The Cold Spring Harbor Laboratory scientists said half of the neurons they measured showed no reaction to incoming stimuli.

The researchers used a new technique that measures the reaction of individual neurons in an unbiased way. Traditional approaches favor the measurement of only the largest and most active neurons.

"This finding challenges the standard model of sound representations in the auditory cortex, which predicts that neural representations of stimuli often engage a large fraction of neurons," said Professor Anthony Zador.

The researchers theorize each neuron in the auditory cortex could have an "optimal stimulus" to which it is particularly sensitized.

"Your entire sensory apparatus is there to make successful representations of the outside world," Zador said. "Sparse representations may make sensory stimuli easier to recognize and remember."

The research, which appeared in the Jan. 28 issue of the journal PLoS Biology, is expected to help scientists better understand how the mammalian brain interprets and learns sounds.