Subcortical Auditory Information Processing after Bilateral Auditory Cortex Ablation Patrick P. Gao, Shu-Juan Fan, Jevin W. Zhang, Iris Y. Zhou, Joe S. Cheng, Yuqi Deng, Dan H. Sanes, and Ed X. Wu Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Hong Kong, HKSAR, China, Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, HKSAR, China, Department of Biology, New York University, New York, NY, United States

Electronic Poster Session: Functional MRI (neuro) - Resting State Connectivity: Applications

INTRODUCTION Resting-state fMRI (rsfMRI) has been increasingly used in understanding brain functional connectivity under normal and pathological conditions. Correlated fluctuations in rsfMRI signal are mostly confined to frequency below 0.1 Hz, so spectral filtering is routinely performed to retain low frequencies before further analysis. Moreover, changes in the connectivity strength or spatial map of these low-frequency fluctuations are the common focus when comparing the normal and diseased brains. However, limited work has studied the underlying spectral alterations. In this study, we investigated the spectral changes of resting-state connectivity in a rat model of complete corpus callosum (CC) transection. METHODS Animal Preparation: Adult Sprague-Dawley rats (230~270g) were subjected to a complete transection of CC (N=6) or sham surgery (N=6). After 7 days of recovery, animals were MRI scanned under mechanical ventilation with isoflurane anesthesia (1-1.5%). MRI Protocols: All MRI experiments were conducted using a 7 T Bruker scanner with a surface coil. Four to six rsfMRI acquisitions were performed using a single-shot GE-EPI sequence with TR/TE=1000/18ms, FOV=32×32mm, 64×64 matrix, nine 1-mmthick slices and a total of 400 data points. Data Analysis: All rsfMRI data was slice-timing corrected, co-registered, detrended and temporally band-pass filtered (0.005Hz2. The time courses of independent components covering CPu, S1 and VC were extracted to calculate power spectra. The time courses without prior band-pass filtering were used for timefrequency analysis using short-time Fourier transform in Matlab. RESULTS AND DISCUSSION CC connects most areas of the cerebral cortex to contralateral homologous areas that share similar functions. In this study, the primary callosal connections of S1 and VC were severed by complete CC transection. Fig. 1 shows that interhemispheric connectivity seen in S1 and VC of sham group was prominently absent in complete CC transection group while the intrahemispheric connectivity was preserved. The interhemispheric connectivity of subcortical network in CPu was not affected and served as an internal control. Fig. 2 shows the power spectral comparison between the two groups. S1 and VC of transection group showed stronger power at relative high frequency (arrows) below 0.1 Hz with respect to that of sham group. Similar results were found using time-frequency analysis of the unfiltered time courses (Fig. 3). Higher power of frequencies around 0.1 Hz was observed in S1 and VC of transection group. These spectral changes were observed in brain regions showing predominately intrahemispheric connectivity and therefore may arise from the faster intrahemispheric communication. In conclusion, these experimental findings indicate that spectral characteristics of rsfMRI connectivity can be modulated by neural disruption and spectral analysis of rsfMRI data may provide a new dimension of information regarding the brain organization and connectivity. REFERENCES [1] Fox MD, et al. Nat Rev Neurosci 2007;8:700-11. [2] Cordes D, et al. AJNR Am J Neuroradiol 2001;22:1326-33. [3] Kaas JH, Epilepsy and the Corpus Callosum 2. eds., pp. 15-27, 1995. [4] Gazzaniga MS Brain 2000;123 ( Pt 7):1293-326. Fig. 2 Spectral comparison between complete CC transection group and sham group. Power spectra were computed from the time courses of ICA components in CPu, S1 and VC. Power spectral density (PSD) was presented in mean ± SD. S1 and VC of transection group showed stronger power at relative high frequency (arrows) with respect to that of sham group.

INTRODUCTION: The retinocollicular and retinogeniculate projections in rodents are a well-established model system for evaluating the mechanisms of retinotopic map formation, neurodegeneration and plasticity during early postnatal visual development (1). However, to date, no tools have yet been available for in vivo, highresolution investigations of retinotopic projections globally along the neonatal visual pathways. Mn has been increasingly used as a T1-weighted contrast agent for in vivo neuronal tract tracing (2) and functional brain mapping at lamina levels in the adult brains (3). In this study, we explore the capability of high-resolution Mnenhanced MRI (MEMRI) for in vivo assessment of retinal projections in the early postnatal rat brains before natural eyelid opening, and compare the Mn enhancements between neonatal and adult visual brain nuclei after intravitreal Mn injection into one eye.

K. C. Chan, I. Y. Zhou, S. J. Fan, J. S. Cheng, and E. X. Wu Laboratory of Biomedical Imaging and Signal Processing, The University of Hong Kong, Pokfulam, Hong Kong, China, People's Republic of, Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam, Hong Kong, China, People's Republic of, Department of Electrical and Electronic Engineering, The Univrsity of Hong Kong, Pokfulam, Hong Kong, China, People's Republic of