Research using MRI

Magnetic Resonance Imaging (MRI) is a powerful tool for obtaining 3-dimensional renderings of the human brain – both its structures (anatomical MRI) and its functioning (functional MRI or fMRI). Another advantage of using MRI for research is that it is a safe technology in that it does not use ionizing radiation (as in X-rays). The MRI facility used by members of Dr. Jolicoeur's team of researchers is located at UNF (Unité de neuroimagerie fonctionelle) de l'Université de Montréal).

MEG overlaid on MRI

MRI is particularly well suited for producing scans of soft tissue and the brain because it sensitive to the concentration of hydrogen atoms such as found in water (H20). Water comprises the majority of the tissue of our bodies. MRI systems incorporate a very strong magnet to align the axes of hydrogen nuclei (protons) to a known starting orientation. A radio frequency transmitter produces an electromagnetic field which flips some of the proton orientations. Once the transmitter is turned off, the protons return to their original state, releasing photons of particular frequencies and at different rates depending on the type of tissue.

Arrays of coils modify the fixed magnetic field by varying degrees at predetermined times during each scan, producing gradients which change in a systematic way, thereby giving each signal a signature frequency that identifies its position in 3-dimensional space. A 3-D anatomical representation of the variations in the soft tissue is then assembled from many 2-D 'slices' of the measured signals (See demonstration).

Functional MRI is essentially measuring the changes in oxygen level of the hemoglobin in blood, whose flow varies with the level of neural activity (deoxygenated blood attentuates the above anatomical signals). This enables scientists to determine what parts of the brain are active and which are inactive.

Several studies in Dr. Jolicoeur's lab have taken advantage of this technology. Some examples are noted below.

In the comprehensive work by Robitaille et al. (2010), Distinguishing between lateralized and nonlateralized brain activity associated with visual short-term memory: fMRI, MEG, and EEG evidence from the same observers.. NeuroImage (in press) the authors used data collected from three different technologies to study VSTM and found both overlaps and differences in the results.

A study that used fMRI as the primary source of data was published by Harrison et al. (2010). The full citation is: Harrison, A., Jolicoeur, P., & Marois, R. (2010). `What' and `where' in the intraparietal sulcus: An fMRI study of object identity and location in visual short-term memory. Cerebral Cortex, in press.

The paper by Robitaille et al.(2009) shows results from an experiment using magnetoencephalography overlaid onto the structures provided by anatomical MRI of subjects, thus providing 3-D representations of the brain activity at different points in time. A layman's summary of this work can be found at Visual Short-Term Memory VSTM and MEG. A snapshot of this kind of overlay is shown above.

Other work that combined MEG and fMRI was presented in:
Grimault, S., Lefebvre, C., Vachon, F., Peretz, I., Zatorre, R., Robitaille, N., & Jolicoeur, P. (2009). Load-dependent brain activity related to acoustic short-term memory for pitch: Magnetoencephalography and fMRI. Published conference proceedings for the Neurosciences and Music - III Conference, in Annals of the New York Academy of Sciences, 1169, 273-277.

In some cases, the results are projected onto an average brain structure obtained from anatomical MRI scans of numerous persons. For an example of this, see the last figure in: the layman's summary, Oscillatory Activity in Visual Short-Term Memory (VSTM), of Grimault et al. (2009).