Elia Formisano

Activation of Heschl’s Gyrus during Auditory Hallucinations

Apart from being a common feature of mental illness, auditory hallucinations provide an intriguing model for the study of internally generated sensory perceptions that are attributed to external sources. Until now, the knowledge about the cortical network that supports such hallucinations has been restricted by methodological limitations. Here, we describe an experiment with paranoid schizophrenic patients whose on- and offset of auditory hallucinations could be monitored within one functional magnetic resonance imaging (fMRI) session. We demonstrate an increase of the blood oxygen level-dependent (BOLD) signal in Heschls gyrus during the patients hallucinations. Our results provide direct evidence of the involvement of primary auditory areas in auditory verbal hallucinations and establish novel constraints for psychopathological models.

Exploring brain function with magnetic resonance imaging.

Since its invention in the early 1990s, functional magnetic resonance imaging (fMRI) has rapidly assumed a leading role among the techniques used to localize brain activity. The spatial and temporal resolution provided by state-of-the-art MR technology and its non-invasive character, which allows multiple studies of the same subject, are some of the main advantages of fMRI over the other functional neuroimaging modalities that are based on changes in blood flow and cortical metabolism. This paper describes the basic principles and methodology of fMRI and some aspects of its application to functional activation studies. Attention is focused on the physiology of the blood oxygenation level-dependent (BOLD) contrast mechanism and on the acquisition of functional time-series with echo planar imaging (EPI). We also provide an introduction to the current strategies for the correction of signal artefacts and other image processing techniques. In order to convey an idea of the numerous applications of fMRI, we will review some of the recent results in the fields of cognitive and sensorimotor psychology and physiology.

Functional fields in human auditory cortex revealed by time-resolved fMRI without interference of EPI noise

The gradient switching during fast echoplanar functional magnetic resonance imaging (EPI-fMRI) produces loud noises that may interact with the functional activation of the central auditory system induced by experimental acoustic stimuli. This interaction is unpredictable and is likely to confound the interpretation of functional maps of the auditory cortex. In the present study we used an experimental design which does not require the presentation of stimuli during EPI acquisitions and allows for mapping of the auditory cortex without the interference of scanner noise. The design relies on the physiological delays between the onset, or the end, of stimulation and the corresponding hemodynamic response. Owing to these delays and through a time-resolved acquisition protocol it is possible to analyze the decay of the stimulus-specific signal changes after the cessation of the stimulus itself and before the onset of the EPI-acoustic noise related activation (decay-sampling technique). This experimental design, which might permit a more detailed insight in the auditory cortex, has been applied to the study of the cortical responses to pulsed 1000 Hz sine tones. Distinct activation clusters were detected in the Heschls gyri and the planum temporale, with an increased extension compared to a conventional block-design paradigm. Furthermore, the comparison of the hemodynamic response of the most anterior and the posterior clusters of activation highlighted differential response patterns to the sound stimulation and to the EPI-noise. These differences, attributable to reciprocal saturation effects unevenly distributed over the superior temporal cortex, provided evidence for functionally distinct auditory fields.

Decay-sampling design for echo-planar functional magnetic resonance imaging (fMRI) of the auditory cortex

The main limiting factor to the application of the functional Magnetic Resonance Imaging (fMRI) to the study of the auditory cortex is the presence of the loud background acoustic noise in the MR scanner during functional measurements. In the present work, we propose an averaged single-trial experimental design for EPI-fMRI (decay-sampling design) which does not require the presentation of stimuli during echo-planar acquisitions and allows for mapping of auditory cortex without the interference of scanner noise. We apply the decay-sampling technique to the study of the cortical responses to amplitude modulated tones in healthy volunteers. Results point out the presence, within the auditory cortex, of neuronal clusters that correspond to different models of responses to the stimulus and to the EPI noise. Furthermore, some of these clusters show a clear tonotopic organization.

An evaluation of the auditory cortex response to simple non - speech stimuli through functional MRI

Publisher Summary The application of functional Magnetic Resonance Imaging (MRI) to the evaluation of the human auditory cortex could appear ideal, particularly because of the high spatial resolution potentialities, the absence of radiation hazards, and the easy repeatability of the examination. The main drawback of this application consists in the necessity of a good suppression of the background noise that has high amplitude and a broad frequency range, especially in case of the Echo-planar acquisitions. The chapter verifies the reliability of functional MRI study of acoustic cortex. The chapter suggest that a precise analysis of both the spatial and the temporal features of functional MRI activation could be performed, leading to a better knowledge of the tonotopical organization of the human auditory cortex and to the extraction of a temporal intra- and inter-hemispheric network, responsible for further processing of the auditory information.