Elsevier

NeuroImage

Volume 164, 1 January 2018, Pages 121-130
NeuroImage

Investigating the spatiotemporal characteristics of the deoxyhemoglobin-related and deoxyhemoglobin-unrelated functional hemodynamic response across cortical layers in awake marmosets

https://doi.org/10.1016/j.neuroimage.2017.03.005Get rights and content

Highlights

  • Deoxyhemoglobin-related and -unrelated fMRI responses examined in awake marmosets.

  • BOLD, T2 * and S0 images were obtained in 5 laminar regions of somatosensory cortex.

  • The middle layers had the highest BOLD and T2* amplitudes and shortest onset times.

  • Functional S0 changes were detected only in superficial layers with a fast onset time.

  • Basis of S0 changes is likely changes in CBV and unmodeled T2 * venous contributions.

Abstract

Blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) has become a major tool to map neural activity. However, the spatiotemporal characteristics of the BOLD functional hemodynamic response across the cortical layers remain poorly understood. While human fMRI studies suffer from low spatiotemporal resolution, the use of anesthesia in animal models introduces confounding factors. Additionally, inflow contributions to the fMRI signal become non-negligible when short repetition times (TRs) are used. In the present work, we mapped the BOLD fMRI response to somatosensory stimulation in awake marmosets. To address the above technical concerns, we used a dual-echo gradient-recalled echo planar imaging (GR-EPI) sequence to separate the deoxyhemoglobin-related response (absolute T2* differences) from the deoxyhemoglobin-unrelated response (relative S0 changes). We employed a spatial saturation pulse to saturate incoming arterial spins and reduce inflow effects. Functional GR-EPI images were obtained from a single coronal slice with two different echo times (13.5 and 40.5 ms) and TR=0.2 s. BOLD, T2*, and S0 images were calculated and their functional responses were detected in both hemispheres of primary somatosensory cortex, from which five laminar regions (L1+2, L3, L4, L5, and L6) were derived. The spatiotemporal distribution of the BOLD response across the cortical layers was heterogeneous, with the middle layers having the highest BOLD amplitudes and shortest onset times. ΔT2* also showed a similar trend. However, functional S0 changes were detected only in L1+2, with a fast onset time. Because inflow effects were minimized, the source of S0 functional changes in L1+2 could be attributed to a reduction of cerebrospinal fluid volume fraction due to the functional increase in cerebral blood volume and to unmodeled T2* changes in the extra- and intra-venous compartments. Caution should be exercised when interpreting laminar BOLD fMRI changes in superficial layers as surrogates of underlying neural activity.

Introduction

The blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) has proven to be a valuable tool to non-invasively probe various brain areas at a macroscopic scale (Fukuda et al., 2016, Goense et al., 2016). With continued improvement in both spatial and temporal resolution of MRI, BOLD fMRI has recently been applied to study the temporal dynamics of the laminar hemodynamic response and its underlying neural activity in both animal models and humans (Hirano et al., 2011, Jin and Kim, 2008, Siero et al., 2011, Silva and Koretsky, 2002). The distinct function of the six cortical layers is the foundation of interlaminar connections, which form a larger and more complex intracortical network at macroscopic scale (Thomson and Bannister, 2003). However, an in-depth understanding of the spatiotemporal characteristics of the laminar BOLD response is essential to interpret the laminar BOLD response as a function of its underlying neural activity, to optimize the BOLD fMRI protocol, and to realize the spatiotemporal limitations of the BOLD fMRI technique.

Several studies have been conducted that characterize the temporal aspects of laminar BOLD contrast in various brain areas and species. Faster laminar BOLD onset times were reported in cortical layer 4 or 5, followed by layer 6, and then layers 1–3 of somatosensory cortex in anesthetized rats (Silva and Koretsky, 2002, Tian et al., 2010, Yu et al., 2014). Cerebral blood volume or flow is reported to have onset time monotonically increasing by cortical depth in anesthetized rats or cats (Hirano et al., 2011, Jin and Kim, 2008, Norup Nielsen and Lauritzen, 2001). However, the aforementioned animal studies suffered from confounds of general anesthesia, which is known to alter hemodynamic regulation and neuronal response (Liu et al., 2013, Masamoto and Kanno, 2012). On the other hand, BOLD fMRI in humans shows promising results concerning faster BOLD onset times in layers 5–6 of the visual cortex (Siero et al., 2013). Nevertheless, insufficient spatiotemporal resolution causes laminar onset times to be contaminated by partial volume of white matter or pial vessels/cerebrospinal fluid (CSF). Consequently, conscious, awake marmosets were adopted in the current study as a nonhuman primate model to bridge the gap between high-resolution fMRI in small animals and low-resolution fMRI in humans (Hung et al., 2015c, Silva et al., 2011). Marmosets are phylogenetically closer to humans, compared to other small animals (Marmoset Genome and Analysis, 2014), and have one of the highest cortical thickness to brain mass ratios among primates (Sun and Hevner, 2014).

