Seperating pitch chroma and pitch height in the human brain

Warren, J.D., Uppenkamp, S., Patterson, R.D. and Griffiths, T.D. (2003). Separating pitch chroma and pitch height in the human brain. Proc. Nat. Acad. Sci. (in press).

 

Motivation

A functional magnetic resonance imaging (fMRI) experiment was performed to establish whether separate mechanisms for processing the two pitch dimensions (pitch height and pitch chroma) exist in the human brain.

 

Stimuli

Figure 1. (a) The pitch helix. The musical scale is wrapped around so that each circuit (red) is an octave. The equivalent change in pitch height with fixed chroma is shown (blue). (b) Examples of sounds with changing pitch height. Each of these harmonic complexes (h1, h2, h3) has a flat spectral envelope in the frequency band 0 - 4 kHz. In h1 (top), all harmonics of the fundamental, f0, have equal amplitude; in h2 (middle), the odd harmonics are attenuated by 10 dB, producing a large increase in pitch height without changing pitch chroma; in h3 (bottom), the odd harmonics are completely attenuated, producing a one octave rise in pitch height with the same chroma (2f0).
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Results

Figure 3. Statistical parametric maps for the group.

(a) Broadband noise contrasted with silence (noise – silence, green) activates extensive bilateral superior temporal areas including both medial and lateral Heschl’s gyrus (HG). The pitch-producing stimuli contrasted with noise (pitch – noise, lilac) produce more restricted bilateral activation in lateral HG, planum polare (PP) and planum temporale (PT).

(b) Pitch chroma change contrasted with fixed chroma (all delta-chroma, red) activates bilateral areas in lateral HG, PP and antero-lateral PT.

(c) Pitch height change contrasted with fixed height (all delta-height, blue) activates bilateral areas in lateral HG and antero-lateral PT.

(d) Voxels in Figures 3b and 3c activated both by pitch chroma change and by pitch height change have been exclusively masked. Pitch chroma change but not height change (delta-chroma only, red) activates bilateral areas anterior to HG in PP; pitch height change but not chroma change (delta-height only, blue) activates bilateral areas in posterior PT. These areas represent distinct brain substrates for processing the two musical dimensions of pitch. The relative magnitude of the BOLD signal change in anterior and posterior areas is shown for each of the contrasts of interest (right). The height of the histogram columns represents the mean size of effect (signal change) relative to global mean signal for the contrasts delta-chroma-only (red) and delta-height-only (blue) at the peak voxels for each contrast in the right hemisphere; vertical bars represent the standard error of the mean size of effect. The histograms demonstrate opposite patterns of pitch chroma and pitch height processing in anterior and posterior auditory areas.

For each contrast (indicated below the panel), activated vowels are rendered on the normalised group mean structural MRI in an ‘axial’ section tilted 0.5 radians to include much of the surface of the superior temporal plane. The statistical criterion was p < 0.05 corrected for multiple comparisons across the whole brain volume. The 90% probability boundaries for primary auditory cortex (6) are outlined (black).

Corrolary: The Helix Circle Sound

The alternating-amplitude sounds make it possible to produce a sequence of notes that follow a circular trajectory around the helix. The pitch chroma increases (or decreases) endlessly without the pitch height rising (or falling).
The waves below are circular. Drop one into your favourite sound player, put it into a loop and leave it on for a while at a moderate level. It goes endlessly up (or down) five notes at a time and then pauses on for two notes and then continues. It proceeds around the chroma circle (green) endlessly, but somehow never leaves the original octave.
This version of the helix circle sound goes up in chromatic steps (12th root of 2) to make the note progression sound musical. If no temporal pattern is imposed on the sequence you tend to hear a pronounced drop (or rise) in the pitch height at the end of the octave. To avoid this, we make add two repetitions of every fifth note which de-emphasises the octave.
  • helix circle rising
  • helix circle falling
This version of the helix circle goes up in chromatic steps (12th root of 2) and to avoid promoting the perception that it drops back an octave after 12 steps, I have interrupted the sequence every five notes and repeated the fifth note twice. Since 5 does not divide into 12, it breaks up the perception of a traditional musical scale.

 

Conclusion

Changes in pitch chroma produce more activation in antero-lateral auditory cortex and changes in pitch height produce relatively more activation in postero-lateral auditory cortex.

 

References

Belin, P. & Zatorre, R.J. (2000) Nature Neurosci. 3, 965–966.
Griffiths, T.D., Büchel, C., Frackowiak, R.S.J. & Patterson, R.D. (1998) Nature Neurosci. 1, 422-427.
Patterson, R. D., Uppenkamp, S., Johnsrude, I., & Griffiths, T.D. (2001) Neuron 36, 767-776.
Patterson, R.D., Milroy, R. & Allerhand, M. (1993) Contemp. Mus. Rev. 9, 69-81.
Warren, J.D., Zielinski, B.A., Green, G.G.R., Rauschecker, J.P. & Griffiths, T.D. (2002) Neuron 34, 139-148.

 

Paper Abstract

Musicians recognise pitch as having two dimensions. On the keyboard, these are illustrated by the octave and the cycle of notes within the octave. In perception, these dimensions are referred to as pitch height and pitch chroma respectively. Pitch chroma provides a basis for presenting acoustic patterns (melodies) that do not depend on the particular sound source. In contrast, pitch height provides a basis for segregation of notes into streams to separate sound sources. This paper reports a functional magnetic resonance experiment designed to search for distinct mappings of these two types of pitch change in the human brain. The results show that chroma change is specifically represented anterior to primary auditory cortex, while height change is specifically represented posterior to primary auditory cortex. We propose that tracking of acoustic information streams occurs in anterior auditory areas, while the segregation of sound objects (a crucial aspect of auditory scene analysis) depends on posterior areas.

 

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