Mayank Mehta et al
In:- Dynamic Coordination in the Brain – Eds. Christoph von der Malsburg, William A. Phillips, & Wolf Singer
This chapter examines aspects of neural synchronies, compression of information in relation to neuron oscillations, and the gamma synchronies involvement in communication between neural assemblies in different areas of the brain.
All layers of the cortex have a variety of inhibitory interneurons using the GABA neurotransmitter. These interneurons control cortical activity through their connections with excitatory neurons. Inhibitory synapses are often located near the soma (main body) of a neuron in a position to influence excitatory inputs flowing from the dendrites to the soma. Both the excitatory and the inhibitory neurons are connected to each other within and between cortical layers. Excitatory-inhibitory networks (E-I networks) are not confined to the cortex, but are widely dispersed in the brain.
When a stimulus arrives, excitatory pyramidal neurons respond and their firing rates may rise as high as 100 Hz. This activation in pyramidal neurons in turn drives the inhibitory neurons to briefly shut down the pyramidal neurons, prior to being synchronously released from inhibition. The inhibitory GABA receptors provide a time constant, and are basic to the 30-100 Hz gamma frequency oscillations that are taken as an indication of activity in the cortex.
Synchronised oscillations at a range of frequencies occur in many brain areas during perception, attention, motor planning and sleep. The 4-12 Hz theta oscillations are present in the hippocampus during spatial exploration, and are present in the visual, parietal, hippocampus and prefrontal areas during working memory. The 10-30 Hz beta frequencies are seen in the visual and motor areas. Gamma oscillations are found in visual and prefrontal areas. Lower frequency oscillations can at times be suppressed in favour of the gamma synchrony, but at other times lower frequency oscillations can facilitate the gamma synchrony.
Synchrony is important for the transmission of information between areas of the brain. For instance inputs from a small number of neurons in the thalamus to a cortical column are more effective if they are synchronised. A small number of neurons is thus sufficient to transmit information. Speed and flexibility of response is also seen as being improved by synchrony. This economy in transmission is particularly important given the energy intensive nature of axon spiking.
Neurons change their firing rate in response to changes in sensory stimuli. The brain has to deal with two influences, its internally generated oscillations and external stimuli. If a neuron is receiving a low level of external stimuli, it will only spike at the peak of an oscillation, but if it is receiving a high level of input, it can spike at any point in the oscillation. The post-synaptic neuron can measure whether the spike occurred at a high or low point of the pre-synaptic oscillation. The activity of pre-synaptic neurons can be thus compressed into an oscillation cycle in downstream neurons and perceived as a single unit. This might also help to bind together different signals so as to determine the relationship between them.
Information is seen as being produced by the activity patterns of groups of neurons. A group of coactive neurons could form within a gamma cycle. These are referred to a cell or neural assembly. The membership of the neural assembly is flexible and can change rapidly. Each gamma cycle may contain an assembly that triggers the formation of another assembly in the next cycle. Decisions are suggested to derive from coordinated activity patterns in different neural assembles dispersed across different regions of the brain. Inhibitory connections between cell assemblies could both synchronise gamma activity between assembles and increase overall firing rates.