Spontaneous Activity

Abstract

Spontaneous activity is an important object of study because it is the basis of two ideas regarding information processing in the CNS. One viewpoint considers spontaneous activity as neuronal noise (Fitzhugh 1957, 1958; Siebert 1965; McGill 1967), the presence of which sets a limit to the detection capabilities and sensitivity of our senses and obscures the stimulus-related activity. The problem of the analysis of messages carried by the nerve fibers for the brain is then similar to the engineering problem of detecting a signal in a noisy communication channel. An increase in the number of independent noisy channels which all carry the same stimulus message but with uncorrelated spontaneous activity will then increase the amount of information transferred at a given intensity or decrease threshold intensity. This can be called ensemble coding. Fitzhugh (1957) calculates that for N independent nerve fibers the threshold intensity is proportional to N^−0.62. An example of such a situation might be present in the auditory system where one inner haircell is innervated by 10–20 afferent nerve fibers with greatly varying spontaneous activity properties and slightly varying thresholds (e.g., Geisler et al. 1985). The other viewpoint reflects the other extreme: spontaneous activity is considered to reflect the main process in the CNS upon which the sensory input either acts as a modulator or is regarded as a mere perturbation of the spontaneous activity (e. g., Rodieck et al. 1962).