Human senses not distinct, but interact in many ways, studies show

November 10, 2003

NEW ORLEANS, Nov. 10 - Until fairly recently, scientists believed that the information gathered by each of the senses -- touch, sight, hearing, smell and taste -- was processed in separate areas of the brain. Research is now revealing, however, that there is a complex interaction between the senses in the brain--an interaction that enables us to understand the world in a unified way.

"Since we perceive the world as a whole and not split up into different sensory modalities, it's important to study how signals from the senses affect each other in the brain," says Colin Blakemore, PhD, of Oxford University.

New research on how the senses interact is revealing some fascinating findings: What we see affects how we perceive odors. Blind people do have a superior sense of touch. And the odd mixing-of-the-senses condition known as synesthesia, in which people claim to "see" sounds or "hear" colors, is a very genuine phenomenon.

Blakemore's colleagues at Oxford, led by Gemma Calvert, DPhil, have recently completed studies that help explain how the brain combines sight and smell to amplify our perception of various odors. Although it's believed that humans can recognize up to 10,000 different odors, we still have a poor sense of smell compared to other animals. To assist our sense of smell, we often rely on additional information from our visual system.

Earlier experiments have shown that when people are asked to smell an odorized liquid that has been tinted with an appropriate color (red for strawberry, for example), their perception of the intensity and pleasantness of the smell is greater than if the liquid is inappropriately tinted (green for strawberry) or not tinted at all. For their current study, the Oxford researchers wanted to find what happens in the brain when odors are matched or mismatched with pictures. Does the smell of an orange elicit a stronger response in the brain if it's co-presented with a picture of an orange rather than a picture of toothpaste, for example?

The study used functional magnetic resonance imaging (fMRI), which measures changes in blood flow in the brain while a task, such as smelling a scent, is carried out by a person lying in the scanner. When brain neurons become involved in a task, they increase their firing rate, which leads to a change in blood flow to the brain area(s) where the activated neurons are located. The fMRI scanner detects that increased blood flow, thus identifying which areas of the brain are involved in the task.

The 12 volunteers in the Oxford study were placed in an fMRI scanner and then shown matched and mis-matched combinations of odors and pictures (a strawberry odor with a picture of a strawberry, for example, or a strawberry odor with a picture of an orange). "We found that areas of the brain involved in smell perception respond differently to the picture-odor combinations," says Calvert.

"Some regions--especially the orbitofrontal cortex and the amygdala, which are both involved in smell--responded more strongly to congruent than to incongruent picture-odor combinations." This finding indicates, she says, that the brain amplifies information gathered from those two senses when the information fits well together.

Calvert and her colleagues plan next to investigate if this multi-sensory effect is stronger for food-related smells than for other smells.

Although it's a common popular belief that blind people have a superior sense of touch, the research on this topic, which has spanned nearly 100 years, has been controversial. New findings from Duquesne University in Pittsburgh, Pennsylvania, may, however, finally put this question to rest. Using rigorous experimental techniques, DU researchers have found that blind people do have a superior sense of touch.

For this study, Daniel Goldreich, PhD, and his colleagues tested 47 sighted and 37 blind volunteers, ranging in age from about 18 to 70. The scientists constructed a special, computer-controlled device designed to tap index fingers with different pieces of plastic. Some pieces were completely smooth. Others contained thin grooves of various widths cut into their surfaces. In general, the narrower the grooves, the more difficult it is to feel them. During the experiment, the volunteers were asked to determine which piece was touching their finger, but without moving the finger. Thus, the test measured what is known as "passive tactile acuity" rather than "active tactile acuity," for which the finger is allowed to move. Each tap lasted for one second, and the force of the tap was light: either 10 grams or 50 grams. By tapping with pieces of different groove width, the scientists were able to determine the minimum groove width that each volunteer could reliably distinguish from a smooth surface.

"On average, blind people were able to distinguish thinner grooves than sighted people," says Goldreich. Although the sense of touch declined with age at a similar rate in the blind and sighted groups, the sense of touch of the average blind person in the study was about as good as that of an average sighted person who was 23 years younger.

"We also found that people who were born blind didn't have a better sense of touch than those who became blind later in life," says Goldreich. Nor did the ability to read Braille enhance a blind person's sense of touch. Goldreich and his colleagues found that blind Braille readers had no better sense of touch than blind nonreaders.

The sighted people in the study were tested with their eyes uncovered and in a lit environment. Goldreich is now investigating whether the tactile acuity of sighted people improves if they are temporarily deprived of vision.

One of the more fascinating mixing of the senses is a condition known as synesthesia, which affects about one in every 2,000 people. People with synesthesia claim that real sensory experiences trigger other entirely inappropriate perceptions. For instance, they might "see" sounds, "hear" colors, or "feel" tastes. The condition is not a new one; it has been known to the scientific community for at least 300 years, although it hasn't been much investigated until relatively recently, and some still doubt whether it is a genuine neurological condition.

A common type of synesthesia is "colored-hearing." People with this condition see specific colors in their "mind's eye" when they hear words, letters or numbers spoken out loud. The term "mind's eye" is used because although these people see the colors in front of them, the colors don't interfere with their normal vision. Another common type of synesthesia is "colored-touch." People with this condition see colors when they feel certain objects.

To learn more about synesthesia, Colin Blakemore's team at Oxford University recently studied an interesting subgroup of people with the condition--people who say that they have been colored-hearing synesthetes all their life, even after becoming blinded by injury or disease to the retina. "We wondered whether they might be just imagining remembered colors rather than having genuine visual sensations, so long after losing their real sight," says Megan Steven, a graduate student at Oxford and the lead author of the study.

Six volunteers with colored-hearing synesthesia who had been blind at least 10 years participated in the study. They were asked to describe the colors they saw when hearing the names of each day of the week, month of the year, letter of the alphabet and/or number from 1-100 (counting by tens after the first 10 digits). Their responses were recorded. Two months after this initial screening, the volunteers were surprised with a second, identical test.

"What we found was an amazing correlation between the two testing days," says Steven. "If a subject said that A was pale green on the first testing day, they would say that the letter was a light or pale green on the surprise testing day two months later. This is strong evidence that they were experiencing a genuine phenomenon--they actually appeared to be seeing colors in their mind's eye, even though they had been blind for at least 10 years." These findings suggest, she adds, that the visual areas of the brain can still remain active after blindness.

Two of the six volunteers also had a special form of colored-touch that Steven coined "colored-Braille," which caused them to see colors when they read Braille letters. The repeat test showed that their responses to the colors they "touched" as they read were also genuine. Since they learned Braille after they started to become blind, this implies that the connections in their brains leading to the color sensations were established through some kind of learning. Synesthesia runs in families and almost certainly depends on a genetic factor. However the particular form that it takes might depend on individual experience.

In a further experiment, the researchers tried to determine what was driving the synesthesia. For the blind people with colored hearing, the meaning of a word, rather than its sound alone, seems to be important. For example, when the word "March" was used in a sentence to mean a particular month of the year, one volunteer saw a "dark greeny blue" color. But when he heard the same word used as a verb ("The soldiers march across the bridge.") he did not see a color. Interestingly, when the same volunteer, who also has colored-Braille synesthesia, read the number 1, the musical note A, or the letter A in Braille--all of which are represented by a single dot in the upper-left corner--he saw the same color (white). His colored-Braille synesthesia appears to depend on the pattern of the dots, not the semantic representation.

In the next step of their research, Steven and her colleagues are using fMRI to investigate which areas of the brain in late-blind synesthetes are activated during colored-hearing and colored-Braille.

Society for Neuroscience

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