Abstract
BackgroundDecoding of frequency-modulated (FM) sounds is essential for phoneme identification. This study investigates selectivity to FM direction in the human auditory system.Methodology/Principal FindingsMagnetoencephalography was recorded in 10 adults during a two-tone adaptation paradigm with a 200-ms interstimulus-interval. Stimuli were pairs of either same or different frequency modulation direction. To control that FM repetition effects cannot be accounted for by their on- and offset properties, we additionally assessed responses to pairs of unmodulated tones with either same or different frequency composition. For the FM sweeps, N1m event-related magnetic field components were found at 103 and 130 ms after onset of the first (S1) and second stimulus (S2), respectively. This was followed by a sustained component starting at about 200 ms after S2. The sustained response was significantly stronger for stimulation with the same compared to different FM direction. This effect was not observed for the non-modulated control stimuli.Conclusions/SignificanceLow-level processing of FM sounds was characterized by repetition enhancement to stimulus pairs with same versus different FM directions. This effect was FM-specific; it did not occur for unmodulated tones. The present findings may reflect specific interactions between frequency separation and temporal distance in the processing of consecutive FM sweeps.
Highlights
To identify complex acoustic stimuli such as speech sounds, the auditory system has to process different components of the sound pattern in a fast and precise way
Successful decoding of frequency variations and FM sweeps is essential for phoneme identification [7,8,9]
While most neurons respond to a broad range of modulation rates and to both upward and downward FM sweeps [12,13], selectivity for the direction of FM sweeps could be found along the tonotopic gradient in the monkey auditory cortex
Summary
To identify complex acoustic stimuli such as speech sounds, the auditory system has to process different components of the sound pattern in a fast and precise way. Recent magnetoencephalography (MEG), functional magnetic resonance imaging (fMRI) and psychophysical studies have investigated the processing of complex sounds such as animal vocalizations or human speech sounds [1,2,3]. These vocalizations vary in a number of properties, for example in terms of amplitude, frequency, and modulation rate [4]. While most neurons respond to a broad range of modulation rates and to both upward and downward FM sweeps [12,13], selectivity for the direction of FM sweeps could be found along the tonotopic gradient in the monkey auditory cortex. This study investigates selectivity to FM direction in the human auditory system
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