(Inside Science) -- The spider's iconic leggy shape can abruptly yank our attention, even when we’re focused on something else, according to a new study. Other shapes such as houseflies and hypodermic needles don’t draw our attention in the same way. This suggests that spiders may be hard-wired into our visual systems, helping us avoid a threat that our ancestors faced for millions of years.

“You can really think of it as sort of a computational circuit that you can program: When you detect a small little mass with radiating segments, you notice it, and bring it to awareness. And then you can avoid it or treat it appropriately,” said Joshua New, assistant professor of psychology at Barnard College in New York City, and first author of the study.

While most spiders are harmless to humans, there is evidence that some dangerous species may have been common during our evolutionary history, according to New.

Other research has found that people can spot spiders quickly when they’re looking for them. But people are also good at spotting animals that don’t pose a threat, as well as modern dangers like guns and hypodermic needles. If we are no better at seeing spiders than we are at seeing other living creatures, then our sensitivity to spiders could just be part of a larger sensitivity to animals and humans in general. If we’re no better at spotting spiders than we are at spotting some historically recent invention, such as needles, both abilities might be learned through experience rather than inborn.

To see if there is something special about spiders, the researchers showed people a cross shape that flashed in the middle of a screen for an eighth of a second. The participants’ task, as far as they knew, was to judge which of the two bars on the cross was longer.

During the first three trials, only the cross appeared. On the fourth trial, another image appeared at the same time. The possible images included a spider, a hypodermic needle, a housefly, and abstract shapes made by rearranging the lines of the spider. People were then asked whether they saw anything besides the cross, and in which quarter of the screen the new image appeared. They also tried to identify the image by picking it out of a lineup.

Sure enough, spiders drew attention more than anything else. When the image was a spider, slightly more than half of the people got all three questions right: They noticed it, located it, and knew what it was. Far fewer people answered all three questions correctly for images that didn’t look like spiders. The study was published online this August in the journal Evolution and Human Behavior.

Lynne Isbell, an evolutionary anthropologist at the University of California, Davis, pointed out that an eighth of a second was long enough for most of New’s participants to consciously identify spider images, making it hard to tell whether deeper processes were at play. Isbell studies how vision in primates has been shaped by the need to spot dangerous snakes.

“To me it would be more interesting if we could really get to the level of unconscious visual detection — before fear stepped in, before conscious recognition stepped in," said Isbell. "Do we do that with spiders, as well as with snakes? That’s what I’m interested in, and I don’t think that the evidence is there yet."

David Rakison, associate professor of psychology at Carnegie Mellon University in Pittsburgh, doesn’t share Isbell’s misgivings. Rakison, who was not involved in the new study, found in an earlier project that five-month-old babies look at spiders longer than they look at other images.

“At least with children, there’s very little conflicting evidence that spiders and snakes have some kind of privileged nature in human visual processing,” said Rakison.

The reason New’s study is persuasive, according to both New and Rakison, is that spider images broke through people’s “inattentional blindness.” Inattentional blindness is the phenomenon where focusing on one thing can make you oblivious to everything else.

It’s surprisingly powerful. In a classic study of the effect, when people focused on counting basketball passes, half of them failed to see a woman strolling through the middle of the scene in a gorilla suit.  

The new study used a similar design, although the basketball game was replaced with a simple cross. Each person had only one chance to spot the image, and they had no opportunity to learn or prepare. This mimics the experiences our ancestors would have had with spiders.   

“You can’t learn that spiders and snakes are dangerous by being bitten by them,” said Rakison. “You can learn through observation, but you certainly don’t want to learn through direct experience, because one experiential case could kill you.”

A pre-programmed spider template could have been just the thing to keep our ancestors from learning the hard way.