Evaluating frequency proximity in stream segregation

Consecutive sounds of similar structure that are close in frequency or pitc h are more likely to be perceived as part of the same sequence than those a t greater frequency separations, The principle of grouping into such percep tual sequences, or auditory streams, is known as frequency proximity. Howev er, the metric by which one frequency difference is judged to be greater or less than another in complex auditory scenes is not yet known. Two experim ents explored the metric for frequency proximity. We presented repeating th ree-tone stimulus patterns at a rate where they are normally heard as two s treams, one containing the highest tone and one containing the lowest. The middle tone joined one stream or the other depending on its frequency. Subj ects reported the perceived allocation of the variable tone by responding o n a 5-point scale. The frequency at which either of these two percepts was equally probable was found to be lower than a logarithmic midpoint or the m idpoints on a cochlear map or the Mel scale; that is, it was unlike metrics arrived at by direct comparisons of tones. Further, the midpoint for high and low tones presented synchronously was lower than that for the same tone s presented sequentially, demonstrating that in addition to a proximity fac tor, some additional factor or factors must operate differently when the lo wer and upper fixed tones are, or are not, presented simultaneously.