Triple code

Is there a universal neural code for sensation, similar to the genetic code, in which the complexity of sense and experience can be reduced to a few simple rules? According to Prof. Ehud Ahissar of the Weizmann Institute’s Neurobiology Department, the answer might be no. He and his team have been studying how rats use their whiskers to sense their environment, and have found that the seemingly simple act of feeling out a 3-D object requires three different types of code.

Rats’ whiskers are highly developed sense organs in the normally nocturnal animals. To get a fix on their surroundings, rats whisk their whiskers back and forth as they move. Researchers had previously shown that whisking is crucial to the act of sensing, but how does the rat’s brain map out a three-dimensional object using this movement?

Sensing begins in the neurons at the whiskers’ bases, which then fire signals off to the brain. The scientists, Marcin Szwed, Knarik Bagdasarian and Ahissar, found that in perceiving each of the three dimensions in the rat’s immediate surroundings – the horizontal, the vertical, and the radial (distance from the whisker base) – the neurons encode information in a completely different form. To sense the horizontal, for instance, the neurons fire with exact temporal precision and the timing of these signals relative to the whisking motion encodes the horizontal placement of an object. The radial, on the other hand, is encoded in quantity, specifically the number of times the neurons fire. The closer an object was to the rat’s snout, the higher the number of neuron-signaling spikes the team recorded. Height seems to be sensed through spacing: Since whisking only takes place in the horizontal plane, the researchers concluded that information about the vertical dimensions of an object is mapped out in the vertical placement of the whiskers, which are arranged grid-like on either side of the snout.

In addition to finding different codes for each dimension, the researchers noted that the nerve cells at the whiskers’ bases seemed to be specialized for the different kinds of encoding. Now the team plans to continue following the whiskers’ signals to find out how the brain interprets all three signals and melds them into one percept.