The Attentional Zoom Effect

In Motter & Belky's 1998 Vision Research paper, titled "The Zone of Focal Attention During Active Visual Search," they describe a radical new paradigm for identifying whether attention functions in a serial or parallel fashion in visual search. What follows is a brief summary of their logic, the experiments, and their conclusions.

For those unfamiliar with the visual search literature, there's been a heated debate about the relatively serial or parallel nature of visual search for quite some time. In simple feature search, one will often experience "pop out" in which an item differing only in a single way from distractors can be found almost immediately, in a way that is not dependent on the number of distractors or lure items. But in conjunctive search, where subjects search for targets that differ from distractors in terms of multiple features, the time it takes to find an object embedded in a field of distractors increases in a nearly linear fashion as the number of distractors increase. For this reason, some have proposed that attention must be deployed in a serial fashion to each item in conjunctive search. The slope of the quasi-linear search time function is interpreted as the amount of time to scan each object.

This search time is also affected by several other factors, including similarity between target-and distractors (aka discriminability) and which stimulus dimensions differentiate the two. Of course, the attentional scanning process need not have a one-to-one relationship with eye movements; in fact, attention might scan surrounding areas covertly during a single eye fixation. The size of the region available to this covert attentional scanning process may depend on stimulus density.

To test these hypotheses, the authors trained two rhesus monkeys to search for targets within a set of distractors. The stimuli were red and green bars, some of which were oriented at 0 degrees and others at 90 degrees. The number of items on a given trial was either 6, 12, 24, 48, or 96, and were always distributed evenly across the field of view. Two types of search tasks were run: feature search (e.g., look for the red bar, or look for the horizontal bar) and conjunction search (e.g., look for the red horizontal bar, or look for the green veritcal bar). Therefore, any given trial might be feature or conjunctive, with one of five array sizes, 44 possible target locations, and four possible target stimuli. Eye movements were tracked with scleral coils and video analysis.

The results showed that at small array sizes, feature search for color and orientation was equal. However, at higher array sizes, orientation actually became faster - suggesting that orientation search is affected by set size. Furthermore, the search time x array size functions were not perfectly linear, but instead was less positive at higher array sizes.

Statistical analysis did not show a correlation of array size with fixation duration, suggesting that attentional scanning may not be serial within each fixation. Yet the authors knew that information must be extracted from the areas surrounding each fixation, because # of fixations was always much less than half the array size. What if subjects were capable of processing the stimuli surrounding a fixated item in parallel with that item?

By determining the probability that a target was found given its distance from the current fixation, the authors were able to derive an empirical measure of the "conspicuity area," or to use the title of their paper, the zone of focal attention. In metaphorical terms, one might view this as the diameter of the attentional spotlight; according to these results, this "spotlight" does not encompass items that are more than 2 times the average nearest-neighbor interstimulus distance from fixation. Subsequent experiments suggest this finding is invariant to total display size, and that the results are essentially similar for more difficult conjunctive search tasks.


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