Elsevier

NeuroImage

Volume 39, Issue 1, 1 January 2008, Pages 223-230
NeuroImage

Statistical mapping of sound-evoked activity in the mouse auditory midbrain using Mn-enhanced MRI

https://doi.org/10.1016/j.neuroimage.2007.08.029Get rights and content

Abstract

Manganese-enhanced MRI (MEMRI) has been developed to image brain activity in small animals, including normal and genetically modified mice. Here, we report the use of a MEMRI-based statistical parametric mapping method to analyze sound-evoked activity in the mouse auditory midbrain, the inferior colliculus (IC). Acoustic stimuli with defined frequency and amplitude components were shown to activate and enhance neuronal ensembles in the IC. These IC activity patterns were analyzed quantitatively using voxel-based statistical comparisons between groups of mice with or without sound stimulation. Repetitive 40-kHz pure tone stimulation significantly enhanced ventral IC regions, which was confirmed in the statistical maps showing active regions whose volumes increased in direct proportion to the amplitude of the sound stimuli (65 dB, 77 dB, and 89 dB peak sound pressure level). The peak values of the activity-dependent MEMRI signal enhancement also increased from 7% to 20% for the sound amplitudes employed. These results demonstrate that MEMRI statistical mapping can be used to analyze both the 3D spatial patterns and the magnitude of activity evoked by sound stimuli carrying different energy. This represents a significant advance in the development of MEMRI for quantitative and unbiased analysis of brain function in the deep brain nuclei of mice.

Introduction

Functional magnetic resonance imaging (fMRI), based on blood oxygen level dependent (BOLD) contrast, provides an effective and increasingly applied method for functional neuroimaging in humans and primates (Ogawa et al., 1990, Bandettini et al., 1992, Ogawa et al., 1993). With a major focus of neurobiology research on the genetic and molecular mechanisms underlying brain function, the mouse has become the preferred mammalian organism for many studies of brain development and disease, utilizing an array of genetic-engineering methods to generate numerous mouse models. A major challenge has been the implementation of fMRI methods with sufficient spatial resolution for analysis of the mouse brain. These challenges led us to investigate alternative MRI methods for assessing brain function and neural activity in mice.

Previously, we developed a 3D Mn-enhanced MRI (MEMRI) method for mapping sound-evoked activity in the mouse brain (Yu et al., 2005), and recently showed the utility of MEMRI for analyzing developmental plasticity in tonotopic maps of the mouse auditory midbrain (Yu et al., 2007). In this approach, neural activity was imaged retrospectively, after the accumulation of paramagnetic Mn ions within synaptically activated neurons of awake, normally-behaving mice. In contrast to low resolution BOLD-fMRI, MEMRI lends itself to study activity patterns in the brains of small animals at high resolution. In addition to our previous auditory studies (Yu et al., 2005, Yu et al., 2007), MEMRI has also been applied to study odor induced activity in the olfactory bulb (Pautler et al., 1998, Chuang et al., 2006), to examine light adaptation in retinal neurons (Berkowitz et al., 2006), to map cocaine-induced neuronal activation (Lu et al., 2007), and to detect hypothalamic activity (Morita et al., 2002, Chaudhri et al., 2006, Kuo et al., 2006) and ischemia-induced excitotoxicity (Aoki et al., 2003). Furthermore, MEMRI has been validated by correlation with BOLD and CBF (cerebral blood flow) functional maps in the somatosensory cortex of rats (Duong et al., 2000). Taken together, these results indicate that MEMRI brain mapping is a powerful new tool for analyzing activity patterns in the brains of small animals.

