Research Methods
Magnetic resonance imaging offers tools to measure white and gray matter architecture in vivo and non-invasively. Diffusion weighted imaging (DWI) measures the incoherent motion of water molecules for every imaged voxel and provides complementary information to conventional MRI on tissue microstructure. DWI has been widely used to investigate white matter because of the relatively coherent organization of axons in fiber bundles that results in a marked diffusion anisotropy, with greater diffusivity occurring along the axonal direction. By measuring the diffusivity in multiple directions, specific algorithms can reconstruct the fiber tracts in the whole brain, modeling structural connectivity in the brain. DWI is particularly well suited for studies of neurological disorders because structural changes in neural tissue, like neuronal cell death and white matter microstructural pathology, are reflected in the shape and size of diffusion profiles.
The coordinated activity of large neuronal ensembles can be measured at the scalp surface with electroencephalography (EEG), providing a convenient way to measure brain activity for research and clinical purposes. Recording EEG activity is completely safe and painless. EEG yields temporal resolution of the order of the millisecond, and can be integrated with during functional magnetic resonance imaging (fMRI) for multimodal imaging investigations.
Functional magnetic resonance imaging (fMRI) is a technique used to measure brain activity. It is based on the detection of changes in the amount of oxygen in the blood as well as its flow. Both are found in response to neural activity. When a brain area is more active it consumes more oxygen. In order to meet this increased demand the blood flow to the active area is increased. FMRI can be used to produce maps of brain activation. These maps show which parts of the brain are involved in a specific process.
The electric fields generated by brain activity are associated with a magnetic field oriented at an orthogonal direction. Importantly, magnetic fields pass unimpeded through the skull, resulting in an undistorted signature of neural activity that can be recorded at the scalp level. Magnetoencephalography (MEG) measures tiny fluctuations in these magnetic fields using an array of extremely sensitive Superconducting QUantum Interference Devices (SQUIDs). This magnetic signal is especially useful for spatial localization of the underlying brain activity, as well as detecting high-frequency oscillations.
Psychophysical testing is a technique to investigate the relationship between (physical) stimuli and perceptual processes. It is based on experiments that seek to determine whether the subject can detect a stimulus, identify it, differentiate between it and another stimulus, or describe the magnitude or nature of this difference. In our lab we usually combine psychophysical testing with neuroscientific methods to observe (MRI, EEG, TMS) and even influence (tDCS, tACS) perception processes in the brain.
Transcranial current stimulation (tCS) encompasses a set of non-invasive brain stimulation methods that use weak currents delivered via electrodes on the scalp to stimulate specific brain areas. The applied current can either be unidirectional direct current (transcranial direct current stimulation; tDCS), sinusoidal alternating current (transcranial alternating current stimulation; tACS) or random noise (transcranial random noise stimulation; tRNS). Transcranial current stimulation can have different modulatory effects on the brain that are dependent on the stimulation paradigm and parameters used. This technique has utility both as a tool in basic science research to provide insight into how the brain functions and as a therapeutic intervention in clinical studies.
Transcranial magnetic stimulation (TMS) is a noninvasive method to cause depolarization or hyperpolarization in the neurons of the brain. TMS uses electromagnetic induction using a rapidly changing magnetic field to induce weak electric currents; this can cause activity in specific or general parts of the brain, that allows studying the functioning and interconnections of the brain.