Sound localization is a fundamental task of the auditory system, and in higher mammals, requires an intact auditory cortex. Many cortical neurons respond over broad regions of space and vary the rate and temporal pattern of their spiking activity with changes in the location of a sound source. The ultimate objective of the proposed studies is to understand the nature and origin of this representation of auditory space in the mammalian auditory cortex. The proposed experiments will address, using an anesthetized cat preparation, how these representations are derived from units' sensitivities to various individual spatial cues and how these sensitivities vary among cortical areas.
Specific Aim 1 will evaluate the individual contributions of interaural time difference (ITD), interaural level-difference (ILD), and monaural spectral cues to the azimuthal sensitivity of neurons in different fields of the auditory cortex. We will test the hypothesis that ILD, and not ITD or monaural spectral shape, is the dominant contributor to azimuth sensitivity in cat auditory cortex. Experiments will estimate the relative importance of various potential azimuthal location cues using free-field stimulation and virtual auditory space (VAS) stimulation, which permits independent control of the sound at each ear. VAS stimuli will use each animal's individual head-related transfer functions, and will be delivered with a transaural system that will allow direct comparison of free-field and VAS stimulation of the same neurons.
Specific Aim 2 will evaluate whether elevation-sensitive neurons in the auditory cortex encode space or merely spectrum. Spatial cues in the vertical dimension are provided by the location-dependent filtering action of the head and pinna. Human listeners can accurately localize noises with a variety of non-flat, wideband spectral profiles. Elevation-sensitive units in auditory cortex have been found, but because their sensitivity was measured using only flat-spectrum source spectra, it is unknown whether such units maintain spatial constancy over changes in source spectra, or whether they merely encode the spectrum of the stimulus at the eardrum. Experiments will evaluate the degree to which neurons' location-related responses are conserved when the source spectrum is varied among a set of wideband, but non-flat, spectra. This research will provide basic understanding of auditory cortical mechanisms needed for evaluation of temporal lobe pathology and for the design of therapeutic responses to injury and disease. ? ?
| Harrington, Ian A; Stecker, G Christopher; Macpherson, Ewan A et al. (2008) Spatial sensitivity of neurons in the anterior, posterior, and primary fields of cat auditory cortex. Hear Res 240:22-41 |
| Macpherson, Ewan A; Sabin, Andrew T (2007) Binaural weighting of monaural spectral cues for sound localization. J Acoust Soc Am 121:3677-88 |
| Stecker, G Christopher; Harrington, Ian A; Macpherson, Ewan A et al. (2005) Spatial sensitivity in the dorsal zone (area DZ) of cat auditory cortex. J Neurophysiol 94:1267-80 |
