Did you perceive the invisible gorilla after all?

The 'invisible gorilla' study is a staple of undergraduate psychology teaching. This classic 1999 work, by Daniel Simons and Christopher Chabris, then at Harvard University, found that when people were asked to watch a video of a basketball game and to count the passes between players, many failed to notice a person in a gorilla suit walking onto the court. This is one of the best-known examples of 'inattentional blindness' — the idea that when our attention is engaged, we can fail to perceive clearly visible visual stimuli.

However, a recent paper in eLife now suggests that we do perceive some of this visual information, after all.

Makaela Nartker at Johns Hopkins University and colleagues ran a series of five studies on a total of about 25,000 participants. For each study, the participants were shown objects on a screen, and asked questions about what they'd seen.

In the first experiment, the participants were asked to look out for crosses, and to indicate whether the horizontal or vertical line of the cross was longer. These crosses appeared only briefly.

On a quarter of occasions, a vertical red line also appeared at either the left or the right of the screen. After these trials, they were asked the standard inattentional blindness question, 'Did you notice anything unusual on the last trial that wasn't there on previous trials - yes or no?'

The team found that just over a quarter of participants responded 'no'. All of the participants were then told that on the previous trial a vertical red line had been present, and they were asked whether it had been on the left or right. The results showed that 64% of the people who said they hadn't seen the line got the answer right — better than chance.

In a subsequent study, in which the line that occasionally appeared was either red or blue, participants who claimed not to have seen anything different performed well above chance at answering which colour it had been.

Taken together, these two studies suggest that 'inattentionally blind' participants had some awareness of the lines, though they said they hadn't seen them.

In their third study, the researchers found that this level awareness can be graded. Of all the participants who said they hadn't seen anything unusual when a line had also been presented, some were more sure about this than others. These people were less likely to give the right answer when asked about the line's location.

For their fourth experiment, the team used a more demanding main task, and the additional object was present for longer. This time, participants had to count how often squares that were coloured either black or white bounced off the outside of a grey rectangle. There were about 28 bounces during each 17-second trial, so this task required significant attention, the team notes. Every so often, another shape — either a triangle or a circle that was either black or white — appeared in one of the top corners of the screen and took a five-second-long journey straight down, before disappearing.

When asked immediately afterwards if they'd noticed anything unusual about trials like this, 57% of participants said they had not. However, these people then did better than chance at identifying the colour and type of the extra shape. This result was replicated in a fifth study, in which the team found that the 'blind' participants had, however, no apparent awareness of the location or direction of movement of the extra shape.  

This work represents the largest ever set of inattentional blindness studies, the team writes. And it certainly challenges the idea that this type of blindness is complete. Instead, it supports the idea that people who are 'inattentionally blind' have some level of awareness of stimuli that they fail to report. "Overall, these data provide the strongest evidence to date of significant residual visual sensitivity in [inattentional blindness]," the team writes, "and even cast doubt on claims that awareness requires attention."

There is, however, plenty of other work showing that we have some level of awareness of information that we are 'blind' to. This includes research on blindsight, and also subliminal visual processing, for example.

It's also worth noting that while sensory data that the brain deems to be 'need to know' is most likely to make it through to our limited conscious awareness, our brains do of course monitor and use other sensory signals — and what's deemed 'need to know' can change. In the original gorilla studies, there's no reason why a person in a gorilla suit wandering onto the court should have made the 'need to know' cut. But if the participants had been watching the basketball game in real life, and the gorilla had been real, even if vision had been the only sense permitted, it's surely probable that everyone would have seen it. (For further info, check out Super Senses by Emma Young, pages 28-9.)

Read the reviewed pre-print in full:
Nartker, M., Firestone, C., Egeth, H., & Phillips, I. (2024). Sensitivity to visual features in inattentional blindness. eLife. 13:RP100337 https://doi.org/10.7554/elife.100337.1

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