Optotype duration and head impulse acceleration: a functional approach to vestibulo-ocular reflex assessment

Abstract Background The functional head impulse test (fHIT) evaluates vestibulo-ocular reflex function during passive head impulses. This study investigated the effects of optotype duration and head acceleration on fHIT outcomes in healthy adults and examined whether age group, sex, and head movement direction were associated with performance. Seventy healthy adults (40 women, 30 men; mean age 36.44 ± 9.5 years; range 18–58 years) were divided into two age groups (18–40 and 41–58 years; n = 35 each). Lateral-plane fHIT was administered at randomized optotype durations of 40, 60, and 80 ms and head accelerations of 4000, 5000, and 6000°/s². Overall within-participant effects were evaluated using a three-way repeated-measures ANOVA with optotype duration, head acceleration, and movement direction as within-subject factors. Pairwise duration comparisons used Wilcoxon signed-rank tests with Bonferroni adjustment, and between-group comparisons used Mann–Whitney U tests. Results Correct answer percentages were generally higher with longer optotype durations. Repeated-measures ANOVA showed significant main effects of optotype duration (F(2, 138) = 77.95, p < 0.001, partial η² = 0.530) and head acceleration (F(2, 138) = 75.68, p < 0.001, partial η² = 0.523), whereas the duration × acceleration interaction was not significant (F(4, 276) = 1.24, p = 0.294, partial η² = 0.018). Significant differences were found between 40 and 60 ms and between 40 and 80 ms at all accelerations ( p < 0.05). Differences between 60 and 80 ms were smaller and were not statistically significant in most acceleration-specific comparisons, a pattern compatible with, but not sufficient to establish, a possible ceiling effect. No statistically significant differences were detected according to age group, sex, or head movement direction ( p > 0.05). Conclusions Optotype duration is associated with fHIT performance in healthy adults, with the clearest differences involving the 40 ms condition. The relatively small differences between 60 and 80 ms may be compatible with a ceiling effect, although this requires further verification. Testing additional optotype durations, including 40 ms, may provide complementary information about functional VOR performance; its diagnostic value, sensitivity, and false-negative implications should be evaluated in populations with vestibular disorders.