Tuesday, January 27, 2009
Testing Basic Assumptions in fMRI : Anticipatory haemodynamic signals in sensory cortex not predicted by local neuronal activity
What if the fMRI signal isn't always linked to neuronal activity for those cool brain studies? What then?
I've actually wondered about this for years since without that basic assumption, what are the studies telling us? Hmmmm. This one below was recently published in Nature, so not just some low tier journal by a couple of screw offs in their back yard.
While this study was only performed on 2 animals, it is very interesting.
To quote the researchers, "These findings (tested in two animals) challenge the current understanding of the link between brain haemodynamics and local neuronal activity. They also suggest the existence of a novel preparatory mechanism in the brain that brings additional arterial blood to cortex in anticipation of expected tasks."
Remember that the main task of the brain is to PREDICT. Maybe those areas that light up in fMRI studies are the ones that are NOT CURRENTLY in use, but WILL BE in use very soon.
So the next time you read a cool fMRI study, keep this one in the back of your brain!
Nature 457, 475-479 (22 January 2008)
Anticipatory haemodynamic signals in sensory cortex not predicted by local neuronal activity
Yevgeniy B. Sirotin1 & Aniruddha Das 1,2,3,4,5,6
1. Department of Neuroscience,
2. Department of Psychiatry,
3. W. M. Keck Center on Brain Plasticity and Cognition,
4. Mahoney Center for Brain and Behavior,
5. Department of Biomedical Engineering, Columbia University, New York, New York 10027, USA
6. New York State Psychiatric Institute, 1051 Riverside Drive, Unit 87, New York, New York 10032, USA
Haemodynamic signals underlying functional brain imaging (for example, functional magnetic resonance imaging (fMRI)) are assumed to reflect metabolic demand generated by local neuronal activity, with equal increases in haemodynamic signal implying equal increases in the underlying neuronal activity1, 2, 3, 4, 5, 6. Few studies have compared neuronal and haemodynamic signals in alert animals7, 8 to test for this assumed correspondence. Here we present evidence that brings this assumption into question. Using a dual-wavelength optical imaging technique9 that independently measures cerebral blood volume and oxygenation, continuously, in alert behaving monkeys, we find two distinct components to the haemodynamic signal in the alert animals' primary visual cortex (V1).
One component is reliably predictable from neuronal responses generated by visual input. The other component—of almost comparable strength—is a hitherto unknown signal that entrains to task structure independently of visual input or of standard neural predictors of haemodynamics. This latter component shows predictive timing, with increases of cerebral blood volume in anticipation of trial onsets even in darkness. This trial-locked haemodynamic signal could be due to an accompanying V1 arterial pumping mechanism, closely matched in time, with peaks of arterial dilation entrained to predicted trial onsets.
These findings (tested in two animals) challenge the current understanding of the link between brain haemodynamics and local neuronal activity. They also suggest the existence of a novel preparatory mechanism in the brain that brings additional arterial blood to cortex in anticipation of expected tasks.
Read the rest HERE