"Neuroscientists have long known that some people have a better sense of touch than others, but the reasons for this difference have been mysterious," said Daniel Goldreich, PhD, of McMaster University in Ontario, one of the studys authors. "Our discovery reveals that one important factor in the sense of touch is finger size."
To learn why the sexes have different finger sensitivity, the authors first measured index fingertip size in 100 university students. Each students tactile acuity was then tested by pressing progressively narrower parallel grooves against a stationary fingertip -- the tactile equivalent of the optometrists eye chart. The authors found that people with smaller fingers could discern tighter grooves.
"The difference between the sexes appears to be entirely due to the relative size of the persons fingertips," said Ethan Lerner, MD, PhD, of Massachusetts General Hospital, who is unaffiliated with the study. "So, a man with fingertips that are smaller than a womans will be more sensitive to touch than the woman."
The authors also explored why more petite fingers are more acute. Tinier digits likely have more closely spaced sensory receptors, the authors concluded. Several types of sensory receptors line the skins interior and each detect a specific kind of outside stimulation. Some receptors, named Merkel cells, respond to static indentations (like pressing parallel grooves), while others capture vibrations or movement.
When the skin is stimulated, activated receptors signal the central nervous system, where the brain processes the information and generates a picture of what a surface "feels" like. Much like pixels in a photograph, each skin receptor sends an aspect of the tactile image to the brain -- more receptors per inch supply a clearer image.
To find out whether receptors are more densely packed in smaller fingers, the authors measured the distance between sweat pores in some of the students, because Merkel cells cluster around the bases of sweat pores. People with smaller fingers had greater sweat pore density, which means their receptors are probably more closely spaced.
"Previous studies from other laboratories suggested that individuals of the same age have about the same number of vibration receptors in their fingertips. Smaller fingers would then have more closely spaced vibration receptors," Goldreich said. "Our results suggest that this same relationship between finger size and receptor spacing occurs for the Merkel cells."
Whether the total number of Merkel cell clusters remains fixed in adults and how the sense of touch fluctuates in children as they age is still unknown. Goldreich and his colleagues plan to determine how tactile acuity changes as a finger grows and receptors grow farther apart.
The research was supported by the National Eye Institute and the Natural Sciences and Engineering Research Council in Canada.
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