Our laboratory uses near-infrared light to study two distinct signals the brain produces in response to stimuli and cognitive activity; the Event Related Optical Signal (EROS), and Near InfraRed Spectroscopy (NIRS). The measurement of the optical parameters can be conducted non-invasively by using near-infrared (NIR) light which penetrates several cm inside the head. With appropriate methodologies it is possible to focus the measurements to relatively small areas (less than 1 cc) and to distinguish signals from different depths. This yields very good spatial resolution. Other advantages of these techniques are safety (because only a very small amount of non-ionizing radiation is used), relatively low cost, and versatility.

EROS is based on the measurement of the changes in optical parameters (scattering and absorption) of active neurons. Some of these changes occur very rapidly, simultaneously with the electrical activity of the neurons. For this reason EROS has very good temporal resolution. For this reason, EROS can be used to analyze the relative timing of activity in different areas, to study the order of recruitment of different cortical areas, and to examine the connections between areas. These are all questions that are difficult to study with other brain imaging methods. Due to its high temporal resolution, EROS allows the use of the full range of paradigms normally used in cognitive psychology.

NIRS is a widely used technique which measures changes in the absorption of two (or more) wavelengths of light, and allows detection and quantification of changes in the concentration of oxy and deoxy-hemoglobin resulting from brain activity. These relatively slow changes (taking several seconds) are also the basis for fMRI measurements, but NIRS is able to separately estimate changes in blood volume and hemoglobin concentration, which fMRI is not able to do. Because NIRS and EROS measurements can be made simultaneously and they measure virtually identical tissue regions, they provide unique opportunities for studying the relation between neuronal activity and the hemodynamic response.

We routinely record EROS, NIRS and ERPs simultaneously. EROS and NIRS are also compatible with functional magnetic resonance imaging (fMRI) and we are developing methods for making these measurements simultaneously.

Current limitations of EROS include its shallow penetration (~3-4 cm), which makes it particularly useful for studying cortical (rather than sub-cortical) activity, and its signal-to-noise ratio, which typically requires averaging data across a number of subjects. The development of EROS is one of the major lines of research in our Lab, and has been funded by the National Institute of Mental Health. Additional relevant research in this area is conducted at the Laboratory for Fluorescence Dynamics (LFD, University of California, Irvine), directed by Dr. Enrico Gratton.