Striking the right tone
Despite efforts by some of the most famous names in physics such as Ohm and von Helmholtz, human perception of acoustic pitch remains an unsolved puzzle. The problem is especially complex because a well-characterizable physical stimulus (sound) is detected by a heretofore poorly understood psychophysiological signal receiver (the ear). The cochlea is the ear’s signal transducer, and many models of it seem to work but then fail empirically, for example, in the case of the missing fundamental: even if the lowest frequency is removed from a sound, humans still perceive the pitch as the lowest frequency from the higher harmonics.
In Physical Review Letters, Stefan Martignoli and Ruedi Stoop of the University of Zurich and the Swiss Federal Institute of Technology, Zurich, Switzerland, report their use of an electronic cochlea to study the problem of perceived pitch. Cochlear models are often tested by applying different shifts to the pitch of the stimulus and then comparing the response with actual psychoacoustic measurements. Martignoli and Stoop follow the details of such signals through their circuit and detect the changes at each step along the path. They find that the pitch-shifting response that humans perceive can be explained as the result of local nonlinearities in the cochlea itself, rather than effects of higher-level neural processing in the brain. – David Voss