Content area
Full Text
R E V I E W S
NoiseRandom or unpredictable fluctuations and disturbances that are not part of a signal.
SpikeAn action potential interpreted as a unitary pulse signal (thatis, it either is or is not present), the timing of which determines its information content. Other properties of the action potential, such as its shape or depolarization levels, are ignored.
Trial-to-trial variabilityThe differences between responses that are observed when the same experiment is repeated in the same specimen (for example, in the same neuron or in the same subject).
Variability is a prominent feature of behaviour. Variability in perception and action is observed even when external conditions, such as the sensory input or task goal, are kept as constant as possible. Such variability is also observed at the neuronal level14. What are the sources of this variability? Here, a linguistic problem arises, as each field has developed its own interpretation of terms such as variability, fluctuation and noise. In this Review, we use the term variability to refer to changes in some measurable quantity, such as spike timing or movement duration. Importantly, the term variability does not indicate that a particular mechanism has generated the variability, and does not suggest whether the variability is beneficial or detrimental. Trial-to-trial variability can arise from two distinct sources. The first source is from the deterministic properties of the system. For example, the initial state of the neural circuitry will vary at the start of each trial, leading to different neuronal and behavioural responses. The variability in the response will be exacerbated if the systems dynamics are highly sensitive to the initial conditions. The second source of variability is noise, which is defined in the Oxford English Dictionary as random or irregular fluctuations or disturbances which are not part of a signal [ ] or which interfere with or obscure a signal or more generally any distortions or additions which interfere with the transfer of information.
Whereas previous reviews have focused on neuronal variability in general, we focus here on work directly relating to noise. Noise permeates every level of the nervous system, from the perception of sensory signals to the generation of motor responses, and poses a fundamental problem for information processing5,6. In recent years the extent to...