Mechanisms of experience dependent self-organization of neuronal assemblies in the mammalian visual system

Neuronal activity and hence sensory signals serve as a shaping factor in the development of the structural and functional organization of the mammalian visual cortex. The electrical responses of cortical neurons control the consolidation and repression of interneuronal connections. The algorithms of this activity dependent selection process closely resemble those proposed by Hebb for adaptative synaptic connections. These local modifications depend in addition on internally generated gating signals. The latter exert a "now print" function by controlling Hebbian modifications as a function of the animal's central state. These permissive gating signals are related to mechanisms which control arousal and visual attention. Evidence as available which suggests that the activation of voltage dependent Ca++ -channels in the dendrites of cortical neurones is the trigger signal for a Hebbian modification. The permissive gating signals appear to control the probability of Ca++ -channel activation. It is concluded that these activity-dependent modifications of neuronal connectivity have an function. They are capable of developing assemblies of cooperating neurones by specifying according to functional criteria which neurones should interact with each other.