This is hidden brain.
I'm Shankar Vedantam.
Nearly half a century ago, volunteers in a psychological experiment were shown a slideshow of a small red car getting into a crash.
Some volunteers were asked, how fast was the car traveling when it passed the yield sign?
There was a catch.
The original slides showed the car passing a stop sign, not a yield sign.
Psychologist Elizabeth Loftus and her colleagues later asked volunteers to identify images they had seen before.
Some volunteers were shown the earlier slide with a stop sign.
Others saw an edited slide where the stop sign was replaced by a yield sign.
Volunteers who had been asked the question about the yield sign mistakenly remembered seeing the picture with a yield sign.
In the intervening decades, hundreds of experiments have shown that what we think we see is really an amalgam of what is out there, how we feel about what we see, our conversations with others about what we see, and so on.
Attention, memory, and emotion all play a role in shaping what we notice and what we fail to observe.
In the late 1990s, the psychologists Dan Simons and Christopher Shabri devised an experiment that produced a nearly unbelievable result.
They had volunteers watch a group of people passing basketballs.
Volunteers were told to count how many times the ball was passed between members of a group.
It was tricky, but if you focused, you could do it.
But the researchers were after something more interesting than counting basketball passes.
In a video, a person wearing a full gorilla suit walks into the middle of the video, turns to face the camera, thumps its chest, and then walks off the other side.
About 9 seconds later, the gorilla seems impossible to miss.
It's a gorilla.