Abstract
Human functional magnetic resonance imaging (fMRI) typically employs the blood-oxygen-level-dependent (BOLD) contrast mechanism. In non-human primates (NHP), contrast enhancement is possible using monocrystalline iron-oxide nanoparticles (MION) contrast agent, which has a more temporally extended response function. However, using BOLD fMRI in NHP is desirable for interspecies comparison, and the BOLD signal’s faster response function promises to be beneficial for rapid event-related (rER) designs. Here, we used rER BOLD fMRI in macaque monkeys while viewing real-world images, and found visual responses and category selectivity consistent with previous studies. However, activity estimates were very noisy, suggesting that the lower contrast-to-noise ratio of BOLD, suboptimal behavioural performance, and motion artefacts, in combination, render rER BOLD fMRI challenging in NHP. Previous studies have shown that rER fMRI is possible in macaques with MION, despite MION’s prolonged response function. To understand this, we conducted simulations of the BOLD and MION response during rER, and found that no matter how fast the design, the greater amplitude of the MION response outweighs the contrast loss caused by greater temporal smoothing. We conclude that although any two of the three elements (rER, BOLD, NHP) have been shown to work well, the combination of all three is particularly challenging.
Highlights
Human functional magnetic resonance imaging typically employs the blood-oxygen-leveldependent (BOLD) contrast mechanism
In the event-related experiment, we sought to probe the emergence of selectivity to different object categories in the ventral visual stream, using a stimulus set that has been successfully employed in non-human primates (NHP) and human studies previously[14,24,26]
Overall, measuring the detailed response patterns elicited by particular stimuli as is standardly done in humans was difficult given the combined challenges of rapid event-related design, BOLD contrast, and NHP functional magnetic resonance imaging (fMRI). These analyses suggest that condition-rich pattern-information analyses, as routinely performed in humans on the basis of BOLD fMRI data, may be more challenging to obtain in monkey BOLD rapid event-related fMRI, with inconsistent behaviour
Summary
Human functional magnetic resonance imaging (fMRI) typically employs the blood-oxygen-leveldependent (BOLD) contrast mechanism. Vanduffel et al.[10] compared the use of BOLD vs MION in block-design experiments in awake macaque monkeys to map the brain areas selective for motion Their results matched monkey electrophysiology and human fMRI results, and showed greater spatial localization and contrast increase in MION relative to BOLD. They found that MION increased the functional sensitivity for stimuli presented at long durations, but brief or rapidly repeated stimulus presentations led to a greater attenuation of the signal compared to BOLD, consistent with a linear model capturing the dispersion of the response over time This suggests that MION might be less sensitive for rapid event-related designs, whose high-temporal-frequency effects might not pass through the nottingham.ac.uk www.nature.com/scientificreports/. Event-related designs have been successfully used in MION fMRI studies previously, for example[13,14,15,16,17]
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