Human Systems Integration Division @ NASA Ames

The decline in smooth pursuit performance following moderate-dose alcohol has been shown in previous studies. Fransson and colleagues (Clin Neurophysiol, 121(12):2134-2142, 2010) showed that a steady-state gain decrease of 2% and 7% at 0.06% and 0.1% blood alcohol concentration (BAC), respectively. The current study examines the detailed features of both the pursuit and saccadic components of the tracking response over a wider range of BACs extending down to zero. Thirteen healthy subjects (8 females, mean age + SD = 25.2 + 2.1 years) participated. Subjects completed a 3-day at-home pre-study schedule that included 8.5 hours in bed at night with the timing verified by actigraphy, call-ins, and self-reported sleep logs. They then participated in a 2-day laboratory study where they consumed a single low-dose of ethanol (40% ABV Vodka with juice) adjusted to target either 0.02 or 0.06% initial BACs, and completed three pre-dose and 6-9 post-dose oculomotor test runs of a 5-minute Rashbass-like ocular tracking task with highly randomized target trajectories (Krukowski & Stone, Neuron, 45(2):315-323, 2005). From this oculomotor test, a set of largely independent measures of oculomotor (Liston & Stone, JOV, 14(14):12, 2014) and pupillary (Tyson, Flynn-Evans, & Stone, JOV, 17(10):660, 2017) performance were computed. For each of these metrics, for each subject, we computed the within-subject % deviation of each test run from their baseline (the average across their three pre-dose runs). We then combined the data across subjects and used linear regression to compute the slope and x-intercept (extrapolated threshold) of the mean % deviation from baseline as a function of BAC. We found significant linear decreases in steady-state pursuit gain (-3.8%/0.01%BAC, p < 0.001) and proportion smooth (-1.0%/0.01%BAC, p < 0.01) with estimated thresholds of +0.013 and -0.007%BAC, respectively. We also found a significant decrease in the precision of visual motion processing for direction (+9.2%/0.01%BAC with a threshold of +0.008%, p < 0.01), but not for speed (p = 0.48). Conversely, we found a significant increase in catch-up saccade amplitude (+9.4%/0.01%BAC, p < 0.001) and rate (+2.38%/0.01%BAC, p < 0.05) during steady-state tracking, with thresholds of +0.009 and -0.019%, respectively. Our study, using high-uncertainty target motion, reveals that the smooth pursuit system is more sensitive to ethanol than previously thought. Ethanol consumption generates a dose-dependent increase in reliance on the saccadic system to maintain steady-state tracking as pursuit becomes substantially less effective, starting at BAC levels around 0.01%.