Navegando por Autor "Liu, Junjie V."
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Artigo fMRI in the awake marmoset: somatosensory-evoked responses, functional connectivity, and comparison with propofol anesthesia(Elsevier, 2013-09) Liu, Junjie V.; Hirano, Yoshiyuki; Nascimento, George Carlos; Stefanovic, Bojana; Leopold, David A.; Silva, Afonso C.Functional neuroimaging in animal models is essential for understanding the principles of neurovascular coupling and the physiological basis of fMRI signals that are widely used to study sensory and cognitive processing in the human brain. While hemodynamic responses to sensory stimuli have been characterized in humans, animal studies are able to combine very high resolution imaging with invasive measurements and pharmacological manipulation. To date, most high-resolution studies of neurovascular coupling in small animals have been carried out in anesthetized rodents. Here we report fMRI experiments in conscious, awake common marmosets (Callithrix jacchus), and compare responses to animals anesthetized with propofol. In conscious marmosets, robust BOLD fMRI responses to somatosensory stimulation of the forearm were found in contralateral and ipsilateral regions of the thalamus, primary (SI) and secondary (SII) somatosensory cortex, and the caudate nucleus. These responses were markedly stronger than those in anesthetized marmosets and showed a monotonic increase in the amplitude of the BOLD response with stimulus frequency. On the other hand, anesthesia significantly attenuated responses in thalamus, SI and SII, and abolished responses in caudate and ipsilateral SI. Moreover, anesthesia influenced several other aspects of the fMRI responses, including the shape of the hemodynamic response function and the interareal (SI–SII) spontaneous functional connectivity. Together, these findings demonstrate the value of the conscious, awake marmoset model for studying physiological responses in the somatosensory pathway, in the absence of anesthesia, so that the data can be compared most directly to fMRI in conscious humansArtigo Investigation of the BOLD and CBV fMRI responses to somatosensory stimulation in awake marmosets (Callithrix jacchus)(Wiley, 2017-12-29) Hirano, Yoshiyuki; Yen, Cecil C.; Liu, Junjie V.; Mackel, Julie B.; Merkle, Hellmut; Nascimento, George Carlos do; Stefanovic, Bojana; Silva, Afonso C.Understanding the spatiotemporal features of the hemodynamic response function (HRF) to brain stimulation is essential for the correct application of neuroimaging methods to study brain function. Here, we investigated the spatiotemporal evolution of the blood oxygen level‐dependent (BOLD) and cerebral blood volume (CBV) HRF in conscious, awake marmosets (Callithrix jacchus ), a New World non‐human primate with a lissencephalic brain and with growing use in biomedical research. The marmosets were acclimatized to head fixation and placed in a 7‐T magnetic resonance imaging (MRI) scanner. Somatosensory stimulation (333‐μs pulses; amplitude, 2 mA; 64 Hz) was delivered bilaterally via pairs of contact electrodes. A block design paradigm was used in which the stimulus duration increased in pseudo‐random order from a single pulse up to 256 electrical pulses (4 s). For CBV measurements, 30 mg/kg of ultrasmall superparamagnetic ironoxide particles (USPIO) injected intravenously, were used. Robust BOLD and CBV HRFs were obtained in the primary somatosensory cortex (S1), secondary somatosensory cortex (S2) and caudate at all stimulus conditions. In particular, BOLD and CBV responses to a single 333‐μs‐long stimulus were reliably measured, and the CBV HRF presented shorter onset time and time to peak than the BOLD HRF. Both the size of the regions of activation and the peak amplitude of the HRFs grew quickly with increasing stimulus duration, and saturated for stimulus durations greater than 1 s. Onset times in S1 and S2 were faster than in caudate. Finally, the fine spatiotemporal features of the HRF in awake marmosets were similar to those obtained in humans, indicating that the continued refinement of awake non‐human primate models is essential to maximize the applicability of animal functional MRI studies to the investigation of human brain function