Intensity is an important physical property of a sound wave and is customarily reported as sound pressure level (SPL). Invasive techniques such as electrical recordings, which typically examine one brain region at a time, have been used to study neuronal encoding of SPL throughout the central auditory system. Non-invasive functional magnetic resonance imaging (fMRI) with large field of view can simultaneously examine multiple auditory structures. We applied fMRI to measure the hemodynamic responses in the rat brain during sound stimulation at seven SPLs over a 72 dB range. This study used a sparse temporal sampling paradigm to reduce the adverse effects of scanner noise. Hemodynamic responses were measured from the central nucleus of the inferior colliculus (CIC), external cortex of the inferior colliculus (ECIC), lateral lemniscus (LL), medial geniculate body (MGB), and auditory cortex (AC). BOLD signal changes generally increase significantly (p<0.001) with SPL and the dependence is monotonic in CIC, ECIC, and LL. The ECIC has higher BOLD signal change than CIC and LL at high SPLs. The difference between BOLD signal changes at high and low SPLs is less in the MGB and AC. This suggests that the SPL dependences of the LL and IC are different from those in the MGB and AC and the SPL dependence of the CIC is different from that of the ECIC. These observations are likely related to earlier observations that neurons with firing rates that increase monotonically with SPL are dominant in the CIC, ECIC, and LL while non-monotonic neurons are dominant in the MGB and AC. Further, the IC's SPL dependence measured in this study is very similar to that measured in our earlier study using the continuous imaging method. Therefore, sparse temporal sampling may not be a prerequisite in auditory fMRI studies of the IC.

In rats, the superior colliculus (SC) is a main destination for retinal ganglion cells and is an important subcortical structure for vision. Electrophysiology studies have observed that many SC neurons are highly sensitive to moving objects, but complementary non-invasive functional imaging studies with larger fields of view have been rarely conducted. In this study, BOLD fMRI is used to measure the SC and nearby lateral geniculate nucleus' (LGN) hemodynamic responses, in normal adult Sprague Dawley (SD) rats, during a dynamic visual stimulus similar to those used in long-range apparent motion studies. The stimulation paradigm consists of four light spots arranged in a linear array and turned on and off sequentially at different rates to create five effective speeds of motion (7, 14, 41, 82, and 164°/s across the visual field). Stationary periods (same light spot always on) are interleaved between the moving periods. The speed response function (SRF), the hemodynamic response amplitude at each speed tested, is measured. Significant responses are observed in the SC and LGN at all speeds. In the SC, the SRF increases monotonically from 7 to 82°/s. The minimum response amplitude occurs at 164°/s. The results suggest that the SC is sensitive to slow moving visual stimuli but the hemodynamic response is reduced at higher speeds. In the LGN, the SRF exhibits a similar trend to that of the SC, but response amplitude during 7°/s stimulation is comparable to that during 164°/s stimulation. These findings are in good agreement with previous electrophysiology studies conducted on albino rats like the SD strain. This work represents the first fMRI study of stimulus speed dependence in the SC and is also the first fMRI study of motion responsiveness in the rat.

