If it weren’t for the “Men in Black” (M.I.B.), your life might be at constant risk from extraterrestrial activity on earth. The M.I.B. are best known for using “neuralizers” to erase witnesses’ memories of alien sightings.

One might therefore wonder whether the M.I.B. considered Dan Simons and Chris Chabris famous gorilla as sufficiently extraterrestrial to neutralize him from about half of the observers who saw the gorilla walking through a scene of basketball playing students but failed to notice its presence. (If you haven’t watched the video, take a moment now).

This famous demonstration of the inability to see salient objects or events that occur right in front of our eyes is known as “Inattentional Blindness” (IB). Critically, IB is based on a lack of attention rather than mere vision deficits. The term had been coined by Arien Mack and Irvin Rock in 1992, followed by their book with the same title at MIT press.

More recently, a study by Drew, Vo and Wolfe showed that not even expert observers like radiologist, who do little else but searching for signs of cancer day in and day out, are immune to IB: When an image of a gorilla — 48 times the size of an average nodule — was inserted into a set of chest CTs, more than 80% of the radiologists failed to see it.

In a new paper published in the Psychonomic Bulletin & Review, Emily Ward and Brian Scholl now ask whether M.I.B. (i.e. “Multiple Inattentional Blindness” or as they call it “Repeated Inattentional Blindness”) is caused by blindness or amnesia. (They did not consider the alternative hypothesis that it might represent activity by the Men in Black.)

One could imagine that the reason for IB is a failure of perception; that is, we simply never actually saw the gorilla in the first place. Or, IB could be attributed to a failure to encode the event into memory yielding what Wolfe had called “inattentional amnesia”. Like in Sperling’s whole vs. partial report, people might have seen the event, but the memory traces had disappeared before more durable memories were established.

So far the main issue with solving the blindness versus amnesia question for IB has been that – according to Wolfe — directly asking subjects if something is consciously perceived without attention “proves to be impossible because the demand to report on [an unexpected event] directs attention to [it]”.

Ward and Scholl now introduced a way to escape this dilemma. They managed to design two experiments that evoke IB repeatedly in the same observers and within the same session, despite the fact that the unexpected event occurred multiple times and despite participants being queried about what they noticed after each critical instance.  What made this possible was the discovery that IB can be caused not only by having no expectation, but also by having the wrong expectation.

How did the researchers do that?

Everything took place on a gray screen horizontally bisected by a purple line, and with a fixation square at its center. On each trial, four black and four white Ls and Ts appeared and moved independently for 14 seconds along linear paths at random velocities. Observers were instructed to fixate the central point and to count how many times the white shapes crossed the midline.

After getting into the mode of counting midline crossings for a few trials, on the fourth trial a gray cross unexpectedly entered the screen on the right, traveling through the whole screen along the midline, passing behind the fixation point, and exiting on the left side. By that time, the cross had been visible for a full 5 seconds. After the cross disappeared, observers were immediately asked whether they noticed “anything … that was different from the first three trials” — and if so, to describe what was different. They were then shown the gray cross and asked if they had noticed it — and if so, to describe where it was and how it moved. There were two more such unexpected events in trials 6 and 9.

By that time, observers were obviously expecting something unexpected to happen. The question was, whether they would be able to report a new unexpected event. So on the 10th and last trial, a black E entered the display from the left and moved horizontally along the midline, behind the fixation point, and then exited on the right. As with the other unexpected trials, observers were again asked the same set of questions.

Ward and Scholl reported that about half of all observers failed to notice the first unexpected event (the gray cross), which nicely replicated the known IB. Interestingly, of these “non-noticers”, almost a third also failed to see the final unexpected event (the black E). This seems quite astonishing considering that an unexpected event had already occurred and been queried about multiple times.

Importantly, for the initial question of whether IB is due to blindness or amnesia, Ward and Scholl point out that observers were asked to report anything unexpected immediately after the E’s offset, where their report could have still been based on the visual “icon”. Their inability to do so provided evidence against the amnesia account. To take this even further, in a follow-up experiment observers were asked to immediately report unexpected events — even mid-trial — which would leave no time for any perceptual decay. Also, the cross and the Es were made more salient by presenting them in novel colors. Still, 13 out of 100 “non-noticers” who had failed to see the first unexpected event also missed the final unexpected event despite the explicit instructions to immediately respond if anything unexpected had happened. Note that this occurred even though the observers were quite happy to respond immediately if the very same unexpected event was repeated.

These findings are theoretically interesting, since previous demonstrations of IB always delayed assessment of perception until after the unexpected event had disappeared, which could be reinterpreted as simply failing to encode visual information into memory. Ward and Scholl therefore conclude that IB reflects not a failure of memory but a failure of “seeing” in the first place.

Finally, this study on Inattentional Blindness once again brings into mind, how easily even salient events can be missed even when we are alert, on the lookout for something odd, and/or experts in our search tasks (like radiologists). So the M.I.B. mission to neutralize memories of alien sightings here on earth might have been far easier than one would have thought.