How can anyone miss a gorilla walking through the middle of the screen?

Many of the viewers did not report seeing it at all. Counting the passes swallowed them so completely that a person in a gorilla suit crossed the screen without registering. The eyes took the scene in; the attention was pointed at another task. The phenomenon is called inattentional blindness, and the evidence that it is a resource and not mere absent-mindedness is that the harder the task, the more people miss.

The setup is simple and misleading in its simplicity. Viewers watch a video of two teams passing balls and are asked to count the passes of one team only. The counting task takes real effort, because you have to follow and ignore at the same time.

Halfway through, a person in a gorilla suit walks in, moves to the center of the picture, stops, beats their chest and leaves. They are on screen for about nine seconds. A substantial share of viewers do not report seeing them at all.

This is not a failure of the eye. The light went in, the retina caught it, and in the physiological sense the picture was there. What did not happen is conscious processing, because attention had been allocated to another task. Conscious vision is not a photograph of everything in front of the eyes but a selection.

The evidence that this is a resource rather than absent-mindedness is that misses rise as the task gets harder. Which makes divided attention a practical problem, not a curiosity. A driver busy with a conversation may not see a motorcycle, and in one famous study an image of a gorilla went into a lung scan and most radiologists did not report it. The miss rate shifts with the instructions, the expectations, and how much the event resembles the task.