University of Toronto researchers have discovered that the main purpose of the startle reflex -- the mechanism that makes people twitch at sudden loud noises -- is to protect the body against blows.
Everyone has experienced the muscle contraction associated with the startle response, a feeling that is most often linked to sounds such as balloons popping or cars backfiring. Yet new studies show that the primary role of the startle reflex is to defend the body against strong impact stimuli, not noises.
"Before now, researchers have concentrated on studying the auditory pathways for the startle reflex, so the discovery that startle is best evoked when noises are combined with tactile stimuli is a surprising conclusion," says psychology professor John Yeomans, the lead author of a study published in the November issue of Neuroscience.
In experiments using a rat model, Yeomans and his colleagues measured the startle response to several combinations of auditory, tactile and vestibular (affecting equilibrium) stimuli. The results showed that the most sensitive sites in the rats' brains for the startle reflex are in the trigeminal nuclei, which relay tactile information from the brain.
The added information from auditory and vestibular pathways must arrive in the brain within 20 milliseconds of the tactile stimulus to have any added effect on the startle response, says Yeomans. "Since head blows activate all three systems within this 20 millisecond time window, blows are the ideal stimulus for evoking startle."
In a paper to be published next month, Yeomans' group argues that humans have the same timing, and that the startle response in rats and humans protects the body against blows.
CONTACT: Professor John Yeomans, Department of Psychology, 416-978-7618, yeomans@psych.utoronto.ca or Jessica Whiteside, U of T public affairs, 416-978-5948.
Journal
Neuroscience