vi
between cortical areas Ofdiffering modalities have been mapped. Therefore if the
mechanics of multisensory integration can be elucidated it will have a great impact on
Ourunderstandingofthebrainasawhole.
Since more than half of the cortex is devoted to primary and associative sensory
areas it is highly desirable to be able to monitor activity in the entire cerebrum while
investigating multimodal interactions. Additionally, since our research is focused on
human cortex, invasive techniques that require direct access to the brain or injection of
radionucliotides or contrasting agents are either impractical or impossible with ethical
and legal constraints. For this reason we used BOLD fMRI as our principle tool since it is
safe and noninvasive, and allows us to monitor activity of the entire brain with the level
of resolution that is necessary to resolve different functional brain areas.
For those unfamiliar with the technology, BOLD fMRI (Blood-Oxygenation Level
Dependant functional Magnetic Resonant Imaging) relies on the hemodynamic
response, a biological response to neural activity observed over a hundred years ago
(Roy and Sherrington 1890). When a signal is passed from one neuron to another, thé
Postsynaptic cell releases nitric oxide (NO) into the synaptic cleft. The NO diffuses into
the surrounding tissue and causes the smooth muscle in any blood vessels it encounters
to relax, increasing blood flow to that area. Thegreaterthenumberof neurons
activated, or the greater the degree of individual activation, the greater the amount of
NO release and the greater the hemodynamic response. Therefore measuring the
hemodynamic response provides a strong correlate to the level of neural activity.