Sequential dependencies in pitch judgments.

Studies that measure pitch discrimination relate a subject's response on each trial to the stimuli presented on that trial, but there is evidence that behavior depends also on earlier stimulation. Here, listeners heard a sequence of tones and reported after each tone whether it was higher or lower in pitch than the previous tone. Frequencies were determined by an adaptive staircase targeting 75% correct, with interleaved tracks to ensure independence between consecutive frequency changes. Responses for this specific task were predicted by a model that took into account the frequency interval on the current trial, as well as the interval and response on the previous trial. This model was superior to simpler models. The dependence on the previous interval was positive (assimilative) for all subjects, consistent with persistence of the sensory trace. The dependence on the previous response was either positive or negative, depending on the subject, consistent with a subject-specific suboptimal response strategy. It is argued that a full stimulus + response model is necessary to account for effects of stimulus history and obtain an accurate estimate of sensory noise.