What They Did
The researchers developed an imaging system for real-time data collection on the diameter of retinal blood vessels. Using this system, they measured the diameter change of an artery providing blood to part of the retina that experienced a 10-Hz flickering light for 20 seconds. The eight participants all showed a significant widening of the relevant artery in response to the flicker.
After a short reaction time, the artery quickly dilates for about 7.7 seconds. The dilation then slows down but continues at the slower rate for about another 13 seconds until the artery reaches its largest diameter. When the flickering ceases, the artery quickly contracts, returning to baseline in about 9 seconds. The researchers also found that the percentage increase in dilation was inversely correlated with the diameter of the artery: larger arteries had a lower percent increase, though all had about the same absolute increase of 3.7 µm. The larger arteries also dilated more slowly.
Finally, the researchers observed the dilation response of a single participant with the flicker duration ranging from 2 to 60 seconds. If the flicker only lasts for 2 seconds, the artery dilates quickly, but it still takes 6 to 7 seconds, longer than the flicker persists. After reaching a peak diameter, the artery immediately begins to contract at a similar rate. With a 60-second flicker, the artery shows the fast and slow dilation processes as in the 20-second condition, but after peaking, it shows pulses of partial contraction and re-dilation approximately every 20 seconds.
Further Exploration
The researchers explain that the correlation between blood flow to and activation of specific parts of the brain or nervous system is well-documented and used in neurological research: for example, it’s the mechanism behind fMRI studies. They also explain that various medical conditions can disrupt that connection, which means parts the brain or nervous system might not get the oxygen they need for various tasks. (see https://www.cognitivefxusa.com/neurovascular-coupling).
The retina is part of the central nervous system: it literally forms out of brain tissue during embryonic development (see https://www.ncbi.nlm.nih.gov/books/NBK10885/). Because we can get higher-resolution imagery with light than with fMRI, looking at the correlation between activation and blood flow in specific areas of the retina might be a way to screen for some medical problems before they become detectable with fMRI.
The new imaging system the researchers developed works by illuminating the area step-by-step with a linear light source, then having pictures taken with a camera “in front of” and “behind” the illuminated area. The light waves from the two locations are out of phase, and the amount of phase difference varies depending on how easily light moves through areas such as blood vessels. Mapping the amount of phase difference to the region of the retina observed allows for images to be generated. The researchers indicate that other retinal imaging technologies also use phase contrast, but their system allows a larger area and a higher frame rate. I’d like to understand more about how it works, but that’s a rabbit hole for another day!
Image credit: Alexander Churkin
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