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Vassilakis, P.N. (1997).  First and second pitch-shift effects as alternative manifestations of a single phenomenon: Introduction.  J. Acoust. Soc. Am. 102(5/2): 3186 (presented at the 134th meeting of the Acoustical Society of America, San Diego, CA).

Abstract
  
The second pitch shift effect describes a drop in the perceived pitch of a complex stimulus when the frequency spacing among its components is increased. Based on existing experimental data describing the first pitch shift effect, the present paper demonstrates that the mathematical model introduced by de Boer [Doctoral dissertation, University of Amsterdam, (1956)] and modified later by Smoorenburg [JASA, 48(4/2): 1055-1060, (1970)], predicts without any further modification the second pitch shift effect as well. The Smoorenburg model also predicts that the perceived pitch of complex stimuli will not always drop when increasing the frequency spacing among components, but may rise depending on the structure of the stimuli. A perceptual experiment was conducted using nine complex stimuli. For eight of the stimuli the model predicted a rise in pitch with increasing frequency spacing while for the ninth stimulus the opposite was predicted. The results of the experiment support the conclusion that the relationship between the direction of pitch motion (rise/drop) and the direction of changes in frequency spacing among components of complex stimuli (increasing/decreasing) can be predicted by the same model that explains the first pitch shift effect, making the second pitch shift effect an unnecessary concept.