project to septa between the barrels and to secondary somatosensory cortex (S2). The paralemniscal pathway runs from the interpolar trigeminal nucleus via posterior nucleus (POm) of the thalamus to S2 and to diffuse targets in barrel cortex especially layer 5. Each pathway also has secondary projections to other layers within barrel cortex and other regions of cortex, including motor cortex. These different pathways are thought to transmit different modalities of sensory information from the whisker. Whisker barrel neurophysiology The whisker barrel cortex contains different types of neurons that receive input from a range of sources that themselves receive and process an array of different types of information. As a result, neurons of the whisker barrel cortex respond to whisker-related input, but in a way that is specific to the neurons type and location. This can manifest in different ways. The simplest way is whether the cortical neuron responds only to the deflection of one whisker, or to the deflection of many whiskers. Neurons in layer 4 barrels tend to strongly or exclusively respond to one whisker, while neurons in other layers are less strongly tuned and can respond to multiple whiskers. Neurons that respond to the deflection of multiple whiskers typically have a primary whisker, to which they respond the most. The difference in response magnitude between deflection of the primary whisker and secondary whiskers can also vary between neurons. Stimulation of multiple whiskers may produce a response that is equal to the sum of the responses if each whisker was stimulated independently, or it may be different. Some neurons show greater responses when multiple neurons are stimulated in sequence, and the sequence may be direction specific. As well as combinations of which whiskers have been stimulated, neurons may also respond to specific types of whisker stimulation. The simplest response, seen in neurons within the layer IV barrel cortex, directly code for whisker displacement. That is to say, that the neuron within a given barrel will fire when the whisker that barrel represents is moved at a rate that is roughly proportional to the angular displacement of the neuron. These neurons also show directional sensitivity; certain neurons will only fire when the whisker is moved in a specific direction. Deflection-based firing neurons can sustain their response throughout the deflection of the whisker. Other neurons respond to the initial deflection, but then quickly return to their previous level of activity. Much