Our brain is active as long as we live (awake or sleeping). 30 to 100 billion neurons (nerve cells) communicate with each other with different speed through one quadrillion (1015) contacts. The irreversible end of all brain activity results in brain death which is the legal definition of death.
Even at rest neuronal activity levels persist for each single neuron in form of spontaneous discharges of neurotransmitters, the currency of nerve cell communication, summing up to an overall brain-specific and thus individual noise level. This noise level is time-dependent - increasing age very likely leads to noise level changes. As a consequence the strength of brain activity results from the composition of the signal-to-noise ratios of various neuronal populations and from the speed of this composition.
Although we know that individual brain activity patterns reflect specific object definitions, we are far from understanding (1) the neuronal algorithm underneath each of these object definitions or else the composition of neuronal microcircuits and (2) how first order object definitions are neuronally linked to a second order context such as experience (historical neuronal changes) and emotions (present perception-based neuronal changes) resulting in individual views of the outer world.
Damages to or deterioration of neuronal populations affect the signal-to-noise ratios and therefore the overall brain activity related to incoming external stimuli (vision, sensation, taste, smell, hearing). This we call mental disorders.
In order to visualize brain activity invasive and non-invasive techniques are used on in vivo and in vitro test systems: electroencephalography (EEG), functional magnetic resonance imaging (fMRI), transcranial magnetic stimulation (TMS), optical imaging, electrophysiology, computational modelling.
Tags: brain, neurons, activity