How the body acts on the basis of sound cues
One of the truly primal mechanisms that we depend on every day of our
lives - acting on the basis of information gathered by our sense of
hearing - is yielding its secrets to modern neuroscience. A team of
researchers from Cold Spring Harbour Laboratory (CSHL) has published
experimental results in the journal Nature which they describe as
surprising. The results fill in a key piece of the puzzle about how
mammals act on the basis of sound cues.
When a pedestrian hears the screech of
a car's brakes, she has to decide whether, and if so, how,
to move in response. Is the action taking place blocks away,
or 20 feet to the left?
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
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, CSHL Professor Anthony Zador and Dr. Petr Znamenskiy
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.
To the striatum
On the simplest level, says 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 says, 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," says Zador.
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.
Subset of neurons
"This is like the difference between a direct democracy and a
representative democracy, of the type we have in the United States,"
Zador explains. "In a direct democracy model of how the auditory cortex
conveys information to the rest of the brain, every neuron activated by
a low - or high-pitched sound would have a 'vote.' Since there is noise
in every perception, some minority of neurons will indicate 'low' when
the sound is in fact 'high,' and vice-versa. In the direct democracy
model, the information sent to the striatum for further action would be
the equivalent of a simple sum of all these votes.
"In contrast - and this is what we found to be the case - the neurons
registering 'high' and 'low' are represented by a specialised subset of
neurons in their local area, which we might liken to members of Congress
or the Electoral College: these in turn transmit the votes of the larger
population to the place - in this case the auditory striatum - in which
decisions are made and actions are taken."