Tobias Overath

Encoding of Spectral Correlation over Time in Auditory Cortex

Natural sounds contain multiple spectral components that vary over time. The degree of variation can be characterized in terms of correlation between successive time frames of the spectrum, or as a time window within which any two frames show a minimum degree of correlation: the greater the correlation of the spectrum between successive time frames, the longer the time window. Recent studies suggest differences in the encoding of shorter and longer time windows in left and right auditory cortex, respectively. The present functional magnetic resonance imaging study assessed brain activation in response to the systematic variation of the time window in complex spectra that are more similar to natural sounds than in previous studies. The data show bilateral activity in the planum temporale and anterior superior temporal gyrus as a function of increasing time windows, as well as activity in the superior temporal sulcus that was significantly lateralized to the right. The results suggest a coexistence of hierarchical and lateralization schemes for representing increasing time windows in auditory association cortex.

Sensitivity to temporal modulation rate and spectral bandwidth in the human auditory system: fMRI evidence

Hierarchical models of auditory processing often posit that optimal stimuli, i.e., those eliciting a maximal neural response, will increase in bandwidth and decrease in modulation rate as one ascends the auditory neuraxis. Here, we tested how bandwidth and modulation rate interact at several loci along the human central auditory pathway using functional MRI in a cardiac-gated, sparse acquisition design. Participants listened passively to both narrowband (NB) and broadband (BB) carriers (1/4- or 4-octave pink noise), which were jittered about a mean sinusoidal amplitude modulation rate of 0, 3, 29, or 57 Hz. The jittering was introduced to minimize stimulus-specific adaptation. The results revealed a clear difference between spectral bandwidth and temporal modulation rate: sensitivity to bandwidth (BB > NB) decreased from subcortical structures to nonprimary auditory cortex, whereas sensitivity to slow modulation rates was largest in nonprimary auditory cortex and largely absent in subcortical structures. Furthermore, there was no parametric interaction between bandwidth and modulation rate. These results challenge simple hierarchical models, in that BB stimuli evoked stronger responses in primary auditory cortex (and subcortical structures) rather than nonprimary cortex. Furthermore, the strong preference for slow modulation rates in nonprimary cortex demonstrates the compelling global sensitivity of auditory cortex to modulation rates that are dominant in the principal signals that we process, e.g., speech.

An information theoretic characterisation of auditory encoding.

The entropy metric derived from information theory provides a means to quantify the amount of information transmitted in acoustic streams like speech or music. By systematically varying the entropy of pitch sequences, we sought brain areas where neural activity and energetic demands increase as a function of entropy. Such a relationship is predicted to occur in an efficient encoding mechanism that uses less computational resource when less information is present in the signal: we specifically tested the hypothesis that such a relationship is present in the planum temporale (PT). In two convergent functional MRI studies, we demonstrated this relationship in PT for encoding, while furthermore showing that a distributed fronto-parietal network for retrieval of acoustic information is independent of entropy. The results establish PT as an efficient neural engine that demands less computational resource to encode redundant signals than those with high information content.

The cortical analysis of speech-specific temporal structure revealed by responses to sound quilts

Speech contains temporal structure that the brain must analyze to enable linguistic processing. To investigate the neural basis of this analysis, we used sound quilts, stimuli constructed by shuffling segments of a natural sound, approximately preserving its properties on short timescales while disrupting them on longer scales. We generated quilts from foreign speech to eliminate language cues and manipulated the extent of natural acoustic structure by varying the segment length. Using functional magnetic resonance imaging, we identified bilateral regions of the superior temporal sulcus (STS) whose responses varied with segment length. This effect was absent in primary auditory cortex and did not occur for quilts made from other natural sounds or acoustically matched synthetic sounds, suggesting tuning to speech-specific spectrotemporal structure. When examined parametrically, the STS response increased with segment length up to ∼500 ms. Our results identify a locus of speech analysis in human auditory cortex that is distinct from lexical, semantic or syntactic processes.

fMRI Evidence for a Cortical Hierarchy of Pitch Pattern Processing

Pitch patterns, such as melodies, consist of two levels of structure: a global level, comprising the pattern of ups and downs, or contour; and a local level, comprising the precise intervals that make up this contour. An influential neuropsychological model suggests that these two levels of processing are hierarchically linked, with processing of the global structure occurring within the right hemisphere in advance of local processing within the left. However, the predictions of this model and its anatomical basis have not been tested in neurologically normal individuals. The present study used fMRI and required participants to listen to consecutive pitch sequences while performing a same/different one-back task. Sequences, when different, either preserved (local) or violated (global) the contour of the sequence preceding them. When the activations for the local and global conditions were contrasted directly, additional activation was seen for local processing in right planum temporale and posterior superior temporal sulcus (pSTS). The presence of additional activation for local over global processing supports the hierarchical view that the global structure of a pitch sequence acts as a “framework” on which the local detail is subsequently hung. However, the lateralisation of activation seen in the present study, with global processing occurring in left pSTS and local processing occurring bilaterally, differed from that predicted by the neuroanatomical model. A re-examination of the individual lesion data on which the neuroanatomical model is based revealed that the lesion data equally well support the laterality scheme suggested by our data. While the present study supports the hierarchical view of local and global processing, there is an evident need for further research, both in patients and neurologically normal individuals, before an understanding of the functional lateralisation of local and global processing can be considered established.

