A striking (and rather shocking) example is when experimenters have asked subjects to watch a short film and to silently count the number of times a ball is passed between people, say those identified by wearing a certain colour clothing. It is frequently the case that a large majority of the observers find it difficult to recognize or notice other notable events in the film, events which would ordinarily draw attention. The focus upon one particular stimulus distracts or interferes with the capacity to effectively “register” other (unexpected) occurrences. This phenomenon is called inattentional or perceptual blindness. (see: expectation, Observer bias)

In a classic experiment research participants viewed the film and were immediately asked to write down their count on paper, after which they were asked a series of questions such as whether they noticed anything unusual appearing in the presentation, whether they saw anyone else other than the six ball-passers, and subsequently whether they happened to spot a gorilla (or a woman carrying an umbrella, depending upon the experimental condition) walk to the centre of the picture, pause and face the camera to thump its chest, and then calmly walk off screen. A remarkably high proportion of those keeping count, as high as seventy-five per cent, fail to notice this spectacle amidst the ‘action’ of the ball-passing! Engagement in the task of watching and counting the passes seems to evoke a kind of “tunnel-vision” (stimulus fixation) in the observers which ‘blots out’ their awareness of the antics of the person in the gorilla suit!

The degree to which observers are beset by this inattentional blindness is reliant upon the difficulty of the monitoring task and how visually similar or dissimilar the highly salient, but unexpected events are to the target in the monitoring task.

A common reaction to such effects is for samplers of the research literature to think that somehow they would be amongst the group who didn't miss the chest-thumping gorilla.

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