How can we locate sounds in closed rooms?

Text source: http://www.bernstein-zentren.de/

Original work: Pecka M, Zahn TP, Saunier-Rebori B, Siveke I, Felmy F, Wiegrebe L, Klug A, Pollak GD, Grothe B (2007) -- Inhibiting the inhibition: a neuronal network for sound localization in reverberant environments. J Neurosci. 27(7):1782-90. (Abstract)

How can we locate sounds in closed rooms?

Scientists from Munich have found the mechanism of echo suppression.

In every natural environment there are echoes - sounds reach the ears not only directly but are also reflected by objects and then reach the hearer from all sides with a short time delay. The fact that we can locate the source of a sound is due to "echo suppression." Information about the direction of the echo is suppressed by the processing of acoustic information in the brain. Scientists from Benedict Grothe's group from the Ludwig-Maximilians Universität Munich and the Bernstein Center for Computational Neuroscience have found out which neuronal circuitry is the basis for these mechanisms in mammals.

We are perfectly able to locate the echo which is produced by shouting in the mountains, which only echoes back after a longer time. In closed rooms however, where the echo is only deferred up to 20 milliseconds the brain suppresses the information about the direction from which the echo comes. Scientists from Grothe's group examined neurons in the "Dorsal Nucleus of the Lateral Lemniscus" (DNLL), an area of the brain, which we know is involved in locating sounds. Sounds which come from the right side are louder in the right ear than in the left one. In the DNLL of the left side of the brain, there are neurons which are stimulated by signals from the right ear and inhibited by signals from the left ear. They only react when sounds come from the right side; therefore we can say that they are "sensitive to direction".

Now scientists could show that these inhibitions by sounds from the left ear continue for up to 20 milliseconds longer than the sounds exists. This is extremely long; normally the duration of the inhibition of a neuron corresponds exactly to the duration of the signal which provoked it. In further experiments scientists could show that this long-term inhibition suppresses the reaction of the cells which are sensitive to direction - they become "deaf" for 20 milliseconds. The source of this long-term inhibition is located in the opposite DNLL. The signal from the left ear makes a detour over the right side of the brain in order to suppress the echo again in the left DNLL. Naturally this process also applies inversely for the neurons, which are sensitive to directions in the right DNLL.

That cells, which are sensitive to direction are deaf to the echo, only partly explains the phenomenon of echo suppression. After all we notice the echo, only the information about the direction is missing. Grothe and his colleagues have shown in a computer model which included further acoustic brain regions, that the echo provokes a neuronal reaction in higher brain regions and that the long-term inhibition in the DNLL only reduces the information about the direction of this perception. In psychophysical experiments with human subjects the scientists could confirm the predictions of their model.