Neural correlates of blood flow measured by ultrasound

Functional ultrasound imaging (fUSI) is an appealing method for measuring blood flow and thus infer brain activity, but it relies on the physiology of neurovascular coupling and requires extensive signal processing. To establish to what degree fUSI trial-by-trial signals reflect neural activity, we performed simultaneous fUSI and neural recordings with Neuropixels probes in awake mice. fUSI signals strongly correlated with the slow (<0.3 Hz) fluctuations in the local firing rate and were closely predicted by the smoothed firing rate of local neurons, particularly putative inhibitory neurons. The optimal smoothing filter had a width of ∼3 s, matched the hemodynamic response function of awake mice, was invariant across mice and stimulus conditions, and was similar in the cortex and hippocampus. fUSI signals also matched neural firing spatially: firing rates were as highly correlated across hemispheres as fUSI signals. Thus, blood flow measured by ultrasound bears a simple and accurate relationship to neuronal firing.

Feedback to distal dendrites links fMRI signals to neural receptive fields in a spiking network model of the visual cortex

The blood oxygenation level-dependent (BOLD) response has been strongly associated with neuronal activity in the brain. However, some neuronal tuning properties are consistently different from the BOLD response. We studied the spatial extent of neural and hemodynamic responses in the primary visual cortex, where the BOLD responses spread and interact over much longer distances than the small receptive fields of individual neurons would predict. Our model shows that a feedforward-feedback loop between V1 and a higher visual area can account for the observed spread of the BOLD response. In particular, anisotropic landing of inputs to compartmental neurons were necessary to account for the BOLD signal spread, while retaining realistic spiking responses. Our work shows that simple dendrites can separate tuning at the synapses and at the action potential output, thus bridging the BOLD signal to the neural receptive fields with high fidelity.

CBF, BOLD, CBV, and CMRO(2) fMRI signal temporal dynamics at 500-msec resolution

PURPOSE

To investigate the temporal dynamics of blood oxygenation level-dependent (BOLD), cerebral blood flow (CBF), cerebral blood volume (CBV), and cerebral metabolic rate of oxygen (CMRO(2)) changes due to forepaw stimulation with 500-msec resolution in a single setting.

MATERIALS AND METHODS

Forepaw stimulation and hypercapnic challenge on rats were studied. CBF and BOLD functional MRI (fMRI) were measured using the pseudo-continuous arterial spin-labeling technique at 500-msec resolution. CBV fMRI was measured using monocrystalline iron-oxide particles following CBF and BOLD measurements in the same animals. CMRO(2) change was estimated via the biophysical BOLD model with hypercapnic calibration. Percent changes and onset times were analyzed for the entire forepaw somatosensory cortices and three operationally defined cortical segments, denoted Layers I-III, IV-V, and VI.

RESULTS

BOLD change was largest in Layers I-III, whereas CBF, CBV, and CMRO(2) changes were largest in Layers IV-V. Among all fMRI signals in all layers, only the BOLD signal in Layers I-III showed a poststimulus undershoot. CBF and CBV dynamics were similar. Closer inspection showed that CBV increased slightly first (P < 0.05), but was slow to peak. CBF increased second, but peaked first. BOLD significantly lagged both CBF and CBV (P < 0.05).

CONCLUSION

This study provides important temporal dynamics of multiple fMRI signals at high temporal resolution in a single setting.