The perception of sounds which vary along an acoustic continuum as belonging to two or more distinct categories without any intervening indeterminate cases. For instance, if a continuum of synthetic stimuli having the spectral attributes of ba and pa is produced by varying the voice onset time (VOT) from strongly negative to strongly positive, the individual tokens will be identified as either ba or pa. The VOT value at which the switch in perception takes place depends on the language background of the hearer. For example, for French subjects the switch will typically take place at 0ms or a positive value close to that, whereas for English subjects the switch typically takes place at a much longer positive VOT, in the region of 60ms.
Such a procedure is known as a categorisation test. A randomised sequence of many presentations of all the tokens is presented to subjects who are asked to identify each token they hear as A or B. Results are usually presented in the form of a categorisation curve. In the (fictitious) example below, the horizontal axis shows the VOT continuum in steps of 20ms from -20 to +140. The vertical axis shows the percentage ba responses summed over all subjects.