Introduction In the mammalian visual system, light received by the eyes is transmitted through the optic nerves. Some of the nerve fibers go to the superior colliculus (SC), which mediates visual reflexes. The majority of other fibers go to the lateral geniculate nucleus (LGN) en route to the primary visual cortex (V1), which is responsible for visual perception[1]. The different parts of the visual system (SC, LGN, V1) interact to permit processing of visual information and responses to stimuli. Interaction between regions can be studied by measuring temporal differences of neural activations in different parts of the visual system. For example, Molotchnikoff et al. used electrical recordings to observe that LGN cells had reduced activation if SC cells were active in the preceding 250ms. This provided evidence that the SC interacts with the LGN to generate saccadic eye movements[2]. The majority of time-resolved studies to date use invasive and local measurements such as electrical recordings. Blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI), which measures local changes in blood oxygenation induced by neuronal activation following a stimulus, is a non-invasive technique that can simultaneously examine a large FOV with high spatial resolution[3]. Recent advancements have shown that BOLD can be used to measure neuronal activity with sub-second temporal precision[4-5]. In this study, we apply BOLD fMRI with repetition time (TR) of 0.2s on rats to measure the difference in response temporal dynamics between the SC and the LGN to monocular visual stimuli, which has not been measured conclusively nor with high temporal resolution[6-7]. Methods Animal preparation: Sprague-Dawley rats (N = 7) between two and three months of age were used in this study. Each animal was anesthetized with 3% isofluorane for induction and 1 – 1.5% for maintenance. Once sedated, animals were placed in a 7T MRI scanner (Bruker PharmaScan) with a brain surface receiver coil (Bruker BioSpin). Respiration rate was monitored with a pressure sensor (SA Instruments) and heart rate and blood oxygen level were monitored with a pulse oximeter (SA Instruments). MRI protocol: Scout images were acquired to determine the sagittal plane. Three 0.8mm thick slices (spaced 0.1mm apart) were positioned such that the middle slice covered both the SC and the LGN. An anatomical scan (3.2cm x 3.2cm, 256 x 256 voxels, TR = 4.2s, TE = 12.9ms) was acquired on the 3 slices. For BOLD experiments, animals were stimulated with a previously employed paradigm of four 40s rest periods with three 20s stimulation periods in between[8]. Stimulation was provided by a 1mm diameter, 0.22 numerical aperture optical fiber placed 1cm from the left eye. The fiber was illuminated by a 40 lux green LED flashed at 1 Hz with a duty cycle of 0.005. Throughout the 220s experiment, 1100 gradient-echo EPI scans (3.2cm x 3.2cm, 64 x 64 voxels, TR = 0.2s, TE = 18ms, α = 30°) were acquired. The experiment was repeated 5 times for each animal with 10 minute rest periods in between. Data analysis: The 1100 images from each experiment were registered using AIR5.2.5[9]. The time series from each pixel was cross correlated with the stimulus paradigm using Simulate6[10] and pixels with correlation coefficient (cc) greater than 0.15 were considered activated by the stimulus. ROIs were drawn over the contralateral SC and the LGN using the anatomical image and average time series were computed from activated pixels in each ROI. Each average time series was cross correlated with the stimulus paradigm and the time shift (TS) from onset of the first stimulus needed to obtain the maximum cc was recorded. The average value of the maximum amplitude (MA) during each of the three stimulation periods was also computed from each average time series. TS and MA from all experiments on an animal are averaged to obtain one pair of TS and MA for the SC and another for the LGN. Statistically significant differences (p < 0.05) between SC and LGN were identified with a signed-rank test. Results Figure 1 shows brain regions in a representative rat activated by the stimulus. Notice the activated pixels in the contralateral SC and LGN. Figure 2 shows time series from the SC and the LGN along with the stimulus paradigm. The boxplots in figure 3 show the distributions of TS observed in the SC and LGN. The SC response is shifted less than that of the LGN (p = 0.016) by 0.8 ± 0.3s. Figure 4 shows the distributions of MA. The SC response has larger amplitude than that of the LGN (p = 0.016) by 0.4 ± 0.1 % BOLD. Discussion and Conclusions The primary finding of this study is that there is an approximately 0.8s difference between the BOLD responses of the rat contralateral SC and LGN to the visual stimuli. In addition, the amplitude of the SC response is larger than that of the LGN. These non-invasive measurements were simultaneously conducted over a large FOV and with high spatial and temporal resolution. In comparison, Pawela et al. observed SC amplitude was larger than that of the LGN, but did not report any differences in temporal dynamics[6]. Wall et al. observed in humans that SC reached peak amplitude 1 – 2s before the LGN, but the latter had larger amplitude[7]. However, the authors used significantly shorter stimulus duration and interstimulus time, possibly coupling the BOLD responses from multiple stimuli. One explanation of our results is that SC neurons respond faster and more frequently to the stimulus compared with LGN neurons. A second possibility involves the fact active SC neurons are known to reduce LGN activation in the proceeding 0.25s[2]. This would cause LGN neurons to be suppressed shortly after the onset of stimulation, leading to its reduced and delayed BOLD response. This effect would be less apparent using shorter stimuli. However, it is important to note that this study used spontaneous SC neuronal firing as the stimulus, which is different from our study where neuronal firing is caused by an external source. Alternatively, there are regional differences in physiological properties (blood volume, blood flow, etc.), which link neuronal activity to the BOLD response[11], between the SC and LGN that are not due to neuronal activity. For example, the SC may have slightly higher blood flow at rest compared to the LGN[12]. There may also be regional differences in vessel dilation rate, which have been observed in the somatosensory cortex and can affect BOLD responses[13]. To isolate the origin of the time shift difference, multimodality studies should be conducted with functional imaging and electrical recording techniques to help separate the hemodynamic and neuronal contributions to the BOLD signals.

— This paper demonstrated our recent use of contrast-enhanced MRI, diffusion tensor/kurtosis imaging, proton magnetic resonance spectroscopy, and functional MRI techniques, for in vivo and global assessments of the structure, metabolism and function of the visual system in rodent studies of ocular diseases, optic neuropathies, developmental plasticity and neonatal hypoxic-ischemic brain injury at 7T. Results suggested the significant values of high-field multiparametric MRI for uncovering the processes and mechanisms of developmental and pathophysiological changes systematically along both anterior and posterior visual pathways, and may provide early diagnoses and therapeutic strategies for promoting functional recovery upon partial vision loss.

The superior colliculus (SC) is a laminated subcortical structure in the mammalian midbrain, whose superficial layers receive visual information from the retina and the visual cortex. To date, its functional organization and development in the visual system remain largely unknown. This study employed blood oxygenation level-dependent (BOLD) functional MRI to evaluate the visual responses of the SC in normally developing and severe neonatal hypoxic–ischemic (HI)-injured rat brains from the time of eyelid opening to adulthood. MRI was performed to the normal animals (n=7) at postnatal days (P) 14, 21, 28 and 60. In the HI-injured group (n=7), the ipsilesional primary and secondary visual cortices were completely damaged after unilateral ligation of the left common carotid artery at P7 followed by hypoxia for 2 h, and MRI was performed at P60. Upon unilateral flash illumination, the normal contralateral SC underwent a systematic increase in BOLD signal amplitude with age especially after the third postnatal week. However, no significant difference in BOLD signal increase was found between P14 and P21. These findings implied the presence of neurovascular coupling at the time of eyelid opening, and the progressive development of hemodynamic regulation in the subcortical visual system. In the HI-injured group at P60, the BOLD signal increases in both SC remained at the same level as the normal group at P28 though they were significantly lower than the normal group at P60. These observations suggested the residual visual functions on both sides of the subcortical brain, despite the damages to the entire ipsilesional visual cortex. The results of this study constitute important evidence on the progressive maturation of visual functions and hemodynamic responses in the normal subcortical brain, and its functional plasticity upon neonatal HI injury.