Ever wondered why simply seeing a tasty snack can make your mouth water?
Scientists have found that 'eating with your eyes' is hardwired into the brain and is deeply rooted in evolution.
The researchers say that in future this behaviour could be shifted to help those with eating disorders.
'We hope discovering a basic mechanism should lead to an understanding of the pathology of human disorders related to appetite, such as bulimia, and anorexia,' study coauthor Professor Koichi Kawakami told MailOnline.
'But these will be future works.'
Professor Kawakami and colleagues at the National Institute of Genetics near Fuji in southern Japan revealed a direct link between seeing food and the motivation to eat.
The study involved monitoring brain activity in zebrafish - an animal with a neuron network similar to humans.
'Feeding behaviour and hunger is regulated by a brain area called the hypothalamus in humans,' Dr Akira Muto, lead author of the study, said.
'Zebrafish, like humans, mostly use vision for recognition of food or prey. It was not known how the hypothalamus receives visual information about prey.'
'We first demonstrated that neurons in the hypothalamus do indeed respond to the sight of prey. Then we discovered "prey detector" neurons in an area called the pretectum.
'Furthermore, we found a direct neural link connecting the prey detector neurons to the hypothalamic feeding centre,' Dr Muto said. The authors explain that there is an evolutionary benefit to animals feeling pangs of hunger when they see prey.
As a baby, it is this response that keeps humans and other animals alive.
It creates motivation for food in juvenile animals who otherwise might not understand what to eat.
The response can be seen in human children who often take small objects as food and explore them with their mouths, Professor Kawakami told MailOnline.
Professor Kawakami and Dr Muto carried out the research using a huge zebrafish facility where thousands of fish tanks are kept.
Each of these tanks contains genetically different fish that can turn on or drive the expression of different genes in different types of cells in the brain or in the body.
'Our study demonstrates how tightly visual perception of food is linked to motivational feeding behaviour in vertebrate animals,' Dr Muto said.
'This is an important step toward understanding how feeding is regulated and can be modulated in normal conditions as well as in feeding disorders.'