PURPOSE. To investigate the relationship between accommodation, pupil size and cognitive demand in free space, under binocular conditions. METHOD. Using a binocular, infrared autorefractometer, accommodation and pupil size in right and left eyes were measured simultaneously. Subjects were seated 40 cm from three forms of visual target of varying cognitive demand: 1) Maltese cross, 2) text and 3) random text and numbers. Subjects were required to fixate target 1 and read targets 2 and 3 aloud until completion. Data represent recordings over 15s and 30s respectively. Subjects were instructed to keep the targets in clear focus at all times. Each target was tested twice, summing to a total of six trials per participant. Trial order was randomly assigned. The subjects were fully corrected for distance viewing during the task. Only contact lens correction was permitted to optimize measuring conditions. No systemic or topical medications affecting accommodation and/or pupil size were used during the experiment. Subjects (n = 16) were between the ages of 18-36 yrs (mean = 25 ± 4.8).
RESULTS. Mean accommodative errors, AE for right, R and Left, L eyes: 1) Maltese cross; R AE = 0.34D, L AE = 0.34D 2) text; R AE = 0.04D, L AE = 0.22D and 3) random text with numbers; R AE = -0.24D, L AE = -0.03D. Paired t-tests reveal a significant difference between AE for targets 1 and 3 (p = 0.014 R, p = 0.045 L) and between targets 2 and 3 (p = 0.041 R, p = 0.002 L). For both right and left eyes, pupil size increased in diameter with an increase in cognitive demand.
CONCLUSIONS. Our results show that 1) binocular accommodative errors measured in a natural setting decrease with an increase in cognitive demand, confirming results from previous monocular studies and 2) pupil size increases with increased cognitive demand. The latter effect implies an increase in sympathetic contribution to the pupil, perhaps attributable to an increase in cognitive demand.