Brain area that amplifies behaviourally significant speech sounds identified

November 22nd, 2007 - 4:14 pm ICT by admin  

Washington, November 22 (ANI): US researchers have identified the brain region that selectively amplifies behaviourally significant speech sounds, such as recognising the unmistakable sound of a spouse calling ones name.

Rajeev Raizada of the Institute for Learning and Brain Sciences, University of Washington, and Russell A. Poldrack of the Department of Psychology at the UCLA say that their experimental approach represents a useful advance because it not only detects brain activity associated with a given task, but also identifies the type of computation the brain is performing during the task.

During the study, the researchers used a set of ten computer-synthesized speech sounds that represented a continuum from the sound ba to the sound da. They designated these on a 1 to 10 scale, with the former being pure ba and the latter being pure da.

The volunteers were first made to hear different pairs of the two sounds, for example 4 and 7, and asked to indicate which pairs they distinguished as different from one another.

The researchers next played the same pairs of sounds to the subjects as the subjects brains were scanned using functional magnetic resonance imaging (fMRI), which is used to measure blood flow in brain regions.

The scans revealed a sensory processing area of the brain known as the left supramarginal gyrus as a distinctive categorical processing area.

According to the researchers, the brain region activated when subjects heard sound pairs that they had earlier reported perceiving as different, while it did not respond significantly to sound pairs that the subjects perceived as the same.

The researchers also found that lower-level auditory regions of the brain or another speech-processing area did not respond significantly to the distinctive sound pairs.

A key aspect of the method proposed here, and one that is especially relevant to categorical perception, is that we were seeking not just neural amplification per se, but in particular selective amplification. Thus, an area such as the left supramarginal gyrus not only amplified differences between the phonetic stimuli, but moreover it specifically amplified only the differences that corresponded to crossing each subjects perceptual category boundary, the researchers wrote.

The way in which the brain selectively amplifies stimulus differences can help to reveal how its representations of the world are structured, they wrote. Such amplification can be said to be involved in a neural representation, as opposed to being just incidental activity, only if it is related to perception and behavior. Used together, these tools can help to reveal when the brain sees the world in shades of gray, and when it sees in black-and-white, the added.

The study has been published in the journal Neuron, published by Cell Press. (ANI)

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