Mapping the bat's belfry.

An abstract artist of the dada movement might have sketched the representation of an animal's body surface that appears on the canvas of its brain surface. And across the wide variety of mammals from rat to primate, there is a remarkably consistent organization to these abstract portraits, with some parts exaggerated, others minimized and the presentation not limited to any natural perspective. But now, a striking variation in the representation has been discovered in the brain of a bat. It appears to reflect postural differences between the flying, upside-down-hanging mammals and those that walk on all fours or upright.

Whether galloping, climbing, sleeping or playing the piano, most mammals hold their forelimbs under or in front of their heads. But bats are the exception. Their hands and arms have been modified into wings, which during flight circle behind the head.

This unusual posture is uniquely reflected in the bat's brain, scientists report in the Feb. 7 NATURE. They applied gentle tactile stimuli to the body surfaces of five bats of the type called greyheaded flying foxes. Then they measured the resulted activity at almost 800 sites in the brain region called the somatosensory cortex. As in other mammals, there are multiple representations of the body surface in this brain region, report M.B. Calford, M.L. Graydon and J.D. Pettigrew of the University of Queensland in St. Lucia, Australia, and Michael F. Huerta and Jon H. Kaas of Vanderbilt University in Nashville, Tenn.

In other mammals, the brain representation called S-I has the head at one end, the tail at the other and the limbs extending, as they generally do in mammalian anatomy, in the same direction as the tongue. The second representation, called S-II, is a smaller, cruder mirror image of S-I.

However, in the bat's bran the representation of forelimbs in both S-I and S-II extends in the opposite direction from that of the tongue. The brain diagram thus seems to depict the animal in flight.

These findings suggest that there is more to the shape of the brain maps than just the accumulation of point-to-point connections from the body surface. Somehow evolution and development have constrained these maps to reflect the customary spatial orientation of the animal's body, whether it be airborne or earthbound. Such constraints may aid the interaction among different maps in the brain. In addition to the somatosensory representations, the visual and auditory systems similarly map input from the environment. The scientists suggest that shaping the maps to reflect body orientation may be necessary to maintain an appropriate relationship between maps.