Fast foresight

时间:2019-03-08 02:12:11166网络整理admin

By Jonathan Knight NERVES in the eye predict the future, say neurologists in Massachusetts. To compensate for the time it takes for visual signals to travel to the brain, the retina transmits an image of where a moving object will be, rather than where it is. A cricketer or baseball player can catch a ball travelling at many metres per second, even though the ball’s image takes up to a tenth of a second to get from the retina to the brain. “People have wondered how we can catch balls as well as we do, given the delays in the neural pathway,” says Markus Meister of Harvard University. To find out how we do it, Michael Berry and others in Meister’s team showed salamanders and rabbits a dark vertical bar on a light background while monitoring the activity of nerve cells in the eye called ganglion cells. These fire when specific patterns of light strike the retina. When the image of a stationary bar struck the retina, a ganglion cell monitoring that region fired about 50 milliseconds later. But when a fast-moving bar travelled across the salamander’s visual field, the same ganglion cell fired two-tenths of a second before the bar got to where the still bar had been. The nerve cell had anticipated the image’s arrival ( Nature, vol 398, p 334). “The effect is even a little stronger in the rabbit retina,” says Berry, “so we’re confident that we’ll find the same thing in humans.” Berry says that ganglion cells monitor large overlapping fields of the retina. Normally, a ganglion cell fires at peak intensity only when an object appears at its centre. But a moving object can cause this peak firing while still at the edge of the field, possibly because neighbouring cells have warned of its arrival. Tracing the delay compensation to the eyes is important because many parts of the brain could have been involved, says Romi Nijhawan, a psychologist at the California Institute of Technology in Pasadena, who first measured the delay in detail. “Nobody knew where it would be,