New “Invisible Gorilla” Study Adds To Proof Of “Inattentional Blindness”

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Additional evidence of the phenomenon known as inattentional blindness has emerged from a new study of sensory focus and memory, this time with a professional group other than cops.

“At Force Science, we write and teach about inattentional blindness in a law enforcement context,” says Dr. Bill Lewinski, executive director of the Force Science Institute. “But this behavioral happening is by no means limited to police work–which, in fact, lends credence to its frequent occurrence in use-of-force encounters.”

Inattentional blindness (IB) refers to the tendency of the human brain to aggressively filter out visual images that are irrelevant to a captivating primary task that an individual is pursuing, even though these images are clearly within the person’s field of view. Later, when the subject claims not to remember these images, he or she may be perceived as lying.

In a police context, the phenomenon may arise, for example, during a shooting when the involved officer is so intently focused on reacting to a threat that he or she fails to notice or later recall other blatantly obvious elements or actions simultaneously present in the surrounding environment, a memory blank that gets interpreted as willful deceit.

You may be familiar with the classic “Invisible Gorilla” film that features two intermingling groups of young people moving around as they pass basketballs back and forth. Viewers are told to concentrate on and count the passes made by one group. Part way through the film, a large person in a black gorilla suit strolls into the scene, stands amidst the basketball crowd, beats on his chest while facing the camera, and then slowly exits.

Later the vast majority of viewers who have never seen the film before say they did not notice the gorilla at all and are stunned upon replay to see how obvious it was. “They were ‘blind’ to him,” Lewinski explains, “because their narrowed focus of attention was on something else within their field of view.”

The new study, conducted by researchers from Harvard University and the Visual Attention Laboratory at Brigham and Womens Hospital in Boston, documents the same effect among highly skilled radiologists, a group known for exceptional expertise in observation that they have honed for years.

Twenty-four top-notch radiologists were asked to participate in common and familiar lung cancer screening. They were to scroll freely through computerized images or “slices” of lung tissue, looking for small circles of light that would signify malignant nodules. Each case series they examined contained an average of 10 nodules, and the doctors were to click the locations of these abnormalities while outfitted with eye-tracking equipment.

In the last of these cases, the researchers had inserted a picture of a black gorilla outlined in white into five slides of tissue sections. “The gorilla was over 48 times the size of the average nodule in the images,” the team writes. In other words, about the size of a matchbook. The gorilla picture was situated near a lung nodule “so that both were clearly visible.”

Detecting the suspect nodules “was challenging, even for expert radiologists,” the researchers note, so they were fully engaged in the search. They scrolled through the slices containing the gorilla an average of 4.3 times, with “ample opportunity” to study them.

Yet when questioned later, 20 out of the 24 expert observers–more than 83%–had failed to notice the unexpected but intrusive gorilla. The eye-tracker revealed that those who missed the beast spent nearly six seconds reviewing the tissue slices with him in them, with more than half looking “directly at the gorilla’s location.”

The researchers note: “This is a clear illustration that…expert searchers are not immune to the effects of IB, even when searching…images within their domain of expertise…. When engaged in a demanding task, attention can act like a set of blinders, making it possible for salient stimuli to pass unnoticed right in front of our eyes….”

In an earlier study by different researchers, radiologists were shown chest x-rays with a clavicle (collarbone) missing. Roughly 60% of the radiologists failed to notice this when they were shown x-rays as if for an annual exam.

“It would be a mistake to regard these results as an indictment of radiologists,” researchers in the current study write. “As a group, they are highly skilled practitioners of a very demanding class of visual search tasks….[E]ven this high level of expertise does not immunize against inherent limitations of human attention and perception.”

Certainly true, Lewinski agrees. He points out in addition that while the test the radiologists were involved in was demanding, “it was not life-threatening for the participants themselves. The level of inattentional blindness can be expected to be even greater among LEOs who are involved in unexpected, rapidly evolving, uncertain, and unpredictable threats to their very existence.”

A full report on the new study, titled “The invisible gorilla strikes again: Sustained inattentional blindness in expert observers,” will be published in the journal Psychological Services. The authors are Trafton Drew, Melissa Vo, and Jeremy Wolfe.

[Our thanks to Force Science staffer Jennifer Dysterheft for her assistance in reporting on this study.]

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