Abstract
Despite decades of microelectrode recordings, fundamental questions remain about how auditory cortex represents sound-source location. Here, we used in vivo 2-photon calcium imaging to measure the sensitivity of layer II/III neurons in mouse primary auditory cortex (A1) to interaural level differences (ILDs), the principal spatial cue in this species. Although most ILD-sensitive neurons preferred ILDs favoring the contralateral ear, neurons with either midline or ipsilateral preferences were also present. An opponent-channel decoder accurately classified ILDs using the difference in responses between populations of neurons that preferred contralateral-ear-greater and ipsilateral-ear-greater stimuli. We also examined the spatial organization of binaural tuning properties across the imaged neurons with unprecedented resolution. Neurons driven exclusively by contralateral ear stimuli or by binaural stimulation occasionally formed local clusters, but their binaural categories and ILD preferences were not spatially organized on a more global scale. In contrast, the sound frequency preferences of most neurons within local cortical regions fell within a restricted frequency range, and a tonotopic gradient was observed across the cortical surface of individual mice. These results indicate that the representation of ILDs in mouse A1 is comparable to that of most other mammalian species, and appears to lack systematic or consistent spatial order.
Highlights
The difference in sound level between the 2 ears provides an important sound localization cue for most mammals, and is the principal basis by which animals with small heads and high-frequency hearing, such as rats and mice, localize sounds in the horizontal plane (Chen et al 1995; Wesolek et al 2010; Lauer et al 2011)
The same broadband noise bursts were presented monaurally to each ear. We presented these stimuli at an average binaural level (ABL) of 60 and 80 dB sound pressure level (SPL) for 6 mice, and 40, 60, and 80 dB SPL for 12 mice
To investigate the micro-organization of spatial tuning in layers II/III of mouse auditory cortex, we performed in vivo 2-photon calcium imaging in 15 animals in which the calcium indicator GCaMP6m had been expressed by injected AAV
Summary
The difference in sound level between the 2 ears provides an important sound localization cue for most mammals, and is the principal basis by which animals with small heads and high-frequency hearing, such as rats and mice, localize sounds in the horizontal plane (Chen et al 1995; Wesolek et al 2010; Lauer et al 2011). Extracellular recordings have been used to explore the sensitivity of neurons to interaural level differences (ILDs) in the primary auditory cortex (A1) of a variety of species. Studies of spatial and ILD sensitivity in a range of species have shown that the majority of A1 neurons prefer contralateral stimuli, though many are omnidirectional (i.e., lack spatial selectivity) and others respond best to ipsilateral or midline locations (e.g., Middlebrooks and Pettigrew 1981; Rajan et al 1990; Irvine et al 1996; Mrsic-Flogel et al 2005; Campbell et al 2006; Razak 2011; Zhou and Wang 2012). It has been reported that neurons in rat A1 exhibit exclusively contralateral hemifield spatial preferences (Higgins et al 2010; Yao et al 2013)
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