Oscillations, Waves, and Interactions - GWDG

Specific signal types in hearing research 67
is based on a “poor-man’s” model of processing in the peripheral parts of the hearing
system, the ultimate test for this concept had to come from physiological experiments.
Griffin et al. [60] used transposed stimuli to measure ITD sensitivity in high-frequency
neurons of the Inferior Colliculus of guinea pigs. Their study basically supported all
assumptions and findings made in the original paper by van de Par and Kohlrausch
[52] and these authors concluded: “Despite limitations in the temporal coding of
neurons to high-frequency sounds, the results presented here demonstrate that under
conditions in which binaural neurons receive appropriate spike patterns, sensitivity
to ITDs conveyed by high-frequency stimuli can be equivalent to that observed in
response to low-frequency stimuli. This suggests, as first conjectured by Colburn and
Esquissaud (1976), that mechanisms underlying ITD sensitivity in low- and highfrequency
channels of the auditory system are, to a first approximation, equivalent.”
([60], p. 3477).
The latest “application” of transposed stimuli comes from the field of audiology.
Long et al. [61] used a preprocessing based on the generation of transposed stimuli
to test sensitivity to interaural differences in listeners wearing bilateral cochlear
implants. They found that all their four subjects showed significantly improved performance
when the signal was presented in a BMLD configuration, i. e. interaurally
out of phase. This observation thus supports the potential of providing temporal
information via the envelope of high-frequency carriers.
5 Conclusion
In this chapter, we have described a number of signal types with specific properties in
their phase characteristics, their finestructure or their envelope, which have allowed
detailed tests of ideas and concepts related to human hearing. In the text, we have
concentrated on the acoustic properties and the consequential perceptual insights.
But it also needs to be pointed out that the ability to generate these stimuli, and
to develop advanced time-domain hearing models, depended heavily on the highly
flexible and nearly bug-free software package that was created in the hearing groups
at the DPI throughout the 1980’s with contributions from many group members.
SI, Signal processing Interactive, was the name for a program concept that allowed
to work with digital signals like a pocket calculator works with numbers. In our
opinion, the possibilities offered by this package were at least as essential for the
scientific progress as were the growing insights in psychoacoustics and physiology.
The choice of signal types in this chapter is certainly biased towards those examples,
to which we have ourselves contributed. The great potential of these stimulus
types lies in the possibility to apply them easily in physiological experiments, and
to use them as input to time-domain models which allow the processing of arbitrary
signal waveforms [36,47,62–66]. This close interplay between psychoacoustics,
physiology and modeling is one of the central themes of a conference series, the
International Symposia on Hearing, which was initiated in 1969 by, among others,
Manfred R. Schroeder and is since then organized every three years. This symposium
never took place in Göttingen, but it can be seen as a late echo of the psychoacoustic
research at the DPI that two recent editions of these symposia, the one in 2000 [67]