Abstract
INTRODUCTION: The literature reports better contrast sensitivity and faster manual reaction times, for stimuli located below eye fixation. In contrast, saccade reaction times to a target tend to be quicker when the target appears above eye fixation. Recently, asymmetry has been reported in only three studies for presaccadic fixation durations (PSFDs) during free-viewing, such that fixation durations are shorter if the eyes are directed above eye fixation. A need exists to determine the robustness of this asymmetry, if it is to be considered in the modelling of free-viewing eye movement behavior. In the present study, we report a meta-analysis of published and unpublished results of the vertical asymmetry in PSFDs. The goal was to determine the robustness, and quantify the extent of PSFD asymmetry during free viewing. METHOD: Twenty sets of eye movement data were obtained from research conducted at three laboratories. Only tasks without gaze-contingent manipulations were included. Twelve of the datasets utilized visual search tasks, seven utilized scene viewing, and one involved the viewing of Rorschach inkblots. Eye movements were tracked using Eyelink systems at 250Hz, 500Hz, or 1000Hz. RESULTS: PSFDs for saccades directed within a 90deg radius above, and below current eye fixations during free viewing were calculated. On average, PSFDs for up-directed saccades were shorter by 26 ms. Standardized extent of asymmetry was quantified by Hedge’s g for each data set. The combined effect size (with a random effects model) was 0.96, z= 8.73, p< .01. CONCLUSION: The meta-analysis shows a statistically significant asymmetry in PSFDs, such that during free viewing, fixation durations are shorter if the eyes are directed above eye fixation. In effect, the asymmetry in PSFDs is important for modelling when the eyes move during visual exploration.