Cortical pathways and synaptic mechanisms for texture discrimination learning in rodents

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Humans mainly use their visual system to understand and interpret the world. It is therefore often difficult for us to imagine the world of touch in any detail or understand its importance. Yet if we were unable to recognise objects that we pick up from the way they feel, we would surely recognise the wealth of information yielded up by this vital sense. Touch is most often appreciated when there is no light and we are feeling our way in the dark, or when we are looking for an object we cannot see in a pocket or a bag. Firefighters rely on their tactile sense when exploring smoke filled rooms; surgeons need to relearn how to feel and grasp objects when they wear surgical gloves (which alter the tactile experience). In these cases, the spatial arrangement of small protuberances and depressions on the surface of an object and the way they yield, or are deflected in time, in other words the texture of the surface, give us a great deal of information about the object's identity. Think of how you might distinguish an old (paper) and a new (plastic) note in your pocket purely by touch.This grant is aimed at understanding how texture information is processed in the brain and how the brain adapts and learns to attribute meaning to particular textures. To do this we will study texture processing in the rodent brain. Rodents are nocturnal animals and are therefore highly reliant on tactile information for identifying objects in their environment. Rodents are experts at touch. In laboratory tasks, we have found that they preferentially use their whiskers to identify different textures. Rodents have a highly stylised array of 40 large whiskers on either side of the snout that they can move back on forth (or whisk), effectively to palpate objects and recognise them. Remarkably, they are able to distinguish between surfaces that differ in particle size by just 18um, a distance that is orders of magnitude smaller than the spacing between whiskers on the face. We can teach the animals to associate a reward with a particular texture and then discover which areas of the brain are involved by silencing those brain areas during the exploration. We can also record neuronal activity from the same brain regions and discover how the neurones encode texture information in those places we suspect to be involved. So far, work on higher order touch processing has generally tended to be conducted in monkeys. If we could establish where the processing streams are located in rodents, there is a possibility that work would proceed at a faster pace in this field and fewer monkeys might need to be studied. The second major part of our study concerns how and where changes occur in the brain when animals learn about new textures or attribute meaning to familiar textures. We can test this idea by asking the rodents to learn to distinguish between two similar textures to acquire a reward. While they are learning, we can image the synaptic connections between neurones in the brain that might be involved. Synapses connect neurones together and allow them to communicate with one another. Specifically, we can see whether new connections are formed and whether they correlate with the memory of the texture. To do this we can make a small window in the brain and make the neurones of interest fluoresce by producing GFP (green fluorescent protein). We can view the dendritic spines (which are one half of the synapse) on the neurones using a 2-photon microscope. We can view whether new dendritic spines are produced when the animal learns. We can also eliminate or erase those spines using a molecular probe that selectively infiltrates new spines and see whether the new spines are indeed necessary for learning the texture discrimination. This will help us understand the physical basis of learning and memory in general.