Acute Effects of Caffeine on Dynamic Accommodative Response and Pupil Size: A Placebo-controlled, Double-blind, Balanced Crossover Study.

Objectives: To evaluate the acute effect of caffeine consumption on the accuracy and variability of accommodation, as well as its impact on pupil size and perceived levels of activation.

Methods: 22 university students (21.68 ± 3.67 years old) ingested a capsule of caffeine (4 mg/kg) or placebo (300 mg of corn-starch) in two different days and counterbalanced order. After 30 min of capsule ingestion, we objectively measured the accuracy and variability of accommodation, and pupil size using the WAM-5500 binocular open-field autorefractometer for 2 min at each of the six viewing distances (5 m, 50 cm, 40 cm, 33 cm, 25 cm, and 20 cm). Subjective levels of activation to check the effectiveness of caffeine/placebo manipulation were also reported.

Results: We found that after 30 min of caffeine/placebo ingestion, participant perceived higher levels of activation in the caffeine condition ( = .047, Cohen´s d = 0.48). Caffeine consumption induced a statistically significant dilator effect on pupil size ( = .011, η = 0.271), and reduced variability of accommodative response ( = .027, η = 0.211). However, no differences were obtained for the accuracy of accommodation ( = .321).

Conclusions: Our data suggest that caffeine consumption reduced the variability of accommodative response and induced pupil dilation. Nevertheless, the accuracy of accommodation was insensitive to caffeine intake. These findings may be explained by the bidirectional relationship between ocular functioning and the nervous system´s state of activation.