Distributions of fixation durations and visual acquisition rates.

In this paper, we investigated fixation durations and distributions of visual acquisition times in two experiments. In the first experiment, we reanalyzed fixation data obtained from previous experiments involving visual search and showed that: (a) the distribution of fixation durations is best described by the lognormal distribution and (b) there is a strong linear relationship between the mean fixation duration and the standard deviation of fixation duration. In the second experiment, we investigated how much of the variability in fixation durations can be explained by variability in visual acquisition rates (which we defined as the combined processes of the sampling of the retinal image and the processing of visual information). We designed a two-alternative forced choice discrimination task that mimics a visual search paradigm, but in which the maximal duration of visual acquisition was manipulated. We found a linear relationship between the mean required stimulus duration and the standard deviation of required stimulus duration that was almost identical to the relationship found in experiment 1 and conclude that variability of fixation durations can be explained by the variability in visual acquisition rates.