking. Repeated observations increased the dataset but did not create hundreds of independent participants. Multiple outcomes, binning, regression and corrected directional tests also shape which effects were detected.

Some lower-concentration observations occurred during recovery from higher exposure. The authors compared overlapping concentrations between dose conditions to examine possible residual and expectation effects, but acknowledge remaining concerns for selected measures. They also discuss potential measurement artifacts affecting small latency and dispersion changes.

The absence of a significant residual effect after breath readings returned to zero does not establish that hangover never affects vision or performance under different circumstances.

How to use the finding in a DUI review

Distinguish smooth pursuit, catch-up saccades, gaze holding and pupil responses rather than treating “eye movements” as one outcome. Identify whether observations came from instrumented laboratory analysis or a roadside examination with a different protocol.

This study did not validate HGN scoring, establish roadside sensitivity or specificity, measure driving performance or determine legal impairment thresholds. It complements evidence that alcohol-related neural and behavioral outcomes need separate interpretation. Compensation in one output does not demonstrate intact processing everywhere, and a group-level laboratory effect does not diagnose an individual driver.

Primary source

Tyson TL, Feick NH, Cravalho PF, Flynn-Evans EE, Stone LS. Dose-dependent sensorimotor impairment in human ocular tracking after acute low-dose alcohol administration. The Journal of Physiology. 2021;599:1225–1242. https://doi.org/10.1113/JP280395.