El procesamiento auditivo es la base de la expresión y el procesamiento relacionado con la música. Estimulación Magnética Transcraneal (TMS) se ha utilizado con éxito para estudiar los sistemas cognitivos, sensoriales y motoras, pero rara vez se ha aplicado a la audición. Aquí investigamos TMS combinado con resonancia magnética funcional para comprender la organización funcional de la corteza auditiva.
Auditory cortex pertains to the processing of sound, which is at the basis of speech or music-related processing1. However, despite considerable recent progress, the functional properties and lateralization of the human auditory cortex are far from being fully understood. Transcranial Magnetic Stimulation (TMS) is a non-invasive technique that can transiently or lastingly modulate cortical excitability via the application of localized magnetic field pulses, and represents a unique method of exploring plasticity and connectivity. It has only recently begun to be applied to understand auditory cortical function 2.
An important issue in using TMS is that the physiological consequences of the stimulation are difficult to establish. Although many TMS studies make the implicit assumption that the area targeted by the coil is the area affected, this need not be the case, particularly for complex cognitive functions which depend on interactions across many brain regions 3. One solution to this problem is to combine TMS with functional Magnetic resonance imaging (fMRI). The idea here is that fMRI will provide an index of changes in brain activity associated with TMS. Thus, fMRI would give an independent means of assessing which areas are affected by TMS and how they are modulated 4. In addition, fMRI allows the assessment of functional connectivity, which represents a measure of the temporal coupling between distant regions. It can thus be useful not only to measure the net activity modulation induced by TMS in given locations, but also the degree to which the network properties are affected by TMS, via any observed changes in functional connectivity.
Different approaches exist to combine TMS and functional imaging according to the temporal order of the methods. Functional MRI can be applied before, during, after, or both before and after TMS. Recently, some studies interleaved TMS and fMRI in order to provide online mapping of the functional changes induced by TMS 5-7. However, this online combination has many technical problems, including the static artifacts resulting from the presence of the TMS coil in the scanner room, or the effects of TMS pulses on the process of MR image formation. But more importantly, the loud acoustic noise induced by TMS (increased compared with standard use because of the resonance of the scanner bore) and the increased TMS coil vibrations (caused by the strong mechanical forces due to the static magnetic field of the MR scanner) constitute a crucial problem when studying auditory processing.
This is one reason why fMRI was carried out before and after TMS in the present study. Similar approaches have been used to target the motor cortex 8,9, premotor cortex 10, primary somatosensory cortex 11,12 and language-related areas 13, but so far no combined TMS-fMRI study has investigated the auditory cortex. The purpose of this article is to provide details concerning the protocol and considerations necessary to successfully combine these two neuroscientific tools to investigate auditory processing.
Previously we showed that repetitive TMS (rTMS) at high and low frequencies (resp. 10 Hz and 1 Hz) applied over the auditory cortex modulated response time (RT) in a melody discrimination task 2. We also showed that RT modulation was correlated with functional connectivity in the auditory network assessed using fMRI: the higher the functional connectivity between left and right auditory cortices during task performance, the higher the facilitatory effect (i.e. decreased RT) observed with rTMS. However those findings were mainly correlational, as fMRI was performed before rTMS. Here, fMRI was carried out before and immediately after TMS to provide direct measures of the functional organization of the auditory cortex, and more specifically of the plastic reorganization of the auditory neural network occurring after the neural intervention provided by TMS.
Combined fMRI and TMS applied over the auditory cortex should enable a better understanding of brain mechanisms of auditory processing, providing physiological information about functional effects of TMS. This knowledge could be useful for many cognitive neuroscience applications, as well as for optimizing therapeutic applications of TMS, particularly in auditory-related disorders.
Se describe un protocolo que combina TMS sin conexión y resonancia magnética funcional para estudiar la organización funcional de la corteza auditiva. En las secciones siguientes, vamos a discutir los aspectos metodológicos a tener en cuenta al llevar a cabo este enfoque.
Adquisición y calendario de post-TMS sesión de fMRI
Orden de adquisición exploraciones y contrapeso de las sesiones de resonancia magnética funcional antes y después de TMS-
<…The authors have nothing to disclose.
CIBC comunión (JA) y NSERC beca (RZ). Damos las gracias a M. Roch Comeau (Brainsight) por su ayuda con respecto a la cámara de infrarrojos, los seguidores de RM compatibles y soporte hardware. También damos las gracias a Brian Hynes (Hybex Innovations Inc.) que diseñó el brazo multi-articulado para que el titular de la bobina y proporcionó algunas de las cifras que aparecen en el video. Y un agradecimiento especial a todos los técnicos de RM y M. Ferreira del Cerebro Centro McConnell Imágenes del Instituto Neurológico de Montreal que nos ayudó a optimizar el diseño del experimento.
Material Name | Type | Company |
Transcranial magnetic stimulation | Magstim super Rapid2 stimulator, Rapid-2 Plus One Module | Magstim Ltd., Wales, UK |
Coil for magnetic stimulation | MRI-compatible 70 mm figure-of-eight-coil | Magstim Ltd., Wales, UK |
Magnetic resonance imaging | 3-T Siemens Trio scanner, 32-channel Head Coil | Siemens, Inc., Germany |
Frameless Stereotaxy | Brainsight | Rogue Research Inc., Montreal, Canada |
Optical measurement system | Polaris Spectra | Northern Digital Inc, Ontario, Canada |
Multi-jointed arm for coil holder | Standard | Hybex Innovations Inc., Anjou, Canada |
MRI-Compatible Insert Earphones | Sensimetrics, Model S14 | Sensimetrics Corporation, MA, USA |