Illuminating Deviance Detection: Optical Imaging and the MMN

• ¹ Department of Psychology, National University of Singapore , Singapore

ERP and ERF source localization experiments, as well as PET and fMRI studies, suggest that the generators of the MMN are located in the temporal and frontal cortices. Specifically, infrequent changes in unattended sounds elicit changes in brain activity in the superior temporal cortex (STC) and inferior frontal cortex (IFC). Although there is general agreement that the role of the STC is change-detection, the role of the IFC is subject to ongoing discussion. For example, posited roles of the IFC have included reorientation of attention, contrast enhancement, and inhibition when a response to sound change is not required.

Non-invasive optical brain imaging in humans provides a window on the brain basis of passive deviance detection that nicely complements other brain measurement modalities. As a relatively silent measurement modality, it is ideal for auditory studies and easily lends itself to simultaneous recording of EEG. Optical measures of changes in concentrations of oxygenated and deoxygenated blood (fNIRS), offer relatively good spatial resolution, but the variant of optical imaging that measures light scattering, known as the Event Related Optical Signal (EROS), combines good temporal (ms range) and spatial resolution (mm range). Hence, the EROS technique is a potentially useful tool for clarifying the roles of the STC and IFC in passive change detection.

My colleagues and I have applied the EROS technique to investigate brain function in pitch, stimulus omission, and duration passive oddball paradigms. We consistently find that deviant stimuli elicit signal changes in the STC and IFC during time windows that correspond to that of the electrical MMN and these measures of deviance detection are highly correlated with the electrical MMN. Moreover, we typically find that signal change occurs first in the STC and then in the IFC. However, this temporal-frontal network is dynamically modulated depending on stimulus characteristics (e.g. magnitude of sound change).