Parallel pathways for representing temporal sensory information

Being nocturnal, rodents rely to a large extent on their whiskers to sense the precise nature of the environment, and the resolution of the system is impressive: using its whiskers, a blind-folded rat can discriminate between objects that differ only in micron-scale surface texture. Since it is well-established that different sensory systems in different mammals have profound similarities, studying the rats whiskers can offer important insight into general questions about the mechanisms of sensation in the brain. When a rat is exploring an object, it sweeps its whiskers back and forth across the surface around 10 times per second. This causes the whiskers to vibrate. These vibrations are known as 'micromotions' and they are important, since it is because different objects evoke different micromotions that the rat is able to tell them apart. A key implication is that a critical function of neurons in the rat's brain is to represent and process whisker motion. But how this happens is not fully understood. In my laboratory, we have recently made progress in understanding how the thalamus - a vital control centre for all the senses - represents whisker motion. Whereas previously it was thought that neurons in the thalamus all respond in the same way to a whisker stimulus, we unexpectedly found diversity. This new discovery opens up many new questions, including: Where in the brain does this diversity originate? Are neurons of different types located in different parts of the thalamus? Are there corresponding types of neuron in the cerebral cortex - a key brain structure, which is the next level after the thalamus in the brain's sensory circuit. In this project, we seek to answer these questions. The wider significance of the research is that it will contribute new insight into sensory systems and how sensory information is processed.

The platform for this proposal is new findings from my lab (Petersen et al., Neuron, in press) which unexpectedly show that neurons in the whisker thalamic relay nucleus represent diverse kinetic features of whisker motion (position, velocity and others). This suggests a new view of sensory processing: that the whisker system consists of parallel channels, specialised for conveying different types of temporal information. It is still unknown what is the origin of these channels, and what their anatomical basis is in the pathways from the ventro-posterior medial and posterior medial nuclei of thalamus to barrel cortex. This project seeks to answer these questions. I propose to do this by a novel combination of methods. To identify the sensory features that neurons encode, we will use electrophysiological recording in conjunction with a novel reverse correlation approach. This is a new computational technique, with which it is possible to determine the sensory features that neurons in the whisker system represent in a systematic fashion that was not previously possible. To identify the anatomical location of these neurons, we will use juxtacellular recording and single-cell labelling with biocytin, in conjunction with cytochrome oxidase histochemistry.