Thanks to dopamine, brownies are in the eye of the beholder

Activity in the eyes seems to reflect what’s going on in the brain.

We all need to eat. But for humans, food does more than keep up alive; it makes us happy, serves as a reward, and often gives us something to look forward to.

Dopamine is intimately involved in our relationship with food, since this neurotransmitter plays a major role in the reward and pleasure circuitry of our brains. It spikes when we are exposed to (or even merely anticipating) something we like, and it is known to be involved in addictive behavior. Understanding changes in the brain’s dopamine signaling can shed light on how and why we experience pleasure, but measuring these changes is expensive and invasive. Currently, researchers use PET scans to assess dopamine-related responses in the brain, but a single scan costs about $2,000 and can take over an hour to complete.

However, dopamine isn’t limited to the brain; there are other dopamine pathways in the body, including circuitry in the retinas, heart, and kidneys. A group of researchers wondered whether dopamine levels in these other pathways might change in concert with those in the brain. Using an inexpensive ophthalmologic technique called electroretinography, or “ERG,” they tested whether dopaminergic responses in the retina reflect reward-related neurotransmitter activity in the brain. Their results are published in the latest issue of the journal Obesity.

ERG is a relatively simple procedure that quantifies the responses of light-sensitive cells in the retina. During the procedure, light is flashed into a person’s eye and a machine measures the resulting dopamine-driven electrical responses. If dopamine pathways in the brain and retina are indeed linked, the signals picked up by the ERG should spike in response to the subject getting a reward, just as dopamine levels in the brain do.

Over four days, subjects in the study ingested four different stimuli: a chocolate brownie, water, and two different doses of methylphenidate, a known dopamine agonist. After each stimulus, the researchers conducted an ERG to assess the electrical activity in the retina. In large doses, methylphenidate is known to produce dopamine spikes in the brain, so the first order of business was to determine whether a similar peak in activity was seen in the retina.

Indeed it was; when the subjects ingested the larger of the two doses of methylphenidate, the ERG measured a significant increase in the electrical signals in the retina. This suggests that the dopamine activity in a person’s retina can reflect the dopamine activity in their brain. When the subjects ate the brownie, the researchers observed an equivalent spike in the retinas’ electrical activity, demonstrating that it’s possible to effectively measure the brain’s pleasure response to a food reward simply via a person’s eyes.

This is a pretty exciting finding, since ERG is a much cheaper and easier technique than PET scanning, at about $150 for a set of 200 scans. The study was essentially a proof-of-concept, since there were only nine participants and the subjects were very homogenous—none were on prescription medication, had depression or diabetes, or smoked. However, the results are both surprising and exciting, as there was no previous evidence that the dopamine pathways in the eyes and brain were synchronized. Understanding the reward circuitry in the brain has implications for understanding addictive behavior, nutrition, and obesity, and ERGs could help make research in these areas easier and more fruitful.

Obesity, 2013. DOI: 10.1002/oby.20101 (About DOIs).