7 May 2009

Hebbian Neural Learning Modification in Bear, Connors, & Michael, Neuroscience: Exploring the Brain

by Corry Shores
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Bear, Connors, & Michael

Neuroscience: Exploring the Brain

Activity-Dependent Synaptic Rearrangement

Imagine a neuron that has a synaptic capacity of six synapses and receives inputs from two pre-synaptic neurons, A and B.

One arrangement would be that each of the presynaptic neurons provides three synapses. Another arrangement is that neuron A provides one synapse and neuron B provides five. A change from one such pattern of synapses to another is called synaptic rearrangement. There is abundant evidence for widespread synaptic rearrangement in the immature brain.

Synaptic rearrangement is the final step in the process of address selection. Unlike most of the earlier steps of pathway formation, synaptic rearrangement occurs as a consequence of neural activity and synaptic transmission. In the visual system some of this activity-dependent shaping of connections occurs before birth in response to spontaneous neuronal discharges. However, significant activity-dependent development occurs after birth and is influenced profoundly by sensory experience during childhood. Thus, we will find that the ultimate performance of the adult visual system is determined to a significant extent by the quality of the visual environment during the early postnatal period. In a very real sense, we learn to see during a critical period of postnatal development. (708d)

Synaptic Segregation

The precision of wiring achieved by chemical attractants and repellents can be impressive. In some circuits, however, the final refinement of synaptic connections appears to require neural activity. A classic example is the segregation of eye-specific inputs in the cat LGN.

Segregation of Retinal Inputs to the LGN.

Segregation is thought to depend on a process of synaptic stabilization whereby only retinal terminals that are active at the same time as their postsynaptic LGN target neuron are retained. This hypothetical mechanism of synaptic plasticity was first articulated by Canadian psychologist Donald Hebb in the 1940's. Consequently, synapses that can be modified in this way are called Hebb synapses, and synaptic rearrangements of this sort are called Hebbian modifications. According to this hypothesis, whenever a wave of retinal activity drives a postsynaptic LGN neuron to fire action potentials, the synapses between them are stabilized. Because the activity from the two eyes does not occur at the same time, the inputs will compete on a "winner-takes-all" basis until one input is retained and the other is eliminated. Stray retinal inputs in the inappropriate LGN layer are the losers because their activity does not consistently correlate wit the strongest postsynaptic response (which is evoked by the activity of the other eye).

Plasticity at Hebb synapses: The target neurons in the LGN have inputs from different eyes. Inputs from the two eyes initially overlap and then segregate under the influence of activity. (a) The two input neurons in one eye (top) fire at the same time. This is sufficient to cause the top LGN target neuron to fire, but not the bottom one. The active inputs onto the active target undergo Hebbian modification and become more effective. (b) This is the same situation as in part a, except that now the two input neurons in the other eye (bottom) are active simultaneously, causing the bottom target neuron to fire. (c) Over time, neurons that fire together wire together. Notice also that input cells that fire out of sync with the target lose their link.

Bear, Mark. F., Barry W. Connors, & Michael A. Paradiso.Neuroscience: Exploring the Brain. London: Lippincott Williams & Wilkins, 2007.

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