24 June 2013

Eyes used to measure pleasure response

by Will Parker

The brain's pleasure response to tasting food can be accurately measured through the eyes using a common, low-cost ophthalmological tool, say Drexel University researchers who believe the method could be used by both food scientists and clinicians. Study leader Jennifer Nasser details the use of electroretinography (ERG) to measure dopamine levels in the retina in the current issue of the journal Obesity.

It's long been known that dopamine is associated with a variety of pleasure-related effects in the brain, including the expectation of reward. Less well known is that in the eye's retina, dopamine is released when the optical nerve activates in response to light exposure.

Nasser found that electrical signals in the retina spiked high in response to a flash of light when a food stimulus (a small piece of chocolate brownie) was placed in participants' mouths. Impressively, the increase was as great as that seen when participants had received the stimulant drug methylphenidate to induce a strong dopamine response. These responses in the presence of food and drug stimuli were each significantly greater than the response to light when participants ingested a control substance.

"What makes this so exciting is that the eye's dopamine system was considered separate from the rest of the brain's dopamine system," Nasser said. "So most people - and indeed many retinography experts told me this - would say that tasting a food that stimulates the brain's dopamine system wouldn't have an effect on the eye's dopamine system."

This study was a small-scale demonstration of the concept, with only nine participants. If the technique is validated through additional and larger studies, Nasser said she and other researchers can use ERG for studies of food addiction and food science.

The low cost and ease of performing electroretinography may make it an appealing method for measuring taste responses. ERG is about $150 per session, and each session generates 200 scans in just two minutes. Procedures to measure dopamine responses directly from the brain are much more expensive and invasive.

"Food is both a nutrient delivery system and a pleasure delivery system, and a 'side effect' is excess calories," Nasser concluded. "I want to maximize the pleasure and nutritional value of food but minimize the side effects."

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Source: Drexel University