Functional connectivity structure of cortical calcium dynamics in anesthetized and awake mice

Abstract

The interplay between hemodynamic-based markers of cortical activity (e.g. fMRI and optical intrinsic signal imaging), which are an indirect and relatively slow report of neural activity, and underlying synaptic electrical and metabolic activity through neurovascular coupling is a topic of ongoing research and debate. As application of resting state functional connectivity measures is extended further into topics such as brain development, aging and disease, the importance of understanding the fundamental physiological basis for functional connectivity will grow. Here we extend functional connectivity analysis from hemodynamic- to calcium-based imaging. Transgenic mice (n = 7) expressing a fluorescent calcium indicator (GCaMP6) driven by the Thy1 promoter in glutamatergic neurons were imaged transcranially in both anesthetized (using ketamine/xylazine) and awake states. Sequential LED illumination (λ = 454, 523, 595, 640nm) enabled concurrent imaging of both GCaMP6 fluorescence emission (corrected for hemoglobin absorption) and hemodynamics. Functional connectivity network maps were constructed for infraslow (0.009–0.08Hz), intermediate (0.08–0.4Hz), and high (0.4–4.0Hz) frequency bands. At infraslow and intermediate frequencies, commonly used in BOLD fMRI and fcOIS studies of functional connectivity and implicated in neurovascular coupling mechanisms, GCaMP6 and HbO2 functional connectivity structures were in high agreement, both qualitatively and also quantitatively through a measure of spatial similarity. The spontaneous dynamics of both contrasts had the highest correlation when the GCaMP6 signal was delayed with a ~0.6–1.5s temporal offset. Within the higher-frequency delta band, sensitive to slow wave sleep oscillations in non-REM sleep and anesthesia, we evaluate the speed with which the connectivity analysis stabilized and found that the functional connectivity maps captured putative network structure within time window lengths as short as 30 seconds. Homotopic GCaMP6 functional connectivity maps at 0.4–4.0Hz in the anesthetized states show a striking correlated and anti-correlated structure along the anterior to posterior axis. This structure is potentially explained in part by observed propagation of delta-band activity from frontal somatomotor regions to visuoparietal areas. During awake imaging, this spatio-temporal quality is altered, and a more complex and detailed functional connectivity structure is observed. The combined calcium/hemoglobin imaging technique described here will enable the dissociation of changes in ionic and hemodynamic functional structure and neurovascular coupling and provide a framework for subsequent studies of neurological disease such as stroke.

Subarachnoid hemorrhage leads to early and persistent functional connectivity and behavioral changes in mice

“…Future studies using genetically-encoded neuronal calcium fluorescence reporters (i.e. GCaMP mice) or invasive electrophysiology [36][37][38][39] It is worth commenting on our choice of a mouse model. There are several mouse models of subarachnoid hemorrhage [40,41]; however, a perfect SAH model does not exist and there is not yet a consensus about which model to use for each experimental condition [42].…”

Rehabilitation promotes the recovery of structural and functional features of healthy neuronal networks after stroke

“…Several studies recently addressed the contamination of the blood volume changes on fluorescence signal of calcium indicators like GCaMP (see, for instance, (Ma et al, 2016a;Ma et al, 2016b;Makino et al, 2017;Murphy et al, 2018;Wright et al, 2017)). The recent paper by Ma et al (Ma et al, 2016b) showed that the hemodynamic contribution is slower than the GCaMP6f signal, with an average temporal delay of 0.86 ± 0.05 s (representing the phase shift between neural activity and total hemoglobin concentration).…”

“…The recent paper by Ma et al (Ma et al, 2016b) showed that the hemodynamic contribution is slower than the GCaMP6f signal, with an average temporal delay of 0.86 ± 0.05 s (representing the phase shift between neural activity and total hemoglobin concentration). More recently, by simultaneously recording (wide-field) calcium dynamics and hemodynamics, Wright et al (Wright et al, 2017) showed that sensory evoked responses corresponding to oxygenated and deoxygenated hemoglobin have (i) a very slow onset after stimulus presentation, (ii) delayed peaks and (iii) low peak magnitudes (see also (Murphy et al, 2016)) compared to GCaMP6 fluorescence. These studies suggest that the hemodynamic fluctuations, much slower than calcium-associated neuronal activity, affect the late phase of calcium transients more than the rise.…”

Experimental and computational study on motor control and recovery after stroke: towards a constructive loop between experimental and virtual embodied neuroscience

“…The spectral content of the signals is analyzed to identify the frequency band which captures the spontaneous brain activity that occurs simultaneously with the motor-evoked events. Even though the slowest dynamics < 0.5 Hz , which has the highest power, is often a marker of the resting state (Wright et al, 2017), in this experiment it also contains the propagation of waves generated during the limb movements on the platform. The mechanisms behind stimulation propagation (Spiegler et al, 2016) are different from the spontaneous oscillations at rest (Deco and Jirsa, 2012) that we try to study and model here, and hence the lowest frequencies are excluded from the analysis.…”

“…In addition, the mice heart rate is between 6 − 8 Hz , whilst the activity above 10 Hz is too close to the Nyquist frequency of 12.5 Hz , defined as half of the sampling rate of the recordings. As a consequence these bands are generally avoided in the analysis of calcium signals, which is consequently often centered at the δ band between around 1 Hz and 5 Hz, (Vanni et al, 2017; Wright et al, 2017).…”