Perseveration consists of the inappropriate repetition of a preceding behaviour when a new adapted response is expected. We have developed statistical tools that make it possible to reveal such perseverations, assess their significance and study their finer characteristics, such as their temporal course and impaired processing level. This approach is illustrated and evaluated through analyses of naming errors produced by three patients with impairments affecting different stages of the processing chain leading from visual perception to speech production. These examples of perseverations include the intrusion not only of whole words (patient R.A.V.) but also of isolated phonemes (patient D.U.M.) or of visual features (patient Y.M.) from previous trials. In all cases, the probability that an error is a perseveration from a previous trial is an exponentially decreasing function of the lag between the two trials considered. This suggests that perseverations reflect a decaying internal variable, such as an internal level of activation of previous utterances. Based on these empirical results, we put forward a tentative mechanism for the generation of perseverations: whenever a given processing level is deprived of its normal input, persistent activity inherited from previous trials is no longer overcome by current input, and is revealed in the form of perseverations.