NATALIE ANGIER
ESSAY


Touch is the mother of all sensory systems - so sensitive that the tip of a finger can detect a bump no larger than the diameter of a bacterial cell. It is an ancient sense in evolution: even the simplest single-celled organisms can feel when something brushes up against them and will respond by nudging closer or pulling away. It is the first sense aroused during a baby’s gestation and the last sense to fade at life’s culmination.

“Touch is so central to what we are, to the feeling of being ourselves, that we almost cannot imagine ourselves without it,”said Chris Dijkerman, a neuropsychologist at the Helmholtz Institute of Utrecht University in the Netherlands.“It’s not like vision, where you close your eyes and you don’t see anything. You can’t do that with touch. It’s always there.”

Long neglected in favor of vision and hearing, the study of touch lately has been gaining new cachet among neuroscientists, who sometimes refer to it as haptics, Greek for touch. They’re exploring the implications of recently reported tactile illusions, of people being made to feel as though they had three arms, for example, with the hope of gaining insight into how the mind works.

Others are turning to haptics for more practical purposes, to build better touch screen devices and robot hands.

“There’s a fair amount of research into new ways of offloading information onto our tactile sense,” said Lynette Jones of the Massachusetts Institute of Technology.“To have your cellphone buzzing as opposed to ringing turned out to have a lot of advantages in some situations, and the question is, where else can vibrotactile cues be applied?”

Touch is our most active sense. Susan J.Lederman, a professor of psychology at Queen’s University in Canada, pointed out that while we can perceive something visually or acoustically from a distance and without really trying, if we want to learn about something tactilely, we must make a move. We must rub the fabric, pet the cat.“Contact is a two-way street, and that’s not true for vision or audition,”Dr.Lederman said.“If you have a soft object and you squeeze it, you change its shape. The physical world reacts back.”

Whereas the sensory receptors for sight, vision, smell and taste are clustered together in the head, touch receptors are scattered throughout the skin and muscle tissue and must convey their signals by way of the spinal cord.

Our hands are brilliant and can do many tasks automatically - button a shirt, fit a key in a lock, play piano. Dr.Lederman and her colleagues have shown that blindfolded subjects can easily recognize a wide range of common objects placed in their hands. But on some tactile tasks, touch has its limits. When people are given a raised line drawing of a common object , they have trouble.

“If all we’ve got is contour information,”Dr.Lederman said,“no weight, no texture, no thermal information, well, we’re very, very bad with that.”