12 August 2005
Certain Images Cause Temporary Blindness
by Kate Melville
Researchers at Vanderbilt University have uncovered an intriguing aspect as to how our brains process certain images. The images that seem to trigger the effect are either gory or erotic - what the researchers term "emotional" images. Following exposure to such images, the test subjects suffered a short vision blackout, failing to process what they saw immediately afterwards.
Most people are familiar with the "rubbernecking" effect that accidents trigger. Even though we know we need to keep our eyes on the road, our emotions of concern, fear and curiosity cause us to slow down and stare out the window at the accident. Psychologist David Zald and his colleagues set out to determine if the rubbernecking effect carries over into more minute lapses of attention through two separate experiments.
Research subjects in the first experiment were shown hundreds of pictures that included a mix of disturbing images along with landscape or architectural photos. They were told to search the images for a particular target image. An irrelevant, emotionally negative or neutral picture preceded the target by two to eight items. The closer the negative pictures were to the target image, the more frequently the subject failed to spot the target. In a subsequent study, yet to be published, the researchers substituted erotic for negative images and found the same effect.
"We observed that people fail to detect visual images that appeared one-fifth of a second after emotional images, whereas they can detect those images with little problem after neutral images," said Zald. "We think that there is essentially a bottleneck for information processing and if a certain type of stimulus captures attention, it can basically jam up that bottleneck so subsequent information can't get through," Zald said. "It appears to happen involuntarily."
Previous research has shown that there are limits to how much information we can hold in our visual short-term memory. We often miss visual images that pass right before our eyes if we are paying attention to something else. The researchers believe that we can also miss what we are searching for if we are shown an unexpected image that impacts us emotionally, a situation the researchers have dubbed "emotion-induced blindness."
The researchers then designed a second experiment to see if individuals can override their emotion-induced blindness by focusing more deliberately on the target for which they are searching. The results, however, were ambiguous. The researchers discovered that they were partially right: specific instructions helped some subjects control their attention, but it didn't help others.
Interestingly, the researchers found that the subject's ability to stay focused was somewhat dependent on aspects of the subject's personality. Those with low harm-avoidance scores were better able to stay focused on the targets than those with high harm-avoidance scores. "We increasingly are suspicious that people who are more neurotic or harm avoidant may not be detecting negative stimuli more than other people, but they have a greater difficulty suppressing that information," Zald said in conclusion.
Source: Media release - Vanderbilt University