Synaptic inputs that are often active together are strengthened during learning so that statistical regularities in synaptic input lead to the post-synaptic neuron being selective to particular stimuli whilst also rendering it invariant to accidental features.Neural hierarchies allow neurons at higher level to capture information gained by many neurons at lower levels. A stimulus that drives a neuron at a high-level in a network hierarchy will almost always be part of a (visual or other) scene together with other stimuli. So although invariance aids object recognition when there are changes to irrelevant aspects of the stimulus, it also causes a problem because a given stimulus will never cover the complete receptive field of a high-level neuron but leave room for competing stimuli. This selective efficacy of subsets of a neuron’s input may be aided if converging neuronal inputs to higher-level neurons are functionally segmented and if only a relevant segment is selected at a time. Pascal Fries believes that whist connectivity provides selectivity and invariance, synchronisation provides the required segmentation and selection of a segment.

Gamma-band synchronization (a group or groups of neurons pulse firing together at the rate of 40-80 Hz) can emerge in a network of excitatory and inhibitory neurons. Inhibitory neurons provide shunting inhibition that stops other neurons from firing. This provides windows for synchrony at the moment inhibition wears off. Excitatory signals can then take advantage. Gamma band oscillations are sufficiently regular to allow prediction of the next excitability peak. As long as the travelling time from the sending to the receiving group is also reliable, their communication windows for input and output are open at the same times. Conduction delays between neurons are typically and order of magnitude shorter than the cycle length of the oscillation allowing sending and receiving to occur within one excitability peak. Packages of spikes can therefore arrive at other neuronal groups in precise synchronization and enhance their impact. Rhythmic inhibition therefore provides rhythmic modulation of excitatory input gain. Fries considers the mechanistic consequences of neuronal oscillations and calls this hypothesis ‘communication through coherence’.

Coincidence detection and rhythmic gain modulation create an exclusive communication link between a target group and a strongly synchronised source group. If there is a synchronization among the neurons in groups A and among the neurons in group B but not between A and B then a down stream group C will either synchronise to A or B but not both at the same time. Strong and precise gamma band synchronisation within group A will trigger many spikes in C and entrain C to the rhythm of A. Once entrained a winner-takes-all effect occur as the result of input gain. A competitive advantage is therefore given to one group of neurons.(read here for more details)

Uhlhaas et al note that the fast switching between synchronized and de-synchronized states observed in the data seems at odds with: the coupling strength that can be achieved through synaptic plasticity, the speed of changes in the functional topology, and mechanisms that could cause changes in transmission delays. Hence they conclude that the most likely option for the modulation of synchrony is to change the dynamical states of the coupled neuronal populations, such as the balance between excitation and inhibition. So in addition to the firing rates, precise timing of individual discharges may be used to gate transmission and synaptic plasticity, to selectively route activity across the cortical network and to define particular relations in distributed activity patterns.

Singer notes that such synchro­nous events are statistically improbable so their information content is high. Therefore they are likely to be very effective in eliciting responses in target populations. For the system to work, individual cells need to be able to rapidly change their synchronisation partners if new associations are required due to a change in Gestalt properties of the scene, and if more than one object is present in a scene several distinct assemblies should form.


« »