A. Excessive cravings do not necessarily involve physical substances. Gambling can become compulsive; sex can become obsessive. One activity, however, stands out for its prominence and ubiquity—the world’s most popular pastime, television. Most people admit to having a love-bate relationship with it. They complain about the “boob tube” and “couch potatoes,” then they settle into their sofas and grab the remote control. Parents commonly fret about their children’s viewing (if not their own). Even researchers who study TV for a living marvel at the medium’s hold on them personally. Percy Tannenbaum of the University of California at Berkeley has written: “Among life’s more embarrassing moments have been countless occasions when I am engaged in conversation in a room while a TV set is on, and I cannot for the life of me stop from periodically glancing over to the screen. This occurs not only during dull conversations but during reasonably interesting ones just as well.”

B. Scientists have been studying the effects of television for decades, generally focusing on whether watching violence on TV correlates with being violent in real life. Less attention has been paid to the basic allure of the small screen—the medium, as opposed to the message.

C. The term “TV addiction” is imprecise and laden with value judgments, but it captures the essence of a very real phenomenon. Psychologists and psychiatrists formally define substance dependence as a disorder characterized by criteria that include spending a great deal of time using the substance; using it more often than one intends; thinking about reducing use or making repeated unsuccessful efforts to reduce use; giving up important social, family or occupational activities to use it; and reporting withdrawal symptoms when one stops using it.

D. All these criteria can apply to people who watch a lot of television. That does not mean that watching television, in itself, is problematic. Television can teach and amuse; it can reach aesthetic heights; it can provide much needed distraction and escape. The difficulty arises when people strongly sense that they ought not to watch as much as they do and yet find themselves strangely unable to reduce their viewing. Some knowledge of how the medium exerts its pull may help heavy viewers gain better control over their lives.

E. The amount of time people spend watching television is astonishing. On average, individuals in the industrialized world devote three hours a day to the pursuit—fully half of their leisure time, and more than on any single activity save work and sleep. At this rate, someone who lives to 75 would spend nine years in front of the tube. To some commentators, this devotion means simply that people enjoy TV and make a conscious decision to watch it. But if that is the whole story, why do so many people experience misgivings about how much they view? In Gallup polls in 1992 and 1999, two out of five adult respondents and seven out of 10 teenagers said they spent too much time watching TV. Other surveys have consistently shown that roughly 10 percent of adults call themselves TV addicts.

F. What is it about TV that has such a hold on US? In part, the attraction seems to spring from our biological ‘orienting response.’ First described by Ivan Pavlov in 1927, the orienting response is our instinctive visual or auditory reaction to any sudden or novel stimulus. It is part of our evolutionary heritage, a built- in sensitivity to movement and potential predatory threats.

G. In 1986 Byron Reeves of Stanford University, Esther Thorson of the University of Missouri and their colleagues began to study whether the simple formal features of television-cuts, edits, zooms, pans, sudden noises—activate the orienting response, thereby keeping attention on the screen. By watching how brain waves were affected by formal features, the researchers concluded that these stylistic tricks can indeed trigger involuntary responses and ‘derive their attention-al value through the evolutionary significance of detecting movement…. It is the form, not the content, of television that is unique.’

H. The orienting response may partly explain common viewer remarks such as: “If a television is on, I just can’t keep my eyes off it,” “I don’t want to watch as much as I do, but I can’t help it,” and “I feel hypnotized when I watch television.” In the years since Reeves and Thorson published then pioneering work, researchers have delved deeper. Annie Lang’s research team at Indiana University has shown that heart rate decreases for four to six seconds after an orienting stimulus. In ads, action sequences and music videos, formal features frequently come at a rate of one per second, thus activating the orienting response continuously.

I. Lang and her colleagues have also investigated whether formal features affect people’s memory of what they have seen. In one of their studies, participants watched a program and then filled out a score sheet. Increasing the frequency of edits (defined here as a change from one camera angle to another in the same visual scene) improved memory recognition, presumably because it focused attention on the screen. Increasing the frequency of cuts—changes to a new visual scene-had a similar effect but only up to a point. If the number of cuts exceeded 10 in two minutes, recognition dropped off sharply.

J. Producers of educational television for children have found that formal features can help learning. But increasing the rate of cuts and edits eventually overloads the brain. Music videos and commercials that use rapid intercutting of unrelated scenes are designed to hold attention more than they are to convey information. People may remember the name of the product or band, but the details of the ad itself float in one ear and out the other. The orienting response is overworked. Viewers still attend to the screen, but they feel tired and worn out, with little compensating psychological reward. Our ESM findings show much the same thing.

