Primer describes current understanding of human taste perception and biology

Despite the significance of taste to both human gratification and survival, a basic understanding of this primal sense is still unfolding. Taste provides both pleasure and protection. Often taken for granted, the sense of taste evaluates everything humans put into their mouths. Taste mediates recognition of a substance and the final decision process before it is either swallowed and taken into the body, or rejected as inappropriate.

A new primer written by scientists at the Monell Center and Florida State University and published in the February 26 issue of Current Biology, provides a clear and accessible overview of recent advances in understanding human taste perception and its underlying biology.

Within the past few years, identification of receptors for sweet, bitter and umami (savory) taste has led to new insights regarding how taste functions, but many questions remain to be answered. The Current Biology primer reviews the current state of knowledge regarding how taste stimuli are detected and ultimately translated by the nervous system into the perceptual experiences of sweet, sour, salty, bitter, and umami.

Such perceptual evaluations are related to the function and ultimately, the consequences, of taste evaluation. These can range from pleasurable emotional reactions, for example the delight a child receives from a sweet candy, to the critical life-dependent response that causes a person to spit out a bitter potential toxin.

Author Paul A.S. Breslin, PhD, a sensory scientist at the Monell Center, observes, “For all mammals, the collective influence of taste over a lifetime has a huge impact on pleasure, health, well being, and disease. Taste’s importance to our daily lives is self-evident in its metaphors – for example: the ‘sweetness’ of welcoming a newborn child, the ‘bitterness’ of defeat, the ‘souring’ of a relationship, and describing a truly good human as the ‘salt’ of the earth.”

25 February 2008

In the first study to use imaging technology to see what goes on in the brain when we scratch, researchers at Wake Forest University Baptist Medical Center have uncovered new clues about why scratching may be so relieving – and why it can be hard to stop. The work is reported online in the Journal of Investigative Dermatology and will appear in a future print issue.

"Our study shows for the first time how scratching may relieve itch,” said lead author Gil Yosipovitch, M.D., a dermatologist who specializes in itch. “It’s important to understand the mechanism of relief so we can develop more effective treatments. For some people, itch is a chronic condition that affects overall health.” The study involved 13 healthy participants who underwent testing with functional magnetic resonance imaging (MRI) technology that highlights areas of the brain activated during an activity. Participants were scratched on the lower leg with a small brush. The scratching went on for 30 seconds and was then stopped for 30 seconds – for a total of about five minutes.

“To our surprise, we found that areas of the brain associated with unpleasant or aversive emotions and memories became significantly less active during the scratching,” said Yosipovitch. “We know scratching is pleasurable, but we haven’t known why. It’s possible that scratching may suppress the emotional components of itch and bring about its relief.” The reduced brain activity occurred in the anterior cingulate cortex, an area associated with aversion to unpleasant sensory experiences, and the posterior cingulate cortex, which is associated with memory. When participants reported that the scratching felt most intense, activation in these areas was lowest.

Yosipovitch said patients occasionally report that intense scratching – to the point of drawing blood – is the only thing that relieves chronic itch. “This is the first real scientific evidence showing that itch may be inhibited by scratching,” he said. “Of course, scratching is not recommended because it can damage the skin. But understanding how the process works could lead to new treatments. For example, drugs that deactivate this part of the brain might be effective.”

The imaging studies also showed that some areas of the brain were made more active by the scratching, including the secondary somatosensory cortex, a sensory area involved in pain, and the prefrontal cortex, which is associated with compulsive behavior. “This could explain the compulsion to continue scratching,” said Yosipovitch.

One drawback to the study is that the scratching occurred in the absence of itch. Yosipovitch’s team is continuing the research by evaluating whether the findings will apply to chronic itch. Understanding more about chronic itch is important, Yosipovitch said, noting that more than 30 million Americans suffer from eczema and that almost half (42 percent) of kidney dialysis patients are bothered by moderate to severe itch. In fact, those kidney dialysis patients with itch have a 17 percent higher mortality rate, likely from a loss of sleep, according to a report in Nephrology Dialysis Transplantation.


Wake Forest University Baptist Medical Center News
31 January 2008