Abstract
Periodicities in sound waveforms are widespread, and shape important perceptual attributes of sound including rhythm and pitch. Previous studies have indicated that, in the inferior colliculus (IC), a key processing stage in the auditory midbrain, neurons tuned to different periodicities might be arranged along a periodotopic axis which runs approximately orthogonal to the tonotopic axis. Here we map out the topography of frequency and periodicity tuning in the IC of gerbils in unprecedented detail, using pure tones and different periodic sounds, including click trains, sinusoidally amplitude modulated (SAM) noise and iterated rippled noise. We found that while the tonotopic map exhibited a clear and highly reproducible gradient across all animals, periodotopic maps varied greatly across different types of periodic sound and from animal to animal. Furthermore, periodotopic gradients typically explained only about 10% of the variance in modulation tuning between recording sites. However, there was a strong local clustering of periodicity tuning at a spatial scale of ca. 0.5 mm, which also differed from animal to animal.
Highlights
Some arrays had a small number of faulty channels, and some channels along the edges of the arrays came to lie outside the bounds of the central nucleus of the inferior colliculus (IC) (Cant and Benson, 2005)
To reveal the tonotopic organization of the ICc across all three spatial dimensions we show in Figure 4A the best frequency (BF) data, FIGURE 3 | (A) Schematic diagram of the electrode array used in this study. (B) Example of a typical multiunit frequency response area (FRA) recorded in ICc
Note that our data do not suggest that ‘‘best fit’’ periodotopic gradients obtained from the pooled data would be orthogonal to the tonotopic gradient, and be aligned with iso-frequency laminae: using the regression weights shown in Table 1 to calculate the angles between the tonotopic and each of the best fit periodotopic axes, we find these to come out as 43.7◦ for the click train stimuli, 51.2◦ for sinusoidally amplitude modulated noises (SAMNs), and 45.9◦ for iterated rippled noises (IRNs)
Summary
The qualities of rhythm and pitch both derive from processing periodicity in acoustic waveforms. The periodicity of a sound may be evident in the time domain as repetitions of a waveform motif, or in the frequency domain as spectral peaks at harmonics of a fundamental frequency F0, where F0 is the inverse of the sound’s period. The spectral cues are thought to be encoded by a place code, carried by the pattern of activity across the tonotopic nerve fibers in the lemniscal auditory pathway, while time domain cues manifest as periodic amplitude modulations (AM) of the stimulus envelope, which are encoded when neurons in the auditory periphery ‘‘phase-lock’’ their discharge patterns to the stimulus modulations. Stages of the mammalian auditory system are thought to use both temporal and spectral cues to varying extent, with increasingly higher stages of the auditory pathway relying less on temporal
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