How our hearing is based on the frequencies of sounds - this is the wonder that scientists are close to understanding.
A team of scientists led by Anthony Zador of the Cold Spring Harbor Laboratory (CSHL) probed how the functional connectivity among neurons within the auditory cortex gives rise to a 'map' of acoustic space.
"What we learned from this approach has put us in a position to investigate and understand how sound responsiveness arises from the underlying circuitry of the auditory cortex," Zador said.
Neuronal organization within the auditory cortex fundamentally differs from the organization within brain regions that process sensory inputs such as sight and sensation.
In the auditory system, the organization of sound receptors in the cochlea - the snail-like structure in the ear - is one-dimensional. Cochlear receptors near the outer edge recognize low-frequency sounds whereas those whereas those near the inside of the cochlea are tuned to higher frequencies.
"Because sound is intrinsically a one-dimensional signal, unlike signals for other senses such as sight and sensation which are intrinsically two-dimensional, the map of sound in the auditory cortex is also intrinsically one-dimensional," explained Zador.
Hysell Oviedo compared neuronal activity in mouse brain slices that were cut to preserve the connectivity along the tonotopic axis vs. activity in slices that were cut perpendicular to it.
To precisely stimulate a single neuron within a slice and record from it, scientists used a powerful tool called laser-scanning photostimulation.
"If you did this experiment in the visual cortex, you would see that the connectivity is the same regardless of which way you cut the slice.
"But in our experiments in the auditory cortex slices, we found that there was a qualitative difference in the connectivity between slices cut along the tonotopic axis vs. those cut perpendicular to it," said Oviedo.
Analogously, in the auditory cortex, neurons within a column get tuned to the same frequency.
"It comes from neurons that we think are tuned to higher frequencies. This is the first example of the neuronal organizing principle not following the columnar pattern, but rather an out-of-column pattern," elaborated Zador.
The findings appeared in the journal Nature Neuroscience.