Speaker-normalized sound representations in the human auditory cortex

The acoustic dimensions that distinguish speech sounds (like the vowel differences in “boot” and “boat”) also differentiate speakers’ voices. Therefore, listeners must normalize across speakers without losing linguistic information. Past behavioral work suggests an important role for auditory contrast enhancement in normalization: preceding context affects listeners’ perception of subsequent speech sounds. Here, using intracranial electrocorticography in humans, we investigate whether and how such context effects arise in auditory cortex. Participants identified speech sounds that were preceded by phrases from two different speakers whose voices differed along the same acoustic dimension as target words (the lowest resonance of the vocal tract). In every participant, target vowels evoke a speaker-dependent neural response that is consistent with the listener’s perception, and which follows from a contrast enhancement model. Auditory cortex processing thus displays a critical feature of normalization, allowing listeners to extract meaningful content from the voices of diverse speakers

Neural Dynamics of Phonological Processing in the Dorsal Auditory Stream

Neuroanatomical models hypothesize a role for the dorsal auditory pathway in phonological processing as a feedforward efferent system (Davis and Johnsrude, 2007; Rauschecker and Scott, 2009; Hickok et al., 2011). But the functional organization of the pathway, in terms of time course of interactions between auditory, somatosensory, and motor regions, and the hemispheric lateralization pattern is largely unknown. Here, ambiguous duplex syllables, with elements presented dichotically at varying interaural asynchronies, were used to parametrically modulate phonological processing and associated neural activity in the human dorsal auditory stream. Subjects performed syllable and chirp identification tasks, while event-related potentials and functional magnetic resonance images were concurrently collected. Joint independent component analysis was applied to fuse the neuroimaging data and study the neural dynamics of brain regions involved in phonological processing with high spatiotemporal resolution. Results revealed a highly interactive neural network associated with phonological processing, composed of functional fields in posterior temporal gyrus (pSTG), inferior parietal lobule (IPL), and ventral central sulcus (vCS) that were engaged early and almost simultaneously (at 80–100 ms), consistent with a direct influence of articulatory somatomotor areas on phonemic perception. Left hemispheric lateralization was observed 250 ms earlier in IPL and vCS than pSTG, suggesting that functional specialization of somatomotor (and not auditory) areas determined lateralization in the dorsal auditory pathway. The temporal dynamics of the dorsal auditory pathway described here offer a new understanding of its functional organization and demonstrate that temporal information is essential to resolve neural circuits underlying complex behaviors

Mapping the speech code: Cortical responses linking the perception and production of vowels

The acoustic realization of speech is constrained by the physical mechanisms by which it is produced. Yet for speech perception, the degree to which listeners utilize experience derived from speech production has long been debated. In the present study, we examined how sensorimotor adaptation during production may affect perception, and how this relationship may be reflected in early vs. late electrophysiological responses. Participants first performed a baseline speech production task, followed by a vowel categorization task during which EEG responses were recorded. In a subsequent speech production task, half the participants received shifted auditory feedback, leading most to alter their articulations. This was followed by a second, post-training vowel categorization task. We compared changes in vowel production to both behavioral and electrophysiological changes in vowel perception. No differences in phonetic categorization were observed between groups receiving altered or unaltered feedback. However, exploratory analyses revealed correlations between vocal motor behavior and phonetic categorization. EEG analyses revealed correlations between vocal motor behavior and cortical responses in both early and late time windows. These results suggest that participants' recent production behavior influenced subsequent vowel perception. We suggest that the change in perception can be best characterized as a mapping of acoustics onto articulatio

Cortical mechanisms of seeing and hearing speech

In face-to-face communication speech is perceived through eyes and ears. The talker's articulatory gestures are seen and the speech sounds are heard simultaneously. Whilst acoustic speech can be often understood without visual information, viewing articulatory gestures aids hearing substantially in noisy conditions. On the other hand, speech can be understood, to some extent, by solely viewing articulatory gestures (i.e., by speechreading). In this thesis, electroencephalography (EEG), magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) were utilized to disclose cortical mechanisms of seeing and hearing speech. One of the major challenges of modern cognitive neuroscience is to find out how the brain integrates inputs from different senses. In this thesis, integration of seen and heard speech was investigated using EEG and MEG. Multisensory interactions were found in the sensory-specific cortices at early latencies and in the multisensory regions at late latencies. Viewing other person's actions activate regions belonging to the human mirror neuron system (MNS) which are also activated when subjects themselves perform actions. Possibly, the human MNS enables simulation of other person's actions, which might be important also for speech recognition. In this thesis, it was demonstrated with MEG that seeing speech modulates activity in the mouth region of the primary somatosensory cortex (SI), suggesting that also the SI cortex is involved in simulation of other person's articulatory gestures during speechreading. The question whether there are speech-specific mechanisms in the human brain has been under scientific debate for decades. In this thesis, evidence for the speech-specific neural substrate in the left posterior superior temporal sulcus (STS) was obtained using fMRI. Activity in this region was found to be greater when subjects heard acoustic sine wave speech stimuli as speech than when they heard the same stimuli as non-speech.reviewe