Habituation: events in the history of its characterization and linkage to synaptic depression. A new proposed kinetic criterion for its identification.

Reports on habituation from the last part of the nineteenth and the first part of this century are reviewed. Publications are selected according to their significance for the understanding of the properties that characterize habituation. Reports that have contributed to the establishment of a causative link between habituation and synaptic depression are also reviewed. A kinetic analysis of experimental cases of habituation is made for different reflexes in a number of species. The results of the analysis indicate the existence of two distinct subcategories of habituation: a slow and a fast version. Each category has a remarkably narrow range of kinetic variability, regardless of species and type of reflex. In one particular reflex, the tentacle withdrawal of Helix pomatia, both versions of habituation are displayed in response to different stimulation paradigms. On the basis of the kinetic analysis, a new criterion for identification of habituation is suggested.