Salk research suggests the existence of specialized neurons that distinguish swagger from sway

LA JOLLA, CA - It doesn't take John Wayne's deliberate, pigeon-toed swagger or Marilyn Monroe's famously wiggly sway to judge a person's gender based on the way they move. People are astonishingly accurate when asked to judge the gender of walking human figures, even when they are represented by 15 small dots of light attached to major joints of the body.

And not only that, when human observers watched the walking motion of a male so-called "point light walker," they were more sensitive to the female attributes when watching the next figure in the sequence. This suggests that the human brain relies on specialized neurons that tell gender based on gait, report researchers at the Salk Institute for Biological Studies in the May 21 advance online edition of Nature Neuroscience.

"Our judgment of gender can adapt within seconds," says senior author Gene Stoner, a neuroscientist in the Vision Center Laboratory at the Salk Institute. "The gaits of males and females may vary geographically or culturally and this mechanism allows us to adapt very quickly to local ways of walking," he adds.

How humans move reflects, in part, gender-specific differences in shape such hip-to-waist ratio and the like. Such inherent differences in gait might then be exaggerated by an individual to emphasize their gender. "Our new data suggests that there are neurons selective for gender based on these motion cues and that they adjust their selectivity on the fly," Stoner explains.

Although much work has been done on how the brain represents so-called low-level features, such as "redness" or "left-moving," scientists have been unable to put their finger on more abstract concepts such as gender. "We wanted to know whether gender is represented in a similar way to low-level visual features such as color, or if it is a more semantic concept such as good and evil," says experimental psychologist and first author Heather Jordan, a former post-doc in the Vision Center Laboratory and now an assistant professor at York University in Toronto.

Individual neurons in the visual cortex are finely tuned to certain attributes of visible objects such as the color red, a certain shape or objects moving in a specific direction. These specialized neurons reveal their existence through a telltale effect called adaptation. For example, if you stare at a red patch and then look at a neutral color you tend to see green. This "adaptation" reflects a mechanism in the brain that exaggerates differences between objects to increase the sensitivity and optimize the output of individual neurons.

"In the past, when adaptation in behavior was observed for specific features, neurophysiologists have subsequently been able to find individual neurons which fire only when they encounter this feature," says Jordan. "We think that the same is true for maleness and femaleness - that there are neurons in the brain that fire if, and only if, they 'see' a male gait and others that fire if, and only if, they 'see' a female gait, explains Jordan.

"We know lots about individual neurons that are sensitive to the direction of moving objects. But in this case, motion provides information about the structure of what is moving," says Stoner.

For their experiments, the Salk researchers morphed the gait of averaged male and female walkers -- resulting in varying degrees of "maleness" and "femaleness" .When the figure consisted of less than 49 percent male contribution, the observers reported seeing a figure that appeared female. Once there was more than 49 percent maleness in the figure, they reported seeing a figure that was mostly male. But these numbers were not stable: Viewing the gait of one gender biased judgments of subsequent gaits toward the opposite gender. "If you want to appear particularly feminine you should walk behind a very masculine-looking male and vice-versa," jokes Jordan.

In addition to Stoner and Jordan, the Salk research team included neuroscientist Mayzar Fallah, a former post-doc in the Systems Neurobiology Laboratory and now an assistant professor at York University in Toronto.

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The Salk Institute for Biological Studies in La Jolla, California is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health, and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine all but eradicated the crippling disease poliomyelitis in 1955, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.


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