Our visual system is exceptionally good at detecting change — as long as the change takes place while we’re looking. If you glance at a scene, then look away for a moment, your ability to detect a change is substantially impaired. Changes that would be obvious when we’re looking can become maddeningly difficult to detect if we’re distracted for even a tenth of a second.
Take a look at this quick movie (QuickTime required) — the picture will alternate flashing with a distractor pattern. Each time the picture flashes, a portion of the picture will change in some way. Can you see what’s changing?
Next, take a look at this version, where the distractor pattern has been removed.
A little easier to spot the difference now? Most people probably didn’t have enough time to spot the change in the first movie, but spotted it instantly in the second movie. This phenomenon, called “change blindness,” has been discussed before on Cognitive Daily. It’s likely that it occurs because the visual system simply doesn’t maintain a detailed description of a scene — why should it bother, when the information is right in front of us?
Instead, visual memory probably only includes the key semantic information about the photo — it’s a narrow cobbled street, with cars parked on one side, and lots of potted tropical plants.
Interestingly, if I had given you a hint about the nature of the change — even just mentioning the word “manhole” before showing the first movie, you would have been able to notice the change much faster. This makes sense — this semantic information can then be compared to with the two versions of the photo, and the change observed.
But how exactly does such a hint direct your attention to the change? Elizabeth Walter and Paul Dassonville presented volunteers with dozens of movies like this, preceded by several different types of hints, in order to see which hints diminish change blindness. The hints were actually “primes,” because observers were never told that the words were related to the change blindness task. Instead, the words were flashed rapidly and observers were required to say the word (if they could) and then proceed with the change blindness movie, pressing a button as soon as they noticed a change.
Half the time, the prime was explicit: the word appeared for 200 milliseconds and was easy for all viewers to read. Half the time, it flashed for just 33 milliseconds, and viewers were unable to read it. This is an implicit prime. Implicit primes can make a lot of tasks related to the prime much easier. If viewers were later asked to unscramble the letters in the word, for example, they’d be faster than if they hadn’t been primed at all.
In the experiment, some of the primes were helpful (“manhole”), some were misdirecting (“awning”), and some were irrelevant (“pizza”). How did the primes affect change blindness? Here are their results:
Participants were significantly faster — they showed less change blindness — when the prime was helpful compared to when it was irrelevant or misdirecting. There was no significant difference between irrelevant and misdirecting primes. Most importantly, even when the prime was implicit, observers were faster to locate the change with helpful versus non-helpful primes.
Even though observers had no memory of the implicit primes, helpful primes still improved their ability to notice a change in the photos. So visual attention can be guided by semantic information even when viewers are not conscious of that information.
This research may mean that in some ways, the vivid memories we have of visual events such as a romantic sunset or a traumatic accident are no different than if we had only experienced those events through a verbal description.
Walter, E., & Dassonville, P. (2005). Semantic guidance of attention within natural scenes. Visual Cognition, 12(6), 1124-1142.