K. Sometimes the memory of the product is very subtle. Many ads today are deliberately oblique: they have an engaging story line, but it is hard to tell what they are trying to sell. Afterward you may not remember the product consciously. Yet advertisers believe that if they have gotten your attention, when you later go to the store you will feel better or more comfortable with a given product because you have a vague recollection of having heard of it.

A EVERYBODY knows that the dinosaurs were killed by an asteroid. Something big hit the earth 65 million years ago and, when the dust had fallen, so had the great reptiles. There is thus a nice if ironic, symmetry in the idea that a similar impact brought about the dinosaurs’ rise. That is the thesis proposed by Paul Olsen, of Columbia University, and his colleagues in this week’s Science.

B Dinosaurs first appeared in the fossil record 230m years ago, during the Triassic period. But they were mostly small, and they shared the earth with lots of other sorts of reptile. It was in the subsequent Jurassic, which began 202 million years ago, that they overran the planet and turned into the monsters depicted in the book and movie “Jurassic Park”. (Actually, though, the dinosaurs that appeared on screen were from the still more recent Cretaceous period.) Dr Olsen and his colleagues are not the first to suggest that the dinosaurs inherited the earth as the result of an asteroid strike. But they are the first to show that the takeover did, indeed, happen in a geological eyeblink.

C Dinosaur skeletons are rare. Dinosaur footprints are, however, surprisingly abundant. And the sizes of the prints are as good an indication of the sizes of the beasts as are the skeletons themselves. Dr Olsen and his colleagues, therefore, concentrated on prints, not bones.

D The prints in question were made in eastern North America, a part of the world the full of rift valleys to those in East Africa today. Like the modern African rift valleys, the Triassic/Jurassic American ones contained lakes, and these lakes grew and shrank at regular intervals because of climatic changes caused by periodic shifts in the earth’s orbit. (A similar phenomenon is responsible for modern ice ages.) That regularity, combined with reversals in the earth’s magnetic field, which are detectable in the tiny fields of certain magnetic minerals, means that rocks from this place and period can be dated to within a few thousand years. As a bonus, squishy lake-edge sediments are just the things for recording the tracks of passing animals. By dividing the labour between themselves, the ten authors of the paper were able to study such tracks at 80 sites.

E The researchers looked at 18 so-called ichnotaxa. These are recognizable types of the footprint that cannot be matched precisely with the species of animal that left them. But they can be matched with a general sort of animal, and thus act as an indicator of the fate of that group, even when there are no bones to tell the story. Five of the ichnotaxa disappear before the end of the Triassic, and four march confidently across the boundary into the Jurassic. Six, however, vanish at the boundary, or only just splutter across it; and there appear from nowhere, almost as soon as the Jurassic begins.

F That boundary itself is suggestive. The first geological indication of the impact that killed the dinosaurs was an unusually high level of iridium in rocks at the end of the Cretaceous when the beasts disappear from the fossil record. Iridium is normally rare at the earth’s surface, but it is more abundant in meteorites. When people began to believe the impact theory, they started looking for other Cretaceous-and anomalies. One that turned up was a surprising abundance of fern spores in rocks just above the boundary layer – a phenomenon known as a “fern spike”.

G That matched the theory nicely. Many modern ferns are opportunists. They cannot compete against plants with leaves, but if a piece of land is cleared by, say, a volcanic eruption, they are often the first things to set up shop there. An asteroid strike would have scoured much of the earth of its vegetable cover, and provided a paradise for ferns. A fern spike in the rocks is thus a good indication that something terrible has happened.

H Both an iridium anomaly and a fern spike appear in rocks at the end of the Triassic, too. That accounts for the disappearing ichnotaxa: the creatures that made them did not survive the holocaust. The surprise is how rapidly the new ichnotaxa appear.

I Dr Olse  and his colleagues suggest that the explanation for this rapid increase in size may be a phenomenon called ecological release. This is seen today when reptiles (which, in modern times, tend to be small creatures) reach islands where they face no competitors. The most spectacular example is on the Indonesian island of Komodo, where local lizards have grown so large that they are often referred to as dragons. The dinosaurs, in other words, could flourish only when the competition had been knocked out.

J That leaves the question of where the impact happened. No large hole in the earth’s crust seems to be 202m years old. It may, of course, have been overlooked. Old craters are eroded and buried, and not always easy to find. Alternatively, it may have vanished. Although the continental crust is more or less permanent, the ocean floor is constantly recycled by the tectonic processes that bring about continental drift. There is no ocean floor left that is more than 200m years old, so a crater that formed in the ocean would have been swallowed up by now.

K There is a third possibility, however. This is that the crater is known, but has been misdated. The Manicouagan “structure”, a crater in Quebec, is thought to be 214m years old. It is huge – some 100km across – and seems to be the largest of between three and five craters that formed within a few hours of each other as the lumps of a disintegrated comet hit the earth one by one.