E.A.B. Over

Inhibition of return is not a foraging facilitator in saccadic search and free viewing

The ability to search and scan the environment effectively is a prerequisite for spatial behavior. A longstanding theory proposes that inhibition of previously attended loci (Inhibition of return; IOR) serves to facilitate exploration by increasing the likelihood to inspect new areas instead of returning to locations that have been inspected before. In this eye movement study we tested whether we could find evidence in favor of this hypothesis. Here we report that IOR does occur during search and free viewing, because we found increased fixation times preceding return saccades (eye movements that return to previously fixated locations). Meanwhile we observed no influence of IOR on the search strategy. Rather than the predicted low number we found many return saccades. Therefore, IOR does not serve as a foraging facilitator in saccadic search and free viewing. We hypothesize that IOR is an intrinsic aspect of shifting attention and gaze direction and furthermore that it is not always advantageous to prevent return saccades.

Coarse-to-fine eye movement strategy in visual search

Oculomotor behavior contributes importantly to visual search. Saccadic eye movements can direct the fovea to potentially interesting parts of the visual field. Ensuing stable fixations enables the visual system to analyze those parts. The visual system may use fixation duration and saccadic amplitude as optimizers for visual search performance. Here we investigate whether the time courses of fixation duration and saccade amplitude depend on the subjects knowledge of the search stimulus, in particular target conspicuity. We analyzed 65,000 saccades and fixations in a search experiment for (possibly camouflaged) military vehicles of unknown type and size. Mean saccade amplitude decreased and mean fixation duration increased gradually as a function of the ordinal saccade and fixation number. In addition we analyzed 162,000 saccades and fixations recorded during a search experiment in which the location of the target was the only unknown. Whether target conspicuity was constant or varied appeared to have minor influence on the time courses of fixation duration and saccade amplitude. We hypothesize an intrinsic coarse-to-fine strategy for visual search that is even used when such a strategy is not optimal.

Saccadic search performance: the effect of element spacing

In a saccadic search task, we investigated whether spacing between elements affects search performance. Since it has been suggested in the literature that element spacing can affect the eye movement strategy in several ways, its effects on search time per element are hard to predict. In the first experiment, we varied the element spacing (3.4°–7.1° distance between elements) and target–distracter similarity. As expected, search time per element increased with target–distracter similarity. Decreasing element spacing decreased the search time per element. However, this effect was surprisingly small in comparison to the effect of varying target–distracter similarity. In a second experiment, we elaborated on this finding and decreased element spacing even further (between 0.8° and 3.2°). Here, we did not find an effect on search time per element for element spacings from 3.2° to spacings as small as 1.5°. It was only at distances smaller than 1.5° that search time per element increased with decreasing element spacing. In order to explain the remarkable finding that search time per element was not affected for such a wide range of element spacings, we propose that irrespective of the spacing crowding kept the number of elements processed per fixation more or less constant.

A quantitative measure for the uniformity of fixation density: The Voronoi method

In order to characterize the uniformity of fixation density, we propose a quantitative measure based on Voronoi diagrams, in which cells are defined around fixation locations. We examined how normalized cell size distributions are related to homogeneous and inhomogeneous fixation densities. Two possible measures for use with the Voronoi method are discussed. Both show good correlation with subjective visual evaluations of the uniformity of fixation densities. Not only are these measures objective and quantitative, they also have a simple intuitive meaning: They may be thought of as reflecting the clustering of fixations.

Saccadic search: On the duration of a fixation

Publisher Summary This chapter answers the question that is it the fixated stimulus element or the fixation history that determines fixation duration. It measured 93,922 fixations to investigate how fixation times are adjusted to the demands of a visual search task. Subjects had to search for an O between Cs. The Cs could have a large gap or a small gap. The proportions of both types of Cs in the displays were varied. The main results are that: fixation time depended on the element fixated, fixation time on large gap Cs decreased with increasing proportion of large gap Cs in the display, and fixation time on large gap Cs depended on the gap size of the previously fixated element. The chapter concludes that fixation time depends both on the fixation history and on the current fixation element. The contribution of both components on fixation time may depend on the task and the amount of useful information in the displays. Pre-programming of fixation times appears to be conservative such that extension of fixation time occurs in the next fixation whereas shortening of fixation time appears to be delayed.

On the role of attentional inhibition and memory during visual search

Although humans have limited memory and visual processing capacity, they are capable of finding partly specified targets in complex and dynamic environments. Nowadays there is much need for such effective artificial searchers (for example in military, security and medical image processing). The way the human brain keeps track of inspected items may inspire designers of artificial systems. The role of inhibition of return (putative attentional memory) and the role of memory in visual search in general are discussed. Based on two eye movement studies we conclude that humans use a smart scanning strategy rather than explicit memory to avoid previously inspected locations. Such strategies could be useful in artificial systems that operate in environments that change frequently.