Learning to See: Mechanisms in Experience-dependent Development
Neuronal activity is an important shaping factor in the self-organization of the developing nervous system.
Postnatal signals from sensory surfaces modulate neuronal activity and hence interfere with the self-organizing processes.
In the mammalian visual cortex these experience-dependent modifications are restricted to a critical period of postnatal development.
The rules which determine the direction of an activity-dependent change of neuronal connectivity resemble those postulated by Hebb for adaptive synaptic connections: Whether a connection is strengthened or weakened depends on the correlation between pre-and postsynaptic activity.
For a change to occur it is a prerequisite that the postsynaptic neuron is active. Hence, only sensory patterns capable of activating cortical neurons can induce modifications.
In addition to appropriate senory activity, internally generated permissive gating signals are necessary to permit experience-dependent modification. Thus, whether a change can occur in response to sensory stimulation does depend on the central state of the nervous system.
Stimulation conditions suitable for inducing long-term modifications are associated with an entry of Ca++-ions into intracellular compartments, suggesting the possibility that Ca++-ions serve as a trigger signal for the processes which cause long-term modifications of excitatory transmission. It is proposed that the experience-dependent modifications of neuronal interactions have an associative function and serve to assemble neurons according to functional criteria. The resulting selective interactions are thought to be the prerequisite for the development of cooperatively-coupled neuron assemblies.
KeywordsVisual Cortex Receptive Field Cortical Cell Visual Experience Striate Cortex
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