Superposition of horseshoe‐like periodicity and linear tonotopic maps in auditory cortex of the Mongolian gerbil

Abstract

The segregation of an individual sound from a mixture of concurrent sounds, the so-called cocktail-party phenomenon, is a fundamental and largely unexplained capability of the auditory system. Speaker recognition involves grouping of the various spectral (frequency) components of an individual's voice and segregating them from other competing voices. The important parameter for grouping may be the periodicity of sound waves because the spectral components of a given voice have one periodicity, viz. fundamental frequency, as their common denominator. To determine the relationship between the representations of spectral content and periodicity in the primary auditory cortex (AI), we used optical recording of intrinsic signals and electrophysiological mapping in Mongolian gerbils (Meriones unguiculatus). We found that periodicity maps as an almost circular gradient superimposed on the linear tonotopic gradient in the low frequency part of AI. This geometry of the periodicity map may imply competitive signal processing in support of the theory of "winner-takes-all".

Neural Sensitivity to Periodicity in the Inferior Colliculus: Evidence for the Role of Cochlear Distortions

“…This study also examines claims made in recent studies suggesting a central neural mechanism of temporal pitch extraction (Biebel and Langner 2002;Langner 1997, 1999;Schulze et al 2002). The data show that low-CF neurons respond to high-frequency AM tones in which all spectral components lie outside the pure-tone response area, confirming the basic observation of Biebel and Langner (2002), and apparently consistent with the conclusion of both studies that periodicity pitch is mapped in the CNS.…”

“…However, low-frequency sounds can be used to assess the presence of cochleargenerated distortions, as was performed in this study. None of the cited studies (Biebel and Langner 2002;Langner 1997, 1999;Schulze et al 2002) purporting to examine the existence of periodotopic representations in the brain examined this possibility. It was found that all of the data that indicate sensitivity of low-CF neurons to high-frequency periodic stimuli can readily be explained by the generation of CTs on the basilar membrane at a frequency corresponding to the AM rate.…”

“…These neurons act as modulation extractors, responding to high-frequency AM tones outside their traditional frequency-versus-level response area, with preferred BMFs matching their CFs (Biebel and Langner 2002). Evidence from primary auditory cortex Langner 1997, 1999;Schulze et al 2002) appears to confirm the existence of periodicity maps in response to high-frequency AM tones including one overlying the low-frequency region of primary auditory cortex. Note that this proposed mechanism can account only for the pitch of unresolved harmonics, since resolved harmonics will not evoke temporally-modulated responses within a single neural filter.…”

Fine functional organization of auditory cortex revealed by Fourier optical imaging

“…The major components of the hemodynamics are cyclic: heart beat, respiration, and vasomotor signal (29). Conventional methods of intrinsic optical imaging have been used for functional mapping of the auditory cortex (30)(31)(32)(33)). Here we demonstrate that a recently developed approach may allow generating functional maps of better quality by effectively removing cyclic artifacts from the continuously recorded signal.…”

“…Periodic stimulation offers the advantage of effective separation of the stimulus-evoked responses from intrinsic noise by using Fourier analysis of the continuous data stream. This technique allows reconstruction of functional maps of much higher spatial resolution and lower noise than those obtained by conventional methods of optical imaging (30)(31)(32)(33).…”