Our fingers run over surfaces; our eyes are in constant motion. This is all a part of “active sensing,” key principles of which have now been uncovered by a Weizmann Institute study.
“We intuitively understand that active sensing should provide the brain with information very different from that which is acquired by mere passive sensing, (e.g. feeling without finger movement),” says Prof. Ehud Ahissar, of the Weizmann Institute Neurobiology Department, “yet current experiments nearly always keep the organs stationary.” Much of his recent research focuses on discovering how the sensory nerves in these organs perform when in motion. Such research, he hopes, will deepen our understanding of perception, and help optimize the design of artificial sensory aids for the deaf and blind.
Rats’ whiskers, which sweep back and forth to locate and appraise objects in the immediate vicinity, are an ideal tool for studying the active aspects of perception. Working with doctoral student Marcin Szwed and Dr. Knarik Bagdasarian, Ahissar recorded the transmissions of neurons that connect whiskers to the brain. Tracking these cells’ responses while whisker hairs actively swept over objects, they saw that two basic types of neurons came into play. The first, which they call whisking neurons, respond solely to the whisking motion itself, regardless of whether the whiskers touch an object or not. The second type, which they dubbed touch neurons, informs the brain about the surface being touched. Some of these cells respond immediately upon contact; others relay further information during prolonged contact; and yet others fire briefly as contact is broken.
“These mechanisms were previously overlooked,” says Szwed, “simply because the cells were rarely recorded during active movement.” These latest findings, published in the Oct. 29 issue of Neuron, indicate that perception is a dynamic dance in which hands, eyes and whiskers move towards the world to actively seek out sensation.
Prof. Ehud Ahissar’s research is supported by the the Carl and Micaela Einhorn-Dominic Institute for Brain Research; the Nella and Leon Benoziyo Center for Neurosciences; the Abramson Family Foundation, Washington D.C.; the Edith C. Blum Foundation, New York, NY; the Irving B. Harris Foundation, Chicago, IL.; and Mrs. Esther Smidof, Switzerland. Prof. Ahissar is the incumbent of the Helen and Sanford Diller Family Professorial Chair in Neurobiology.