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The Journal of Neuroscience, February 9, 2005, 25(6):1503-1513; doi:10.1523/JNEUROSCI.4007-04.2005

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Behavioral/Systems/Cognitive
Representation of Tone in Fluctuating Maskers in the Ascending Auditory System

Liora Las,^1 * Edward A. Stern,^2 * and Israel Nelken¹

¹Department of Neurobiology, Edmund Safra Campus, Givat Ram, and Interdisciplinary Center for Neural Computation, Hebrew University, Jerusalem 91904, Israel, and ²Department of Neurology, Massachusetts General Hospital, Harvard Medical School, Charlestown, Massachusetts 02129

Humans and animals detect low-level tones masked by slowly fluctuating noise very efficiently. A possible neuronal correlate of this phenomenon is the ability of low-level tones to suppress neuronal locking to the envelope of the fluctuating noise ("locking suppression"). Using in vivo intracellular and extracellular recordings in cats, we studied neuronal responses to combinations of fluctuating noise and tones in three successive auditory stations: inferior colliculus (IC), medial geniculate body (MGB), and primary auditory cortex (A1). We found that although the most sensitive responses in the IC were approximately isomorphic to the physical structure of the sounds, with only a small perturbation in the responses to the fluctuating noise after the addition of low-level tones, some neurons in the MGB and all A1 neurons displayed striking suppressive effects. These neurons were hypersensitive, showing suppression already with tone levels lower than the threshold of the neurons in silence. The hypersensitive locking suppression in A1 and MGB had a special timing structure, starting >75 ms after tone onset. Our findings show a qualitative change in the representation of tone in fluctuating noise along the IC-MGB-A1 axis, suggesting the gradual segregation of signal from noise and the representation of the signal as a separate perceptual object in A1.

Key words: auditory cortex; auditory thalamus; inferior colliculus; physiology; cat; comodulation masking release

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Received Sep 28, 2004; revised December 17, 2004; accepted December 23, 2004.

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