Re: purely spectral pitch

[Katrin Krumbholz <katrin.krumbholz@xxxxxxxxxxxxxxxxx>]

Dear Peter,

I think, I have to defend John's position a bit:

Let me start off be explaining Roy and my experiments in more detail.
Our study on pitch perception in binaural listening conditions was
devided into two parts. The first part deals with dichotic pitches,
namely Huggins pitch and its complex relative, Multiple-Phase-Shift
(MPS) pitch. As you know, a MPS pitch can generated by introducing
narrow phase transitions at harmonic frequencies in an otherwise diotic
noise. The auditory nerve responses to a spectrally unresolved harmonic
complex tone still mediate information about the repetition rate of the
stimulus: when harmonics merge within the cochlear filters, they interfere
and produce a temporal modulation (beating) at the output of the filters.
This is not the case in a spectrally unresolved MPS stimulus (phase
transitions merged within cochlear filters), because the signals at the
two ears are just noises, so there is no beating between harmonics.
Thus, even if dichotic and monaural pitches would be processed with
different contiguity or integration windows, there would be no temporal
information to process in a spectrally unresolved MPS stimulus!

We measured the lower limit of pitch (LLP) for MPS stimuli and found
that the pitch limit is about 65 Hz, which (as expected) approximately
corresponds to Shackleton and Carlyon's (1994) estimate of the limit of
spectral resolution in the relevant frequency region (between about
200 and 1200 Hz). For a comparison, we also measured the LLP for homo-
and antiphasic (S0 and Spi) harmonic tones in a diotic noise masker.
The loudness of the harmonic tones was matched to the loudness of the
MPS pitch. The LLP for the homophasic harmonic tones was about 35 Hz,
i. e. well below the limit of spectral resolution. In contrast, the LLP
for antiphasic harmonic tones was essentially equal to the LLP for MPS
stimuli. This finding is a bit astonishing, because the antiphasic
tones should produce temporal modulation of the cochlear filter resonses
when harmonics are unresoved (i. e. below about 65 Hz in our frequency
region), and this modulation information should theoretically be
interpretable by a temporal pitch mechanism. Due to the low SNR
of the antiphasic tones, the temporal modulation would be burried
under a lot of noise but there should be no reason why the temporal
information shouldn't be recovered by binaural unmasking. Or should there?

Yes, namely binaural sluggishness. In the second part of our study we
measured the temporal modulation transfer function (TMTF) for
homophasic and antiphase bandpass noise (filtered to the frequency
region of interest, namely between 200 and 1200 Hz) in a diotic noise
masker. The sensation level of the antiphasic noise was about 12 dB
and the sensation level of the homophasic noise was set so that the
modulation detection threshold at low modulation rates would be roughly
equal in both interaural phase conditions. The result was that the TMTF
for the antiphasic noise had a much lower 3-dB down cutoff rate than
the TMTF for the homophasic noise, and in the antiphasic noise, modulation
was completely inaudible at rates above 30 Hz, which would be relevant
for pitch perception.

Thus, the reason for the similar LLP for MPS stimuli
and for antiphasic harmonic tones is that the temporal modultion in
the cochlear filter resonses to a spectrally unresolved antiphasic
harmonic tones is NOT recovered by binaural unmasking, becuase
binaural processing is too slow. Now, if the binaural system is too
slow to follow the envelope modulations in the cochlear filter resonses
to unresolved harmonic tones, it is probably even more too slow to follow
the temporal fine structure in cochlear filter responses to spectrally
resolved harmonics. At least with the current concepts of binaural
sluggishness in mind (temporal integration window with duration of the
order of 100 ms), there is no reason to believe that envelope and
fine-structure temporal information should be treated differently by
the binaurla system.

So under the assumption (and I concede that this is an assumtion)
that fine-structure and envelope are both temporally smeared to
the same extent, the conclusion would be that dichotic pitches and
the pitch of binaurally unmasked harmonic (or pure) tones are PURELY
spectral pitches.

Finally just one short remark on your comments about frequency
selectivity and the distinction between physiological and
phsychophysical data: Ted Evans has produced a lot of quite
compelling evidence that the frequency selectivity of the
mammalian auditory system, as measured in phsychophysics, is
already established at the level of the cochlear!

Best regards,
   Katrin Krumbholz