Imagine hammering a nail into a wall: Your dominant hand swings the hammer while the other holds the nail steady.
In a new theory, Cornell psychology scholars propose that everyday tasks like this are responsible for a fundamental aspect of perception in the brain: why one side is specialized to process high-frequency visual information, and the other low frequencies.
Longstanding research has shown that for most people, the brain's left hemisphere responds fastest to rapidly changing (high-frequency) input, like hammering, while the right hemisphere processes more static (low-frequency) events, like holding a nail. But scientists lacked an explanation for why this is so.
With their "action asymmetry hypothesis," the Cornell team explains this phenomenon and - demonstrated for the first time in large studies - that high-frequency visual specialization is reversed in left-handed people.
"We found the same pattern you always find in righties, whose left hemispheres are specialized for high-frequency visual perception - and the exact opposite in lefties," said Daniel Casasanto, associate professor in the Department of Psychology and College of Human Ecology, and director of the Experience and Cognition Lab. "These data support our theory that the way perceptual systems are organized in the brain depends on the way we perform actions with our hands."
Casasanto is the senior author of "Frequency Asymmetries in Vision: the Action Asymmetry Hypothesis," published June 27 in the Journal of Experimental Psychology: General. The first author is Owen Morgan, M.A. '23, a doctoral student in the field of psychology.
Theories about the so-called hemispheric asymmetry for visual perception have suggested that it may develop in the womb, or that it could be linked to language, since the left hemisphere processes the high-frequency components of language. The new study contradicts those theories, since handedness usually does not cause any reversal in fetal development, or in the hemisphere that processes language.
Building on a body of work Casasanto calls the body specificity hypothesis - showing that people's brains and minds are organized according to specifics of how their bodies interact with the world - Casasanto and Morgan investigated the role of motor action. They repeated experiments conducted previously to establish hemispheric asymmetry for frequency perception, but with a key difference. They enlisted lefties - subjects routinely left out of prior work that prioritized homogeneous samples.
"In this case," Casasanto said, "testing lefties and comparing them to the righties is the key to figuring out how perception is actually organized in the brain, and why it's organized that way."
A pair of experiments including nearly 2,000 participants - roughly equal numbers of right- and left-handers, and some mixed - confirmed which hemisphere handled high-frequency visual processing. That was measured by reaction times when pairs of "hierarchically constructed" target shapes were flashed on a screen - for example, a diamond composed of small triangles next to a triangle composed of small squares.
A third experiment confirmed that both righties and lefties used their left hemisphere to process high-frequency sounds in language. That ruled out the possibility that so-called language laterality could explain the hemispheric differences in visual perception.
Why might right- and left-handers process high-frequency vision in different sides of the brain? The authors propose two mechanisms. First, once a hemisphere becomes responsible for high-frequency action, it may be efficient for the brain to make connections between similar motor, vision and hearing systems in the same side. Or perhaps dominant hands continuously feed high-frequency sights and sounds into that side's visual field.
"The fact of this action asymmetry causes asymmetries in the visual and auditory input that we give ourselves," Casasanto said. "Then the hemisphere that is used to getting either high- or low-frequency input may become specialized for that kind of information."
The studies did not find a strong reversal in low-frequency visual input. The researchers speculate this may be because either hand could perform many low-frequency tasks, such as holding a nail, specialization might be reduced.
After showing that high-frequency visual processing reverses with handedness, providing initial support for the action asymmetry hypothesis, Casasanto plans in future research to investigate whether that is also true for hearing. The team also plans to test frequency specialization in stroke patients who have lost the use of their dominant hand, to see whether visual perception gets reorganized according to their new habits of hand action.
"That's our hypothesis: that asymmetries in hand action give rise to asymmetries in perception in vision and audition," he said. "The way you perform actions with your hands influences a bunch of different cognitive functions, including language and emotion - and, we know now, visual perception."
