Sound Repetition Rate in the Human Auditory Pathway: Representations in the Waveshape and Amplitude of fMRI Activation

Abstract

Sound repetition rate plays an important role in stream segregation, temporal pattern recognition, and the perception of successive sounds as either distinct or fused. This study was aimed at elucidating the neural coding of repetition rate and its perceptual correlates. We investigated the representations of rate in the auditory pathway of human listeners using functional magnetic resonance imaging (fMRI), an indicator of population neural activity. Stimuli were trains of noise bursts presented at rates ranging from low (1-2/s; each burst is perceptually distinct) to high (35/s; individual bursts are not distinguishable). There was a systematic change in the form of fMRI response rate-dependencies from midbrain to thalamus to cortex. In the inferior colliculus, response amplitude increased with increasing rate while response waveshape remained unchanged and sustained. In the medial geniculate body, increasing rate produced an increase in amplitude and a moderate change in waveshape at higher rates (from sustained to one showing a moderate peak just after train onset). In auditory cortex (Heschl's gyrus and the superior temporal gyrus), amplitude changed somewhat with rate, but a far more striking change occurred in response waveshape-low rates elicited a sustained response, whereas high rates elicited an unusual phasic response that included prominent peaks just after train onset and offset. The shift in cortical response waveshape from sustained to phasic with increasing rate corresponds to a perceptual shift from individually resolved bursts to fused bursts forming a continuous (but modulated) percept. Thus at high rates, a train forms a single perceptual "event," the onset and offset of which are delimited by the on and off peaks of phasic cortical responses. While auditory cortex showed a clear, qualitative correlation between perception and response waveshape, the medial geniculate body showed less correlation (since there was less change in waveshape with rate), and the inferior colliculus showed no correlation at all. Overall, our results suggest a population neural representation of the beginning and the end of distinct perceptual events that is weak or absent in the inferior colliculus, begins to emerge in the medial geniculate body, and is robust in auditory cortex.

Listening in Silence Activates Auditory Areas: A Functional Magnetic Resonance Imaging Study

“…Such a strong contralateral selectivity in these temporal auditory regions has been reported previously using monaural stimulation (Jäncke et al, 2002) and in selective orientation to one ear during dichotic listening (Hall et al, 2000;Jäncke et al, 2003). Our results probably refer to processes distinct from those corresponding to an increased activity in the superior temporal gyrus after regular patterns of sound (Hughes et al, 2001;Mustovic et al, 2003), at the offset of long-duration sound (Jäncke et al, 1999;Harms and Melcher, 2002;Seifritz et al, 2002), or induced by a brief period of silence (Kraemer et al, 2005) interpreted as auditory imagery. Auditory imagery has been also shown to increase activity in the auditory areas (Bunzeck et al, 2005;Zatorre and Halpern, 2005) and in the right inferior frontal areas (Hoshiyama et al, 2001).…”

Human Cortical Activity during Streaming without Spectral Cues Suggests a General Neural Substrate for Auditory Stream Segregation

“…The ABBB pattern used in the present study offers both advantages of the triplet pattern, because the only difference is that the silent gap between triplets has been filled with another B tone. Filling the gap was important for the fMRI part of this study, in which we examined the time course as well as the magnitude of blood oxygen level-dependent (BOLD) responses, because actual (or perceived) gaps lead to changes in BOLD time course (Harms and Melcher, 2002;Wilson et al, 2007). By ensuring no physical temporal gaps, we were able to investigate the effect perceived temporal gaps introduced when the percept shifts from one to two streams.…”

Illusory Vowels Resulting from Perceptual Continuity: A Functional Magnetic Resonance Imaging Study

“…According to previous studies (Herdener et al, 2007;Hart et al, 2003;Harms & Melcher, 2002), the locus of peak activation can depend on the actual rate of stimulation and can shift from HG for high rates of stimulation to the STG lateral and posterior of HG for lower rates. For instance, Hart et al (2003) compared sustained harmonic complexes and tones to complexes with sound onset rates of 5 Hz.…”

“…For example, in a preliminary report, Cusack, Carlyon, Johnsrude, and Epstein (2001) showed greater activation in HG for tones presented to the two ears when there was a small onset disparity between them, resulting in the percept of two sounds, than when they started synchronously, leading to the percept of a single sound. More recently, Harms, Guinan, Sigalovsky, and Melcher (2005) and Harms and Melcher (2002) have shown greater activation in HG for bursts of white noise presented with fast (8-10 Hz) onset rates, compared to slower rates, whereas the same pattern was not observed in the STG. Other studies have also shown relatively greater activation in HG to sounds with high onset rates, with a different pattern of results observed in regions closer to the STG (Herdener et al, Figure 1.…”