About Functional Magnetic Resonance Imaging

Functional magnetic resonance imaging (fMRI) is an in-vivo technique that reveals regions of the brain that become active during certain cognitive-behavioral-affective tasks relative to a specified baseline.  Increased oxygen delivery to activated regions based on locally increased blood flow/volume/velocity is revealed using the blood oxygen level dependent (BOLD) effect.  The BOLD effect arises from changes in the local 'magnetic susceptibility' (the extent to which an applied magnetic field is distorted as it interacts with a material) between oxygenated blood (diamagnetic) and deoxygenated blood (paramagnetic). The MRI signal is inversely proportional to deoxygenated blood. The BOLD signal can be assessed for each voxel (volume element) of a brain slice.

For a more in depth explanation of the biophysics underlying fMRI, we recommend: The Basics of MRI: http://www.cis.rit.edu/htbooks/mri/inside.htm 

Introduction and Purpose

Using neuroimaging techniques, and targeting blood flow, glucose metabolism, or D2 receptors, we have previously documented abnormalities in the functioning of the striato-thalamo-orbitofrontal circuit during acute drug intoxication, short-term and protracted withdrawal and during drug craving. This same circuit has been repeatedly implicated in salience attribution/reward processing and inhibitory control in animal and human (neurophysiology, lesion, neuroimaging) studies. Our goal in the current fMRI study was to study the involvement of this circuit in relative reward processing and inhibitory control in cocaine addicted human subjects.  

Method

Cocaine addicted subjects and controls performed a Monetary Incentive Go-No/Go task: subjects either responded or refrained from responding during a trigger stimulus under three salience conditions: high money, low money, and no money.

Results

While the reward circuit has been engaged in the contrast between the high and low money conditions in the control subjects, it was not responsive to this contrast in the cocaine subjects.  Instead, it was involved in the contrast between low money and no money in the drug addicted group.  The high monetary reward was also associated with higher ERP P300 peak amplitudes in another group of controls, implicating stimulus emotional evaluation during this condition.  Indeed, the BOLD results were accompanied by linear increases in self-reported interest, excitement, and value of the higher monetary reward in the control subjects but not in the cocaine group.

Conclusions

These results suggest that drug-related rewards in drug addicted individuals are assigned a constant reward value, possibly reflecting a functional change in the orbitofrontal cortex.

Presented as poster at the International Society for Magnetic Resonance in Medicine, 2003 (Toronto, Canada); to be presented at the Department of Energy Workshop on "Frontiers in Neuroimaging at DOE National Laboratories", Oct 27-29, 2003 (Cambridge, MA); and at a symposium on "The Orbitofronal Cortex in Drug Addiction" at the American College of Neuropsychopharmacology, 2003 (San Juan, Puerto Rico).