Researchers from the Howard Hughes Medical Institute in the U.S.A. and the U.S. National Institute of Health mapped out four sensory hotspots in the brain, devoted to processing four of the five basic tastes – sweet, salty, bitter and umami.
The findings, published in Science, demonstrate that neurons responding to specific tastes are arranged discretely in a ‘gustotopic map.’
“This work further reveals coding in the taste system via labelled lines, and it exposes the basic logic for the brain representation of the last of the classical five senses,” said Charles Zuker of the Howard Hughes Medical Institute.
The authors added that they did not find a hotspot for the sour taste, possibly because it lies outside the area they sampled, or because acid stimuli also act on other pathways such as pain and touch.
Over the last 50 years, researchers have produced comparable maps for the other senses, identifying networks of neurons responsible for vision, hearing and touch in the cortex, and showing that the sense of smell involves a sparser pattern of neuronal activity. Taste, however, has remained a mystery.
“The way that we perceive the sensory world has been something that's fascinated humanity throughout our whole existence,” said Nicholas Ryba of the National Institute of Dental and Craniofacial Research, who collaborated on the new study.
Previous research had measured the electrical activity of small clusters of neurons to see which areas of a mouse's brain were activated by different tastes. In such experiments the areas responding to different tastes seemed to blend together, and scientists therefore concluded that neurons appeared to process all tastes broadly.
However Zuker and his colleagues had previously identified unique taste receptors and taste receptor cells for each taste – uncovering a “one taste, one cell class” coding scheme. They reported that activating individual receptor cells triggered innate behaviours in mice: attraction to sweet, umami, and low salt and aversion to bitter, sour, and high salt.
With this clear link between taste and ‘hardwired’ behaviours in mind, the researchers wondered why different tastes would be processed by the same neurons in the brain, and suspected that that previous experiments had missed something.
Zuker and his colleagues used a technique known as two-photon calcium imaging to identify individual cells responding to taste in the brains of mice. They then traced the neural connections into the primary taste cortex in the brain – the insula.
They observed that when a mouse is given something bitter to taste, or the receptors on its tongue that sense bitter are stimulated, many neurons in one small, specific area of the brain light up. Then, when the mouse is given something salty, an area a few millimetres away is activated.
Zucker and his team explained that each taste corresponded to a different hotspot in the brain, with no overlapping areas – in fact, they noted that there was space between all of them.
“This work further reveals coding in the taste system via labelled lines, and it exposes the basic logic for the brain representation of the last of the classical five senses,” said Zuker.
Zuker explained that now the team has discovered a brain map for taste qualities, they next want to uncover “how taste combines with other sensory inputs like olfaction and texture, and the internal state—hunger and expectation, for example—to choreograph flavour, taste memories, and taste behaviours.”
Volume 333, number 6047, pages 1262-1266, doi: 10.1126/science.1204076
“A Gustotopic Map of Taste Qualities in the Mammalian Brain”
Authors: X. Chen, M. Gabitto, Y. Peng, N.J.P. Ryba, C.S. Zuker