<P> Neurons also exhibit plasticity in their intrinsic excitability (intrinsic plasticity). </P> <P> Despite the common use of Hebbian models for long - term potentiation, there exist several exceptions to Hebb's principles and examples that demonstrate that some aspects of the theory are oversimplified . One of the most well - documented of these exceptions pertains to how synaptic modification may not simply occur only between activated neurons A and B, but to neighboring neurons as well . This is due to how Hebbian modification depends on retrograde signaling in order to modify the presynaptic neuron . The compound most commonly identified as fulfilling this retrograde transmitter role is nitric oxide, which, due to its high solubility and diffusibility, often exerts effects on nearby neurons . This type of diffuse synaptic modification, known as volume learning, counters, or at least supplements, the traditional Hebbian model . </P> <P> Hebbian learning and spike - timing - dependent plasticity have been used in an influential theory of how mirror neurons emerge . Mirror neurons are neurons that fire both when an individual performs an action and when the individual sees or hears another perform a similar action . The discovery of these neurons has been very influential in explaining how individuals make sense of the actions of others, by showing that, when a person perceives the actions of others, the person activates the motor programs which they would use to perform similar actions . The activation of these motor programs then adds information to the perception and helps predict what the person will do next based on the perceiver's own motor program . A challenge has been to explain how individuals come to have neurons that respond both while performing an action and while hearing or seeing another perform similar actions . </P> <P> Christian Keysers and David Perrett suggested that, while an individual performs a particular action, the individual will see, hear, and feel himself perform the action . These re-afferent sensory signals will trigger activity in neurons responding to the sight, sound, and feel of the action . Because the activity of these sensory neurons will consistently overlap in time with those of the motor neurons that caused the action, Hebbian learning would predict that the synapses connecting neurons responding to the sight, sound, and feel of an action and those of the neurons triggering the action should be potentiated . The same is true while people look at themselves in the mirror, hear themselves babble, or are imitated by others . After repeated experience of this re-afference, the synapses connecting the sensory and motor representations of an action would be so strong that the motor neurons would start firing to the sound or the vision of the action, and a mirror neuron would have been created . </P>

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