neurons. This means that neurotransmitters can be released in the absence of an input signal. Synaptic bombardment: The large number of incoming spikes add a fluctuating amount of charge to the cell, which depends on the structure of the incoming spike trains and affects the cell's excitability. Chaos: Chaotic dynamics can occur in single cells (due to periodic inputs or bursting due to intrinsic currents). Simple networks of neurons can also exhibit chaotic dynamics. Even if the chaos is deterministic, it can amplify noise from the other sources to macroscopic levels due to sensitive dependence on initial conditions. Connectivity noise: Noise that arises from the number of connections and non-uniformity that a neuron has with other neurons within a neuronal network. There is a stronger presence of sub-threshold noise when the interconnectivity is strengthened, or the number of connection to other neurons is increased. The opposite remains true, too. If the interconnectivity of the neurons is decreased so then is the level of sub-threshold noise. Environmental stimuli: Noise can be produced on a larger scale due to fluctuations in CO2, which lead to variations in blood flow. The level of CO2 in the blood allows for either vasoconstriction or vasodilation, which can encroach, or expand, into nearby neural networks producing noise. Recording methods Global recording The external noise paradigm assumes "neural noise" and speculates that external noise should multiplicatively increase the amount of internal noise in the central nervous system. It is not clear how "neural noise" is theoretically distinguished from "neural signal." Proponents of this paradigm believe that adding visual or auditory external noise to a stimuli, and measure how it affects reaction time or the subject's performance. If performance is more inconsistent than without the noise, the subject is said to have "internal noise." As in the case of "internal noise," it is not clear on what theoretical grounds researchers distinguish "external noise" from "external signal" in terms of the perceptual response of the viewer, which is a response to the stimulus as a whole. Electroencephalogram or EEG can be used to measure the brain since the signal-to-noise ratio is poor, so noise produced by the brain can be detected in vivo. Local field potentials can be used to test the noise present in large neuronal networks. This measure of voltage can be used to determine the interconnectivity by the level of noise. Local recording Local recording has