The Use of fMRI to Assess Multisensory Integration

Abstract

Although scientists have only recently had the tools available to noninvasively study the neural mechanisms of multisensory perceptual processes in humans (Calvert et al. 1999), the study of multisensory perception has had a long history in science (James 1890; Molyneux 1688). Before the advent of neuroimaging techniques, such as functional magnetic resonance imaging (fMRI) and high-density electrical recording, the study of neural mechanisms, using single-unit recording, was restricted to nonhuman animals such as monkeys and cats. These groundbreaking neurophysiological studies established many principles for understanding multisensory processing at the level of single neurons (Meredith and Stein 1983), and continue to improve our understanding of multisensory mechanisms at that level (Stein and Stanford 2008).It is tempting to consider that neuroimaging measurements, like blood oxygenation level–dependent (BOLD) activation measured with fMRI, are directly comparable with findings from single-unit recordings. Although several studies have established clear links between BOLD activation and neural activity (Attwell and Iadecola 2002; Logothetis and Wandell 2004; Thompson et al. 2003), there remains a fundamental difference between BOLD activation and single-unit activity: BOLD activation is measured from the vasculature supplying a heterogeneous population of neurons, whereas single-unit measures are taken from individual neurons (Scannell and Young 1999). The ramifications of this difference are not inconsequential because the principles of multisensory phenomena established using single-unit recording may not apply to population-based neuroimaging data (Calvert et al. 2000). The established principles must be tested theoretically and empirically, and where they fail, they must be replaced with new principles that are specific to the new technique.