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1.  A brain basis for musical hallucinations☆ 
The physiological basis for musical hallucinations (MH) is not understood. One obstacle to understanding has been the lack of a method to manipulate the intensity of hallucination during the course of experiment. Residual inhibition, transient suppression of a phantom percept after the offset of a masking stimulus, has been used in the study of tinnitus. We report here a human subject whose MH were residually inhibited by short periods of music. Magnetoencephalography (MEG) allowed us to examine variation in the underlying oscillatory brain activity in different states. Source-space analysis capable of single-subject inference defined left-lateralised power increases, associated with stronger hallucinations, in the gamma band in left anterior superior temporal gyrus, and in the beta band in motor cortex and posteromedial cortex. The data indicate that these areas form a crucial network in the generation of MH, and are consistent with a model in which MH are generated by persistent reciprocal communication in a predictive coding hierarchy.
doi:10.1016/j.cortex.2013.12.002
PMCID: PMC3969291  PMID: 24445167
Musical hallucinations; Magnetoencephalography; Auditory cortex; Gamma oscillations; Beta oscillations; Predictive coding
2.  Predictive coding and pitch processing in the auditory cortex 
Journal of cognitive neuroscience  2011;23(10):10.1162/jocn_a_00021.
In this work, we show that electrophysiological responses during pitch perception are best explained by distributed activity in a hierarchy of cortical sources and, crucially, that the effective connectivity between these sources is modulated with pitch-strength. Local field potentials were recorded in two subjects from primary auditory cortex and adjacent auditory cortical areas along the axis of Heschl's gyrus (HG) while they listened to stimuli of varying pitch strength. Dynamic Causal Modelling was used to compare system architectures that might explain the recorded activity. The data show that representation of pitch requires an interaction between non-primary and primary auditory cortex along HG that is consistent with the principle of predictive coding.
doi:10.1162/jocn_a_00021
PMCID: PMC3821983  PMID: 21452943
3.  Do cortical gamma oscillations promote or suppress perception? An under-asked question with an over-assumed answer 
Cortical gamma oscillations occur alongside perceptual processes, and in proportion to perceptual salience. They have a number of properties that make them ideal candidates to explain perception, including incorporating synchronized discharges of neural assemblies, and their emergence over a fast timescale consistent with that of perception. These observations have led to widespread assumptions that gamma oscillations' role is to cause or facilitate conscious perception (i.e., a “positive” role). While the majority of the human literature on gamma oscillations is consistent with this interpretation, many or most of these studies could equally be interpreted as showing a suppressive or inhibitory (i.e., “negative”) role. For example, presenting a stimulus and recording a response of increased gamma oscillations would only suggest a role for gamma oscillations in the representation of that stimulus, and would not specify what that role were; if gamma oscillations were inhibitory, then they would become selectively activated in response to the stimulus they acted to inhibit. In this review, we consider two classes of gamma oscillations: “broadband” and “narrowband,” which have very different properties (and likely roles). We first discuss studies on gamma oscillations that are non-discriminatory, with respect to the role of gamma oscillations, followed by studies that specifically support specifically a positive or negative role. These include work on perception in healthy individuals, and in the pathological contexts of phantom perception and epilepsy. Reference is based as much as possible on magnetoencephalography (MEG) and electroencephalography (EEG) studies, but we also consider evidence from invasive recordings in humans and other animals. Attempts are made to reconcile findings within a common framework. We conclude with a summary of the pertinent questions that remain unanswered, and suggest how future studies might address these.
doi:10.3389/fnhum.2013.00595
PMCID: PMC3778316  PMID: 24065913
gamma oscillations; magnetoencephalography; electroencephalography; perception; inhibition; tinnitus; epilepsy
4.  Single-subject oscillatory gamma responses in tinnitus 
Brain  2012;135(10):3089-3100.
This study used magnetoencephalography to record oscillatory activity in a group of 17 patients with chronic tinnitus. Two methods, residual inhibition and residual excitation, were used to bring about transient changes in spontaneous tinnitus intensity in order to measure dynamic tinnitus correlates in individual patients. In residual inhibition, a positive correlation was seen between tinnitus intensity and both delta/theta (6/14 patients) and gamma band (8/14 patients) oscillations in auditory cortex, suggesting an increased thalamocortical input and cortical gamma response, respectively, associated with higher tinnitus states. Conversely, 4/4 patients exhibiting residual excitation demonstrated an inverse correlation between perceived tinnitus intensity and auditory cortex gamma oscillations (with no delta/theta changes) that cannot be explained by existing models. Significant oscillatory power changes were also identified in a variety of cortical regions, most commonly midline lobar regions in the default mode network, cerebellum, insula and anterior temporal lobe. These were highly variable across patients in terms of areas and frequency bands involved, and in direction of power change. We suggest a model based on a local circuit function of cortical gamma-band oscillations as a process of mutual inhibition that might suppress abnormal cortical activity in tinnitus. The work implicates auditory cortex gamma-band oscillations as a fundamental intrinsic mechanism for attenuating phantom auditory perception.
doi:10.1093/brain/aws220
PMCID: PMC3470708  PMID: 22975389
tinnitus; gamma oscillations; mutual inhibition; auditory cortex; magnetoencephalography
5.  Gamma band pitch responses in human auditory cortex measured with magnetoencephalography 
Neuroimage  2012;59(2-5):1904-1911.
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 Heschl's 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.
Highlights
► High gamma-band correlates of pitch perception identified with MEG beamforming. ► Results correlate strongly with invasive electrode recordings of same responses. ► Validation of accuracy of MEG beamformer approach.
doi:10.1016/j.neuroimage.2011.08.098
PMCID: PMC3236996  PMID: 21925281
Pitch; Auditory; Magnetoencephalography; Gamma; Beamformer; Perception
6.  Direct Recordings of Pitch Responses from Human Auditory Cortex 
Current Biology  2010;20(12):1128-1132.
Summary
Pitch is a fundamental percept with a complex relationship to the associated sound structure [1]. Pitch perception requires brain representation of both the structure of the stimulus and the pitch that is perceived. We describe direct recordings of local field potentials from human auditory cortex made while subjects perceived the transition between noise and a noise with a regular repetitive structure in the time domain at the millisecond level called regular-interval noise (RIN) [2]. RIN is perceived to have a pitch when the rate is above the lower limit of pitch [3], at approximately 30 Hz. Sustained time-locked responses are observed to be related to the temporal regularity of the stimulus, commonly emphasized as a relevant stimulus feature in models of pitch perception (e.g., [1]). Sustained oscillatory responses are also demonstrated in the high gamma range (80–120 Hz). The regularity responses occur irrespective of whether the response is associated with pitch perception. In contrast, the oscillatory responses only occur for pitch. Both responses occur in primary auditory cortex and adjacent nonprimary areas. The research suggests that two types of pitch-related activity occur in humans in early auditory cortex: time-locked neural correlates of stimulus regularity and an oscillatory response related to the pitch percept.
Highlights
► We report direct recordings of electrical activity from human auditory cortex ► We distinguish activity related to stimulus regularity and to perceived pitch ► Both are demonstrated in primary cortex and adjacent “core” areas
doi:10.1016/j.cub.2010.04.044
PMCID: PMC3221038  PMID: 20605456
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