Abstract
Harmonic sounds, such as voiced speech sounds and many animal communication signals, are characterized by a pitch related to the periodicity of their envelopes. While frequency information is extracted by mechanical filtering of the cochlea, periodicity information is analyzed by temporal filter mechanisms in the brainstem. In the mammalian auditory midbrain envelope periodicity is represented in maps orthogonal to the representation of sound frequency. However, how periodicity is represented across the cortical surface of primary auditory cortex (AI) remains controversial. Using optical recording of intrinsic signals, we here demonstrate that a periodicity map exists in primary AI of the cat. While pure tone stimulation confirmed the well-known frequency gradient along the rostro-caudal axis of AI, stimulation with harmonic sounds revealed segregated bands of activation, indicating spatially localized preferences to specific periodicities along a dorso-ventral axis, nearly orthogonal to the tonotopic gradient. Analysis of the response locations revealed an average gradient of − 100° ± 10° for the periodotopic, and −12° ± 18° for the tonotopic map resulting in a mean angle difference of 88°. The gradients were 0.65 ± 0.08 mm/octave for periodotopy and 1.07 ± 0.16 mm/octave for tonotopy indicating that more cortical territory is devoted to the representation of an octave along the tonotopic than along the periodotopic gradient. Our results suggest that the fundamental importance of pitch, as evident in human perception, is also reflected in the layout of cortical maps and that the orthogonal spatial organization of frequency and periodicity might be a more general cortical organization principle.
Highlights
Acoustic signals in speech, music, and animal communication are characterized by a pitch
1986; Vanzetta and Grinvald, 1999, 2008; Logothetis et al, 2001). It has been previously demonstrated by microelectrode mapping procedures that there exists a close match between the spatial distributions of reflectance changes of intrinsic optical signals with a map of local field potentials measured in the same animal (Godde et al, 1995)
Together these data indicate that the broad spatial activations typically seen in optical imaging data do not reflect methodological constraints but rather the pattern of subthreshold and spiking activation (Narayan et al, 1994), and a true property of cortical representations
Summary
Acoustic signals in speech, music, and animal communication are characterized by a pitch Sound sources such as strings and tubes, as well as vocal chords, vibrate periodically and emit so-called harmonic sounds, which are characterized by periodic envelope modulations. Spike trains in the auditory nerve show phase coupling to these beats and transfer temporal information about the fundamental frequency (Palmer, 1982), even when the fundamental frequency component is ‘missing’. Irrespective of their spectral content, all harmonic sounds with the same envelope periodicity have the same pitch. A more or less salient periodicity pitch is obtained only in the spectral range below 5 kHz, where auditory nerve fibers faithfully transmit temporal information and the pitch saliency improves when the neuronal analysis has access to at least partly resolved components of a harmonic sound (Langner, 1992; Pressnitzer et al 2001)
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