An increase in neural activity can lead to a decrease in blood oxygenation/flow

Functional magnetic resonance imaging (fMRI) is one of the most popular measures of human brain function. It is based upon changes in blood oxygenation, called the blood oxygenation level dependent activity (BOLD) response. The BOLD response is commonly interpreted as a correlate of neural activity, based on the premise that greater neural activity leads to increased blood flow/oxygenation (increases in blood flow lead to increases in oxygenation — Fox and Raichle, 1985). Although there is some evidence to justify this premise (e.g., Logothetis, 2002), an experiment from Anna Devor and colleagues (2008) shows that this is not necessarily the case. Using anesthetized rats, Devor et al. measured the electrical, hemodynamic, and metabolic response of forepaw stimulation in primary sensory cortex. They found that forepaw stimulation increased electrical activity and energy consumption in both hemispheres. However, while blood flow/oxygenation increased in the hemisphere contralateral to the paw that was stimulated, blood flow/oxygenation showed an overall decrease in the ipsilateral hemisphere (see figure). Although Devor et al. were using spectral and laser spectral imaging to measure blood flow and oxygenation and not fMRI, fMRI experiments in humans have shown that contralateral and ipsilateral hand stimulation also lead to an increased and decreased BOLD response, respectively (Hlushchuk & Hari, 2006).

Devor et al. (2008) Figure 2
Devor and colleagues are not sure why the relationship between blood flow/oxygenation varies depending on the side of stimulation. They speculate that the activity of inhibitory neurons may release signals that promote blood vessel constriction while the activity of excitatory neurons do the opposite. This could explain their results as paw stimulation leads to increased activity of inhibitory neurons in ipsilateral sensory cortex. Another explanation they suggest is that astrocyte activity in cortex ipsilateral to stimulation may lead to reduced blood flow/oxygenation. Regardless of the mechanism, their findings clearly demonstrate that the relationship between neural activity and BOLD is more complicated than a simple correlation with metabolic rate or electrical activity, which complicates trying to relate fMRI data to those of direct measures of brain function like EEG/MEG (e.g., Dale et al., 2000).

-David Groppe

REFERENCES

Dale AM, Liu AK, Fischl BR, Buckner RL, Belliveau JW, Lewine JD, & Halgren E (2000). Dynamic statistical parametric mapping: combining fMRI and MEG for high-resolution imaging of cortical activity. Neuron, 26 (1), 55-67 PMID: 10798392

Devor A, Hillman EM, Tian P, Waeber C, Teng IC, Ruvinskaya L, Shalinsky MH, Zhu H, Haslinger RH, Narayanan SN, Ulbert I, Dunn AK, Lo EH, Rosen BR, Dale AM, Kleinfeld D, & Boas DA (2008). Stimulus-induced changes in blood flow and 2-deoxyglucose uptake dissociate in ipsilateral somatosensory cortex. The Journal of neuroscience : the official journal of the Society for Neuroscience, 28 (53), 14347-57 PMID: 19118167

Fox PT, & Raichle ME (1985). Stimulus rate determines regional brain blood flow in striate cortex. Annals of neurology, 17 (3), 303-5 PMID: 3873210

Hlushchuk Y, & Hari R (2006). Transient suppression of ipsilateral primary somatosensory cortex during tactile finger stimulation. The Journal of neuroscience : the official journal of the Society for Neuroscience, 26 (21), 5819-24 PMID: 16723540

Nikos K. Logothetis (2002). The neural basis of the blood-oxygen-level-dependent
functional magnetic resonance imaging signal
Philosophical Transactions of the Royal Society B DOI: 10.1098/rstb.2002.1114

ResearchBlogging.org

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~ by eeging on July 3, 2011.

One Response to “An increase in neural activity can lead to a decrease in blood oxygenation/flow”

  1. Hi,

    This is a little self-promoting, but it is relevant I promise! I’ve just published a paper showing a dissociation between performance measures and BOLD in humans – thought you might be interested: http://www.sciencedirect.com/science/article/pii/S030439401000649X
    It’s a slightly messy paper for lots of reasons, and so the interpretation has a lot of caveats, but we’re working on some tighter studies now.

    Cheers,

    Matt.

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