Whisking whiskers tell rats about surroundings

Abstract

Watch a rat's twitching nose and you may think that it's taking a good sniff. However, Mitra Hartmann from Northwestern University, USA, explains that much of the time the animals are deftly exploring their surroundings by whisking their whiskers to and fro. Hartmann is fascinated by how animals perceive objects and how rats interpret deflections as their whiskers brush past obstacles. She explains that measuring the interactions between a rat's whiskers and a structure could shed light on how the rodents build an understanding of their surroundings. However, directly measuring the whisker interaction patterns for all of the possible poses that a rat's head could strike would be almost impossible in real life. So, having previously built a cyber-rat simulation – complete with 62 cyber-whiskers laid out in a grid – to learn more about the rodent's sixth sense, Hartmann teamed up with Jennifer Hobbs and Blythe Towal to calculate the contact patterns as the simulated rat's whiskers explored objects that it encountered.Explaining that the cyber-rat's head can be positioned in any orientation relative to an object, Hartmann and Hobbs placed a wall and floor in the simulation and embarked on the Herculean challenge of calculating the chance that the rat's whiskers would encounter the surfaces after positioning the head in 83,509 different positions (at distances up to 60 mm from the wall, looking over a range of angles from vertically up to vertically down and looking from 90 deg right to 90 deg left). Next, Hartmann and Hobbs positioned the rat side-on to the wall, explaining that this is the orientation that rats tend to assume naturally in a burrow, and simulated a tunnel while calculating the probability of the whiskers contacting the wall and floor while whisking.Analysing the calculations, Hartmann and Hobbs were delighted to see trends emerging in the contact patterns. When the rat faced the wall head on, the adjacent whiskers in the same row or column were most likely to contact the object. However, when the rat turned side-on to the wall, the duo could see that clusters of whiskers laid out in a triangle were most likely to touch the wall. And when the rat was placed in a burrow, the triangular arrangement became even more pronounced.But how many of the head positions that Hobbs and Hartmann had simulated would a flesh-and-blood rat use when thrown into the complete unknown? This time, Towal placed a rat on a perch near to a wall in the dark and filmed the first few seconds as the animal explored the new location with its whiskers. Measuring the position of the animal's head from the movies, the trio was pleased that the rat's behaviour strongly reflected their simulations. ‘We found that the rat tends to put its head at the angles where it is most likely to maximise the number of whiskers in contact with the ground and with a structure in front of it in an uncertain environment’ says Hartmann.Adding that motion is the key to an animal building an understanding of its surroundings, Hartmann explains that sensory systems have been shaped by the way that an animal moves through the environment. She stresses that we can only understand how the brain constructs a map of the objects in the vicinity by studying sensory signals in the context of the movements that generated them. ‘If you are going to understand what the brain does, you have to study it in awake animals that are free to explore the world the way they evolved,’ she says.