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

Abstract

The brain continuously disentangles competing sounds, such as two people speaking, and assigns them to distinct streams. Neural mechanisms have been proposed for streaming based on gross spectral differences between sounds, but not for streaming based on other nonspectral features. Here, human listeners were presented with sequences of harmonic complex tones that had identical spectral envelopes, and unresolved spectral fine structure, but one of two fundamental frequencies ( f 0 ) and pitches. As the f 0 difference between tones increased, listeners perceived the tones as being segregated into two streams (one stream for each f 0 ) and cortical activity measured with functional magnetic resonance imaging and magnetoencephalography increased. This trend was seen in primary cortex of Heschl's gyrus and in surrounding nonprimary areas. The results strongly resemble those for pure tones. Both the present and pure tone results may reflect neuronal forward suppression that diminishes as one or more features of successive sounds become increasingly different. We hypothesize that feature-specific forward suppression subserves streaming based on diverse perceptual cues and results in explicit neural representations for auditory streams within auditory cortex.

Grouping Mechanisms in Music

“…Later, Gutschalk, Oxenham, Micheyl, Wilson, and Melcher (2007) presented human subjects with ABBB sequences consisting of harmonic complex tones with varying fundamental frequencies but identical spectral envelopes. As the pitch separation between the alternating tones increased, the subjects were more likely to hear two streams.…”

“…A number of studies exploring the neural correlates of streaming based on pitch have found either no difference in activation between the left and right hemispheres, or activation primarily in the right hemisphere (Cusack, 2005;Gutschalk et al, 2005Gutschalk et al, , 2007Snyder et al, 2006;Wilson et al, 2007). In contrast, Deike, Gaschler-Markefski, Brechmann, and Scheich (2004) and Deike, Scheich, and Brechmann (2010) found activation primarily in the left hemisphere when subjects were asked to segregate A from B tones continuously in sequences where the tones differed in timbre or in pitch.…”

Tracking Vocal Pitch through Noise: Neural Correlates in Nonprimary Auditory Cortex

“…jneurosci.org as supplemental material). Such continuity illusion-related reductions were presumably caused by forward suppression (Wehr and Zador, 2005) or adaptation (Ulanovsky et al, 2003(Ulanovsky et al, , 2004 because these mechanisms in AC have been implicated also in possibly related auditory phenomena (Bregman et al, 1999) such as the integration of simple (Micheyl et al, 2005) or complex (Gutschalk et al, 2007) sound streams. It is conceivable that, in the case of continuity illusions, the early portion of the interrupted sound partially suppresses the neural response to the middle portion, provided that this middle portion is masked (see Introduction).…”

Active stream segregation specifically involves the left human auditory cortex

“…This enhancement also correlated with behavioral reports of stream segregation. Similarly, Gutschalk et al (2007) described an increase of the P1m component for the A tone with increasing separation in fundamental frequencies (f 0 ) between harmonic complex tones in repeating ABBB_ sequences where A and B denote tones with different f 0 . In the second experiment of the earlier MEG study, Gutschalk et al (2005) directly compared magnetic fields evoked by B tones of a certain perceptually ambiguous ABA_ sequence when the subjects perceived two separate streams and when the subjects perceived one integrated stream.…”

“…They found a larger N1m amplitude during the perceptual organization of two separate streams both when the subjects followed the A or B tones and a larger P1 amplitude when the subjects followed the B tones. Gutschalk et al (2005Gutschalk et al ( , 2007 and Snyder et al (2006) suggested that reduced forward suppression may be the neuronal mechanism underlying the enhanced long latency components/fields during the perception of two segregated streams. The dipoles of these components/fields were located in the nonprimary auditory cortex.…”