Adjacent speech, and even nonspeech, contexts influence phonetic categorization. Four experiments
investigated how preceding sequences of sine-wave tones influence phonetic categorization. This
experimental paradigm provides a means of investigating the statistical regularities of acoustic
events that influence online speech categorization and, reciprocally, reveals regularities of the sound
environment tracked by auditory processing. The tones comprising the sequences were drawn from
distributions sampling different acoustic frequencies. Results indicate that whereas the mean of the
distributions predicts contrastive shifts in speech categorization, variability of the distributions has
little effect. Moreover, speech categorization is influenced by the global mean of the tone sequence,
without significant influence of local statistical regularities within the tone sequence. Further
arguing that the effect is strongly related to the average spectrum of the sequence, notched noise
spectral complements of the tone sequences produce a complementary effect on speech
categorization. Lastly, these effects are modulated by the number of tones in the acoustic history and
the overall duration of the sequence, but not by the density with which the distribution defining the
sequence is sampled. Results are discussed in light of stimulus-specific adaptation to statistical
regularity in the acoustic input and a speculative link to talker normalization is postulated.