of this activity is also modulated by the behaviour of the animal - rats and mice actively move their whiskers to explore their environment, and the response of a neuron to a particular stimulus can vary depending on what the animal is doing. Experience-dependent plasticity Because the barrel cortex has a well-organised structure that relates clearly to the whisker pad, it has been used extensively as a tool to study sensory processing and development, and the phenomenon of experience-dependent plasticity - changes in the activity, connectivity, and structure of neural circuits in response to experience. Neurons in the barrel cortex exhibit the property of synaptic plasticity that allows them to alter the vibrissae to which they respond depending on the rodent's history of tactile experience. Experience-dependent plasticity is commonly studied in the barrel cortex by partially depriving it of sensory input, either by lesioning elements of the afferent pathway (e.g. the trigeminal nerve) or by ablating, plucking, or trimming some of the facial whiskers. The anatomical structure of the barrels is only affected by lesioning elements of the pathway, but innocuous forms of deprivation can induce rapid changes in the cortical map into adulthood, without any corresponding changes in the barrel structures. Because of their different effects, it seems these two paradigms work by different mechanisms. Some forms of plasticity in the barrel cortex display a critical period. Plucking whiskers in neonatal rats causes a long-lasting expansion of the representation of the spared whisker in layer 4. However, layer 4 plasticity rapidly diminishes if sensory deprivation begins after day 4 of life (P4) whereas representations in layer 2/3 remain highly plastic into adulthood. Two cortical processes run alongside each other when barrel cortex is deprived of sensory input from some whiskers to produce representational plasticity. In deprived cortex, neuronal responses to spared whiskers are enhanced and responses to deprived whiskers are weakened. These two processes have different time courses, with the weakening of deprived response preceding the strengthening of spared response, implying that they have different underlying mechanisms. These two effects combine to produce an expansion of the cortical representation of spared whiskers into the representation of adjacent deprived whiskers. It is likely that several different mechanisms are involved in producing experience-dependent plasticity in a whisker deprivation protocol (adapted from Feldman and Brecht, 2005 ): Almost immediately, loss of input to a deprived barrel column leads to a loss of