Cortical activity patterns have been studied previously in the mouse brain by optical imaging techniques, using voltage- and calcium-sensitive dyes and two-photon microscopy for increased penetration (Grinvald et al., 1986, Denk et al., 1995, Svoboda et al., 1997, Mrsic-Flogel et al., 2003). However, even with two-photon microscopy the depth of optical penetration is limited to several hundred microns, with the result that deep brain activity cannot be analyzed using optical imaging. We still lack knowledge of how sensory stimuli are represented in important subcortical brain nuclei, such as the inferior colliculus (IC), the auditory midbrain. Neuroimaging methods such as MEMRI could enable the study of activity induced in deep brain regions of awake mice.

In this study, we focused on the auditory midbrain of mice, examining the sensitivity and specificity of MEMRI to detect activity patterns after sound-stimulation with defined frequency and amplitude components. The basic circuitry encoding acoustic signals in the central auditory system has been studied extensively (Popper and Fay, 1992, Pollak et al., 2003). Neuronal recordings demonstrated that auditory neurons in the ascending auditory pathway respond to a limited range of frequencies, and are arranged tonoptically (Aitkin and Moore, 1975, Merzenich et al., 1975, Romand and Ehret, 1990). This tonotopic organization is most applicable for sound stimuli near threshold activation of auditory neurons (Phillips et al., 1994). However, it is not only the auditory neurons with threshold response that are activated during acoustic perception, but also neurons with different frequency sensitivity firing together to represent each perceived sound stimulus. Therefore, analysis of activity patterns resulting from supra-threshold acoustic stimulation can therefore provide critical new insights into auditory processing.

In this study, sound-evoked activity patterns were represented using statistical p-value maps derived from MEMRI images. The methods used to generate these maps were similar to the voxel-wise statistical methods currently used in BOLD fMRI. Gaussian filtering was implemented to minimize false positive (Type I) errors produced by multiple comparisons. The p-value threshold was chosen to restrict false positives while still preserving the biological meaningful results. We found that setting the p-value threshold at 0.05, in combination with Gaussian filtering, resulted in 40-kHz activity patterns located in the ventral IC, in excellent agreement with previous electrophysiological results (Romand and Ehret, 1990). The 3D p-maps showed that both the volume of the 40-kHz activity regions and the peak MEMRI signal enhancement increased in proportion to the stimulus amplitude. Our results clearly demonstrate that MEMRI statistical mapping provides a sensitive and high-resolution method for mapping activity in deep brain nuclei of mice.

Section snippets

Animals

All mice used in these studies were maintained under protocols approved by the Institutional Animal Care and Use Committee of New York University School of Medicine. The procedure to prepare the mice for MEMRI was described previously (Yu et al., 2005). Briefly, ICR mice were injected intraperitoneally (IP) with 0.4 mmol/kg body weight of MnCl2 in saline at postnatal day (P)19 (P0 denotes the day of birth), exposed to 24-h of defined sound stimulation or quiet, and then anesthetized with

Statistical mapping of 40-kHz activity patterns in the mouse IC

Previously, we demonstrated that 40 kHz pure-tone stimulation enhanced the ventral IC regions of MEMRI images (Yu et al., 2005), which was in excellent agreement with the tonotopic map established in mice by electrophysiology (Romand and Ehret, 1990). In this study, our goal was to develop an unbiased and quantitative statistical mapping method to represent 3D sound-evoked MEMRI activity patterns in the mouse IC. Student’s t-test was used for voxel-by-voxel comparisons of the IC signal

Discussion

Our previous studies demonstrated that MEMRI provides a sensitive method to detect accumulated sound-evoked activity in the mouse IC (Yu et al., 2005, Yu et al., 2007). In this study, we developed and applied a statistical parametric mapping method for quantitative analysis of MEMRI activity patterns. We first analyzed the spatial distribution of activity patterns induced by 40-kHz pure tones at different sound amplitudes. The resulting statistical maps showed activity localized to the ventral

Acknowledgments

This research was supported by grants from the National Institutes of Health (NS038461, DC006892). We thank Dr. Youssef Zaim Wadghiri for advice on the MRI imaging protocols used in these studies. We thank the National Academy of Sciences for permission to reprint some data from our publication: Yu et al. (2007).

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