Another issue that commonly appears in high temporal resolution BOLD fMRI experiments is the large inflow contribution in BOLD contrast (Duyn et al., 1994). Inflow might account for more than half of the BOLD contrast when there is not enough time for the MR signal to recover (Glover et al., 1996). To separate the inflow contribution from the real BOLD contrast, a dual gradient-recalled echo planar imaging (GR-EPI) sequence was utilized to isolate spin-spin relaxation time (T2*) change from non-T2* change, which contained the inflow effect manifested as change in spin-lattice relaxation time (T1) (Speck and Hennig, 1998). The difference in T2* is presumably proportional to change in the amount of deoxyhemoglobin in the blood and directly related to BOLD contrast (Kim and Ogawa, 2012). In addition, inflow contribution was suppressed by saturating the incoming blood below the circle of Willis in marmosets. Therefore, any remaining non-T2* signal would be either from outside of the vasculature and/or exceptions in modeling T2* by dual echoes.

The main goal of the current study was to investigate the spatiotemporal characteristics of the laminar BOLD hemodynamic response function in the primary somatosensory (S1) cortex of awake marmosets. To accomplish our goal, the present study implemented several strategies to avoid shortcomings in prior studies, such as confounding effects of anesthesia, insufficient spatiotemporal resolution, and inflow contribution. Our results show early onset times of laminar BOLD and deoxyHb component in layers 3–4 of S1, whereas the non-deoxyHb component, which was insensitive to T2* changes, was significant only in layers 1 and 2. The non-deoxyHb component in layers 1 and 2 exhibited a significantly faster onset time when compared to the deoxyHb component. Since inflow contribution was suppressed by saturation of arterial spins below the circle of Willis, this could only be explained by the shrinkage of the cerebrospinal fluid (CSF) compartment due to expansion of the arterial blood compartment. However, CSF partial volume could not fully explain the peak amplitude of the deoxyHb component in layers 1 and 2. The gap could be filled by the contribution of the slower intravenous signal that was not properly modeled by the dual-echo fitting.

Section snippets

Methods

All procedures were approved by the Animal Care and Use Committee of the National Institute of Neurological Disorders and Stroke. Four male adult common marmosets (Callithrix jacchus) weighting 350–500 g and aged 4–5 years old were housed in pairs in dedicated cages and maintained on a diurnal 12-hour light cycle. Their diet consisted of ad libitum Zupreem canned marmoset food, Purina 5040 biscuits, unfiltered water, and P.R.A.N.G. rehydrator. In addition, the animals were fed daily with

Results

During fMRI experiments, marmosets were constantly monitored by an MR-compatible camera inside the magnet. The individualized 3D-printed helmet/chin pieces were very effective in providing comfortable yet effect head restraint, and for the vast majority of their time inside the magnet, the marmosets were resting with their eyes closed. Occasionally, they would open their eyes to check their surroundings or momentarily adjust their body position. Supplementary Fig. S1 shows an example session

Sources of the non-deoxyHb component in superficial layers

In the present work, we show that the BOLD fMRI response to somatosensory stimulation has distinct and heterogeneous spatiotemporal characteristics across the layers of somatosensory cortex in awake marmosets. The largest and fastest BOLD signal changes were found in L1+2–L4, whereas the smallest and slowest responses were found in L5–L6 of the somatosensory cortex. The heterogeneous laminar distribution of the BOLD fMRI response was largely associated with changes in the deoxyHb component (T2

Conclusions

A heterogeneous spatiotemporal distribution of relative BOLD signal changes in response to functional stimulation was observed across the layers of the primary somatosensory cortex in awake marmosets. The mid-upper cortical layers tended to have stronger relative BOLD signal changes and faster onset times than the lower ones. When the laminar BOLD signal changes were decomposed into their deoxyhemoglobin-related (absolute ΔT2*) and deoxyhemoglobin-unrelated (relative S0 changes) components, the

Acknowledgments

We thank Xianfeng (Lisa) Zhang, and Jennifer Lynn Ciuchta for their technical support in animal preparation. Also, many thanks go to the Scientific and Statistical Computing Core for their technical support in using AFNI to process the data. This research was supported by the Intramural Research Program of the NIH, NINDS (Alan P. Koretsky, Scientific Director).

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    Present address: Department of Physics, Universidade Federal do ABC, Santo Andre, Sao Paulo, 09210, Brazil.

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