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

Abstract

In natural environments, a sound can be heard as stable despite the presence of other occasionally louder sounds. For example, when a portion in a voice is replaced by masking noise, the interrupted voice may still appear illusorily continuous. Previous research found that continuity illusions of simple interrupted sounds, such as tones, are accompanied by weaker activity in the primary auditory cortex (PAC) during the interruption than veridical discontinuity percepts of these sounds. Here, we studied whether continuity illusions of more natural and more complex sounds also emerge from this mechanism. We used psychophysics and functional magnetic resonance imaging in humans to measure simultaneously continuity ratings and blood oxygenation level-dependent activity to vowels that were partially replaced by masking noise. Consistent with previous results on tone continuity illusions, we found listeners' reports of more salient vowel continuity illusions associated with weaker activity in auditory cortex (compared with reports of veridical discontinuity percepts of physically identical stimuli). In contrast to the reduced activity to tone continuity illusions in PAC, this reduction was localized in the right anterolateral Heschl's gyrus, a region that corresponds more to the non-PAC. Our findings suggest that the ability to hear differently complex sounds as stable during other louder sounds may be attributable to a common suppressive mechanism that operates at different levels of sound representation in auditory cortex.

Hearing an Illusory Vowel in Noise: Suppression of Auditory Cortical Activity

“…lateralized, presumably nonprimary, AC region on alHG (Riecke et al, 2011a) that may be sensitive to vocal pitch (Lattner et al, 2005). Consistently, this region revealed offset suppression associated with vowel restoration in our study.…”

“…The offset suppression could also be observed in the anterolateral portion of Heschl's gyrus (alHG) (Fig. 6 B, C, bottom), a region that has been associated with vowel continuity illusions in an fMRI study using similar methods as here (Riecke et al, 2011a).…”

“…This region was in the vicinity of a region that showed a corresponding effect for the restoration of a simple tone ( p Ͻ 0.001; circle) in an accompanying EEG study (Riecke et al, 2009b). The red patch delineates the alHG that showed a corresponding effect for restored vowels ( p Ͻ 0.05) in a similar fMRI study (Riecke et al, 2011a). STS, Superior temporal sulcus; HG, Heschl's gyrus.…”

“…Thus, suppression of onset/offset-evoked central responses may be a prerequisite for auditory restoration. This putative mechanism could also be involved in the restoration of ecologically valid sounds: the no-discontinuity rule has been validated behaviorally for speech (Başkent et al, 2009), and continuity illusions of vowels and speech have been associated with changes in hemodynamic activity in temporal and parietal cortices (Shahin et al, 2009;Riecke et al, 2011a), although neural on/off responses could not be resolved in these studies. Therefore, evidence for a role of onset/ offset suppression for the restoration of ecologically valid sounds is sparse (Shahin et al, 2012).…”

Reduced low‐frequency power and phase locking reflect restoration in the auditory continuity illusion

“…Weaker BOLD responses were consistently observed when participants reported the continuity illusion (vs. veridical discontinuity) given the same acoustic input. For the restoration of more complex speech signals, higher-order and language-relevant areas in both the frontal and auditory cortex are recruited (Heinrich et al, 2008;Shahin et al, 2009;Riecke et al, 2011). Notably, the influence of top-down processing in the auditory continuity illusion is restricted, even for speech-like stimuli: the BOLD activity in the superior and medial temporal gyri, associated with illusory vowel continuity, is not influenced by attention.…”