IT'S true. Pornography can make you blind. Look at a smutty picture and, according to research by Steven Most, of Yale University, and his colleagues, you will suffer from a temporary condition known as emotion-induced blindness.

Dr Most made this discovery while studying the rubbernecking effect (when people slow down to stare at a car accident). Rubbernecking represents a serious lapse of attention to the road, but he wondered if the initial reaction to such gory scenes could cause smaller lapses. The answer is, it does. What he found was that when people look at gory images—and also erotic ones—they fail to process what they see immediately afterwards. This period of blindness lasts between two-tenths and eight-tenths of a second. That is long enough for a driver transfixed by an erotic advert on a billboard to cause an accident.

The researchers conducted two experiments. In the first, the results of which will be published in next month's Psychonomic Bulletin and Review, the experimental subjects were shown a sequence of images. Some of these images were gory (violent injuries and mutilated bodies) while others were photographs of landscapes and buildings—things thought to be emotionally neutral. The subjects were asked to watch out for a picture that had been rotated, without being told what was in it. Between the horror picture and this target were two to eight neutral pictures. The closer the grim picture was to the target, the less likely the subjects were to spot the target. When the gory pictures were substituted by erotic ones, the outcome was the same.

Dr Most thinks that the explanation for this temporary blindness is that there is an information-processing bottleneck in the brain when it is presented with important stimuli. When the human brain was evolving, such stimuli would not have been two-dimensional images. They would, rather, have been part of the real world. Gory scenes would have had survival value (ie, “am I going to be next?”), while erotic ones would have had reproductive value. Paying attention to the landscape would have been a distraction.

In the age of photography, though, it is the image that is the distraction, and if the distracted individual is travelling at speed in a car, such distraction could be fatal. So the team carried out a second series of experiments, still unpublished, that were intended to discover whether their subjects could override this emotion-induced temporary blindness by using what they rather grandiloquently called an “attentional strategy” (ie, focusing harder on the target image). This was arranged by asking the subjects to find not any rotated photo, but a rotated photo of a building, in the array of images. The fact that they had to pay attention to both content and orientation meant they focused harder. As the researchers had expected, in this version of the experiment subjects were, on average, better at spotting the target image.

But that average concealed some interesting differences that depended on a subject's personality. The researchers knew from previous studies that the more neurotic someone is, the worse he is at controlling his attention, so they decided to see how a measure of neuroses known as the harm-avoidance scale correlated with their results. The harm-avoidance scale is a measure of a person's reaction to negative or frightening stimuli. They found that the lower a subject's score on this scale was, the more successful he was at detecting the target. This information might be useful when considering the reliability of witnesses to crimes.