310 — Active vision varies across the cardiac cycle

The heartbeat acts as a “fundamental internal oscillator,” a mechanism that is the most constant “clock” in the human body throughout life. And so it makes sense that the body would develop around this oscillator, leveraging it as a mechanism by which we understand the world around us.

And it also makes sense that the activities of our brains — and thus our bodies — vary across the cardiac cycle: Research has shown that reaction-time decreases when a stimulus is presented late in the cardiac cycle (specifically, at diastole, when the ventricles are relaxed) versus early (during systole).

The logical next question is whatever or not the body’s outputs are also in synchronized flux with the cardiac cycle.

The experimental design is simple: Give people a bunch of images to memorize, and only give them a few milliseconds to look at each image (so that it fits well within a single cardiac oscillation). But give the subject control over when the image is shown.

Specifically, the subjects are given a button that, when pressed, advances to the next picture in the series.

What the researchers found surprised me: Subjects preferentially chose to advance to the next image during systole (I expected there to be no correlation). This was clearly done without the subject being aware that this was true; instead, the subject self-paced — and this tendency arose spontaneously.

Despite this, cardiac cycle had no influence on memory performance.

It’s not clear what the mechanism is that is behind this phenomenon: Even though baroreceptors in the arteries transmit cardiac cycle information to the brain, it’s not obvious what causes this preferential treatment of stimuli during systole over diastole.