MRI was extended to the measurement of changes in oxidative metabolism in the normal human during functionally induced changes in cellular activity. A noninvasive MRI method that is model-independent calibrates the blood oxygen level dependent (BOLD) signal of functional MRI (fMRI) against perfusion-sensitive MRI, using carbon dioxide breathing as a physiological reference standard. This calibration procedure provides a regional measurement of the expected sensitivity of the fMRI BOLD signal to changes in the cellular activity of the brain. Maps of the BOLD signal calibration factor showed regional heterogeneity, indicating that the magnitude of functionally induced changes in the BOLD signal will be dependent on both the local change in blood f low and the local baseline physiology of the cerebral cortex. BOLD signal magnitude is shown to be reduced by 32% from its expected level by the action of oxygen metabolism. The calibrated fMRI technique was applied to stimulation of the human visual cortex with an alternating radial checkerboard pattern. With this stimulus oxygen consumption increased 16% whereas blood f low increased 45%. Although this result is consistent with previous findings of a significant difference between the increase in blood f low and oxygen consumption, it does indicate clearly that oxidative metabolism is a significant component of the metabolic response of the brain to functionally induced changes in cellular activity.Functionally related changes in neuronal activity in the normal brain are reliably accompanied by changes in local cerebral blood flow (CBF) (1). The degree to which the cerebral metabolic rate for oxygen (CMR O2 ) also changes with activityrelated increases and decreases in neuronal activity remains controversial. Although some reports show little or no taskinduced increase in CMR O2 (2-4), others have shown varying degrees of coupling of oxidative metabolism to glucose consumption and blood flow (5-7), both of which increase dramatically with task activation (3,4,8). Some have suggested that the reason CBF changes more than CMR O2 during functionally related increases in neuronal activity (decreases have not been addressed) is to enhance the diffusion-limited delivery of oxygen to the tissue (9-11).Current functional MRI (fMRI) methods rely on the fact that CBF changes more than CMR O2 , producing localized changes in tissue oxygen content. These localized changes in tissue oxygen content change magnetic fields in a manner that can be detected with MRI. This signal has been termed the blood oxygen level dependent (BOLD) signal of fMRI (12, 13). The fMRI BOLD signal has been viewed as a reasonable marker of functionally related changes in neuronal activity. The magnitude of the BOLD signal, of course, is dependent on the relationship between changes, if any, in CMR O2 and the changes in CBF. The greater the increase in CMR O2 for any increase in CBF, the smaller the BOLD signal becomes and vice versa.In this paper we examine the degree to which the relationship betw...

Understanding the relationship between fMRI signal changes and activated cortex is paramount to successful mapping of neuronal activity. To this end, the relative extravascular and intravascular contribution to fMRI signal change from capillaries (localized), venules (less localized) and macrovessels (remote, draining veins) must be determined. In this work, the authors assessed both the extravascular and intravascular contribution to blood oxygenation level-dependent gradient echo signal change at 1.5 T by using a Monte Carlo model for susceptibility-based contrast in conjunction with a physiological model for neuronal activation-induced changes in oxygenation and vascular volume fraction. The authors compared our Model results with experimental fMRI signal changes with and without velocity sensitization via bipolar gradients to null the intravascular signal. The model and experimental results are in agreement and suggest that the intravascular spins account for the majority of fMRI signal change on T2*-weighted images at 1.5 T.

Social cognitive career theory proposes that contextual supports and barriers play key roles in the career choice process, yet little research has examined hypotheses involving these variables. Participants (111 college students) completed measures of math/science-related course self-efficacy, coping efficacy, outcome expectations, interests, goals, and perceived contextual supports and barriers. Findings indicate that self-efficacy and outcome expectations were jointly predictive of interests and choice intentions. Support and barrier percepts produced only weak direct relations to choice, though barrier percepts were found to moderate interest-choice relations. A model portraying barriers and supports as linked to choice indirectly (via their impact on self-efficacy) produced better fit to the data than did a model specifying barriers and supports as directly linked to choice. Implications for future research and counseling are discussed.