Washington: New experimental results have shed light on how mammals act on the basis of sound cues.
It`s well known that sounds detected by the ears wind up in a part of the brain called the auditory cortex, where they are translated - transduced - into information that scientists call representations.
These representations, in turn, form the informational basis upon which other parts of the brain can make decisions and issue commands for specific actions.
What scientists have not understood is what happens between the auditory cortex and portions of the brain that ultimately issue commands, say, for muscles to move in response to the sound of that car`s screeching brakes.
To find out, a team of researchers from Cold Spring Harbor Laboratory (CSHL) trained rats to listen to sounds and to make decisions based on those sounds. When a high-frequency sound is played, the animals are rewarded if they move to the left. When the sound is low-pitched, the reward is given if the animal moves right.
On the simplest level, said CSHL Professor Anthony Zador, "we know that sound is coming into the ear; and we know what`s coming out in the end - a decision," in the form of a muscle movement. The surprise, he said, is the destination of the information used by the animal to perform this task of discriminating between sounds of high and low frequency, as revealed in his team`s experiments.
"It turns out the information passes through a particular subset of neurons in the auditory cortex whose axons wind up in another part of the brain, called the striatum," Zador explained.
The classic series of experiments that provided inspiration and a model for this work, performed at Stanford University by William Newsome and colleagues, involved the visual system of primates, and had led Zador to expect by analogy that representations formed in the auditory cortex would lead to other locations within the cortex.
These experiments in rats have implications for how neural circuits make decisions, according to Zador. Even though many neurons in auditory cortex are "tuned" to low or high frequencies, most do not transmit their information directly to the striatum. Rather, their information is transmitted by a much smaller number of neurons in their vicinity, which convey their "votes" directly to the striatum.
The results were published in the journal Nature.