The human auditory cortex

1 Introduction: Why Human Auditory Cortex? David Poeppel and Tobias Overath I The Methods 2 Architecture, Connectivity, and Transmitter Receptors of Human Auditory Cortex Stephanie Clarke and Patricia Morosan 3 Invasive Research Methods Matthew A. Howard III, Kirill V. Nourski, and John F. Brugge 4 Recording Event-Related Brain Potentials: Application to Study Auditory Perception Claude Alain and Istvan Winkler 5 Magnetoencephalography Srikantan Nagarajan, Rodney A. Gabriel, and Alexander Herman 6 Hemodynamic Imaging: Functional Magnetic Resonance Imaging Thomas M. Talavage, Ingrid S. Johnsrude, and Javier Gonzalez Castillo II The Principal Computational Challenges 7 Coding of Basic Acoustical and Perceptual Components of Sound in Human Auditory Cortex Deborah Hall and Daphne Barker 8 Auditory Object Analysis Timothy D. Griffiths, Christophe Micheyl, and Tobias Overath 9 Speech Perception from a Neurophysiological Perspective Anne-Lise Giraud and David Poeppel 10 Cortical Processing of Music Robert J. Zatorre and Jean Mary Zarate 11 Multisensory Role of Human Auditory Cortex Virginie van Wassenhove and Charles E. Schroeder 12 Redefining the Functional Organization of the Planum Temporale Region: Space, Objects, and Sensory-Motor Integration Gregory Hickok and Kourosh Saberi 13 Toward a Theory of Information Processing in Auditory Cortex Peter Cariani and Christophe Micheyl

Gamma band pitch responses in human auditory cortex measured with magnetoencephalography

We have previously used direct electrode recordings in two human subjects to identify neural correlates of the perception of pitch (Griffiths, Kumar, Sedley et al., Direct recordings of pitch responses from human auditory cortex, Curr. Biol. 22 (2010), pp. 1128–1132). The present study was carried out to assess virtual-electrode measures of pitch perception based on non-invasive magnetoencephalography (MEG). We recorded pitch responses in 13 healthy volunteers using a passive listening paradigm and the same pitch-evoking stimuli (regular interval noise; RIN) as in the previous study. Source activity was reconstructed using a beamformer approach, which was used to place virtual electrodes in auditory cortex. Time-frequency decomposition of these data revealed oscillatory responses to pitch in the gamma frequency band to occur, in Heschls gyrus, from 60 Hz upwards. Direct comparison of these pitch responses to the previous depth electrode recordings shows a striking congruence in terms of spectrotemporal profile and anatomical distribution. These findings provide further support that auditory high gamma oscillations occur in association with RIN pitch stimuli, and validate the use of MEG to assess neural correlates of normal and abnormal pitch perception.

Auditory Object Analysis

The concept of what constitutes an auditory object is controversial (Kubovy & Van Valkenburg, 2001; Griffi ths & Warren, 2004; Nelken, 2004). It is more difficult to examine the sound pressure waveform that enters the cochlea and “see” different objects in the same way that we “see” objects in the visual input to the retina. However, in both the auditory system and the visual system, objects can be understood in terms of the “images” they produce during the processing of sense data. The idea that objects are mental events that result from the creation of images from sense data goes back to Kant (1929). Visual images, representations in the visual brain corresponding to objects, can be understood as having two spatial dimensions.

Brain network activity subserving tinnitus and normal perception

cancelled P1011 INCREASED METABOLISM AT THE BASAL FOREBRAIN IN INCIPIENT STAGES OF ALZHEIMER’S DISEASE M.-J. Kim, K.-M. Lee Neurology, Seoul National University College of Medicine, Seoul, Republic of Korea Objective: To test whether the cholinergic dysfunction in Alzheimer’s disease (AD) is a primary event, or a retrograde event secondary to neuronal loss at the cholinergic targets. Methods: In vivo neuroimaging markers of neural activity using 18F-fluorodeoxyglucose (FDG) were obtained from 13 patients with mild cognitive impairment (MCI), 20 with early AD, and 14 normal elderly. For a sufficient resolving power for small nuclei in the basal forebrain, we employed a high-resolution research tomograph (HRRT)-PET and a novel method of spatial normalization across subjects based on subcortical volumes of interest defined structures close to the basal forebrain. The markers were correlated with the Mini Mental Status Examination (MMSE) score of the patients. Results: MCI and early AD patients showed a higher uptake of FDG at the basal forebrain than the normal elderly. The uptake showed an “inverted-U” relationship with the MMSE score, in contrast to the linear relationship observed at the precuneus. A cross-voxel analysis over the whole brain revealed that FDG uptake at the medial part within the basal forebrain was significantly correlated with that in cortical and subcortical pathways of cholinergic innervation. Conclusions: Our results demonstrate that neuronal activity at the basal forebrain, as reflected by glucose metabolism, increases in incipient phases of AD, and decreases only with further progression of the disease. This initial increase may represent compensation against neurodegeneration that has started at the target areas and provide cognitive reserve against functional impairment in early stages of the disease.

Introduction: Why Human Auditory Cortex?

This volume concentrates on current approaches to understanding the human auditory cortex. Why auditory? The reasons why one would, could, and should study auditory processing ought to require little comment or motivation. That being said—and given the critical importance of hearing and speech for human communication and welfare—one might wonder why hearing research, in general, has remained the less popular stepchild and ugly duckling in the context of sensory neuroscience, in particular vision. (This question is discussed in more detail later.).