They say if you are trying to find out if love is true, it is all in the eyes.
But it seems there is no love truer than the love of chocolate, according to new research.
Scientists researching the brain's pleasure response to tasting food, have discovered it can be measured through the eyes.
The release of the brain's pleasure chemical dopamine has the effect of making pupils dilate - a sign that has been commonly associated with feelings of love and lust.
In the eye's retina, dopamine is released when the optical nerve activates in response to light exposure.
Dr Jennifer Nasser, associate professor of Nutrition Sciences in Drexel University's College of Nursing and Health Professions, led the study using electroretinography - which records the electrical response to light by the retina.
She and her colleagues found that electrical signals in the retina spiked high in response to a flash of light when a small piece of chocolate brownie was placed in participants' mouths.
The increase of dopamine was as great as that seen when participants had received the stimulant drug - methylphenidate or RItalin, which is used to treat patients with Attention Deficit Disorder (ADD).
These responses in the presence of food and drug stimuli were each significantly greater than the response to light when participants were given a plain control substance, water.
'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.'