Ask Alvaro Pascual-Leone what touch is and you will get a clear—and captivating—definition.
“We’re tempted to think that touch is like the other senses,” says Pascual-Leone, an HMS professor of neurology at Beth Israel Deaconess Medical Center, “simply a specialization of our nervous system that allows us to capture such aspects of reality as pain, pressure, and temperature. But as we have found, this doesn’t allow us to fully appreciate the role of touch in brain development. When we see, hear, smell, and taste, our mind’s fingers touch our mind’s eyes, ears, nose, and tongue, shaping the way we interpret those sensations.”
Pascual-Leone refers here to work his team has done to assess the plasticity of the human visual cortex. For their study, the researchers initially monitored visual-cortex activity in normally sighted participants as they performed a tactile exercise. Then for five days and nights, some participants wore light-blocking blindfolds. During this period, all participants underwent intensive tactile training that included Braille instruction.
After five days, the researchers found that, when compared with the sighted group, the blindfolded participants showed a greater ability to discriminate among Braille characters, a behavioral improvement linked to activity in their visual cortices. This ability diminished within one day, leading the researchers to marvel at the capacity of the visual cortex to reallocate its resources rapidly to accommodate nonvisual information—and then return to original functioning.
“Maybe the visual cortex is visual because we have vision,” says Pascual-Leone. “It may be functionally capable of processing any other type of information. And perhaps this plasticity is true of every part of the brain.”
Pascual-Leone’s research on the excitability of the visual cortex underscores this concept. His team measured the minimum level at which the visual cortices of participants could perceive a flash of light delivered alone and when coupled with a glancing touch to the hand.
Then the researchers administered both stimuli at levels below human perception—and found that the participants could still accurately report the light flashes. Without the below-threshold touch, however, the lights remained unseen. The visual cortex had changed, says Pascual-Leone, increasing its ability to interpret the light stimuli by combining two imperceptible sensations—one visual and one touch-based.