Shulman and Rothman PNAS, 1998 In this period of intense research in the neurosciences, nothing is more promising than functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) methods, which localize brain activities. These functional imaging methodologies map neurophysiological responses to cognitive, emotional, or sensory stimulations. The rapid experimental progress made by using these methods has encouraged widespread optimism about our ability to understand the activities of the mind on a biological basis. However, the relationship between the signal and neurobiological processes related to function is poorly understood, because the functional imaging signal is not a direct measure of neuronal processes related to information transfer, such as action potentials and neurotransmitter release. Rather, the intensity of the imaging signal is related to neurophysiological parameters of energy consumption and blood flow. To relate the imaging signal to specific neuronal processes, two relationships must be established… The first relationship is between the intensity of the imaging signal and the rate of neurophysiological energy processes, such as the cerebral metabolic rates of glucose (CMRglc) and of oxygen (CMRO2). The second and previously unavailable relationship is between the neurophysiological processes and the activity of neuronal processes. It is necessary to understand these relationships to directly relate functional imaging studies to neurobiological research that seeks the relationship between the regional activity of specific neuronal processes and mental processes.

Shulman and Rothman PNAS, 1998 Psychology CMRglc NeuronalNeuroenergetics MentalImage Signal Neuroscience CMRO2 CBF

Let’s back up… What do we know for sure about fMRI?

280 million Hb molecules per red blood cell Hemoglobin Molecule

L. Pauling and C. Coryell The Magnetic Properties and Structure of Hemoglobin, Oxyhemoglobin and Carbonmonoxy hemoglobin, PNAS, vol. 22, pp , Different magnetic properties of hemoglobin and deoxyhemoglobin

Hemoglobin Molecule

Baseline Task from Mosley & Glover, 1995 Blood Oxygenation Level Dependent Imaging

Brain or Vein?

Large Vessel Contributions to BOLD Contrast Virchow-Robin Space

Intravascular Perivascular Extravascular

3 z = 1.64 Small Large Courtesy of Dr. Allen Song, Duke University Isotropic Diffusion Weighted Spiral Imaging at 4T

9 sec a b

Diffusion-weighted (b factor = 54) Diffusion-weighted (b factor = 108) Subject 41057, Slice 12, 4.0 Tesla ADC masked by BOLD activation BOLD activation (b factor = 0)

Diffusion-weighted (b factor = 54) Diffusion-weighted (b factor = 108) Subject 41037, Slice 183, 4.0 Tesla ADC masked by BOLD activation BOLD activation (b factor = 0)

Diffusion-weighted (b factor = 54) Diffusion-weighted (b factor = 108) Subject 41037, Slice 177, 4.0 Tesla ADC masked by BOLD activation BOLD activation (b factor = 0)

ADC masked by BOLD activation Subject 41037, Slice 177, 4.0 Tesla

Negative dips

Vanzetta and Grinvald, Science, 286: , 1999 Phosphorescence Decay Time (Oxyphor R2 oxygen tension-sensitive phosphorescent probe)

Vanzetta and Grinvald, Science, 286: , 1999 Phosphorescence Decay Time (Oxyphor R2 oxygen tension-sensitive phosphorescent probe)

Vanzetta and Grinvald, Science, 286: , 1999 Oxy Hb deoxy Hb

Berwick et al, JCBFM, 2002 Optical imaging of rat barrel cortex Hb02= oxyhemoglobin, Hbr = deoxyhemoglobin, Hbt = total blood flow

N. Logothetis, Nature Neuroscience, 1999 Functional Imaging of the Monkey Brain

Hu, Le, Ugurbil MRM, 1997 Early Response in fMRI

Hu, Le, Ugurbil MRM, 1997 Early Response in fMRI

What triggers blood flow?

Arterioles ( microns) precapillary sphincters Capillaries (5-10 microns) Venules (8-50 microns)

Tissue factors K + H + Adenosine Nitric oxide

C. Iadecola, Nature Neuroscience, 1998 Commentary upon Krimer, Muly, Williams and Goldman-Rakic, Nature Neuroscience, 1998 Neuronal Control of the Microcirculation

Krimer, Muly, Williams, Goldman-Rakic, Nature Neuroscience, 1998 Pial Arteries 10  m NoradrenergicDopamine

Krimer, Muly, Williams, Goldman-Rakic, Nature Neuroscience, 1998 Dopamanergic terminals associated with small cortical blood vessels 10  m

Krimer, Muly, Williams, Goldman-Rakic, Nature Neuroscience, 1998 Dopamanergic terminals associated with small cortical blood vessels 2  m 400 nm

Krimer, Muly, Williams, Goldman-Rakic, Nature Neuroscience, 1998 Perivascular iontophoretic application of dopamine s40-60 s

Let’s back up again… Why isn’t all the oxyHb used up?

Uncoupling…

glucose pyruvate Glucose 6 phosphate Fructose – 1,6-phosphate TCA cycle lactate Net +2 ATP Net +36 ATP glucose O2 CO2 + H20

Shulman and Rothman PNAS, 1998

Proposed pathway of glutamate / glutamine neurotransmitter cycling between neurons and glia, whose flux has been quantitated recently by 13 C MRS experiments. Action potentials reaching the presynaptic neuron cause release of vesicular glutamate into the synaptic cleft, where it is recognized by glutamate receptors post-synaptically and is cleared by Na + -coupled transport into glia. There it is converted enzymatically to glutamine, which passively diffuses back to the neuron and, after reconversion to glutamate, is repackaged into vesicles. The rate of the glutamate-to-glutamine step in this cycle (Vcycle), has been derived from recent 13 C experiments.

Sibson et al. PNAS, 1998

Heeger, Nature Neuroscience 2002

Ito et al. JCBFM, 2001

Relationship of BOLD to neuronal activity

Attwell and Laughlin, JCBFM, 2001 Brain Energetics

Attwell and Laughlin, JCBFM, 2001 Brain Energetics

Rees et al. Nature Neuroscience 2000

Heeger, Nature Neuroscience 2000

Lauritzen, JCBFM, 2001

Climbing Fiber Stimulation

Lauritzen, JCBFM, 2001 Climbing Fiber Stimulation

Lauritzen, JCBFM, 2001 Parallel Fiber Stimulation

Lauritzen, JCBFM, 2001 Harmaline IP synchronizes inferior olive

Smith et al. PNAS, 2002

Hyder et al. PNAS, 2002

Spatial co-localization?

How neuronal activity changes cerebral blood flow is of biological and practical importance. The rodent whisker-barrel system has special merits as a model for studies of changes in local cerebral blood flow (LCBF). Whisker-activated changes in flow were measured with intravascular markers at the pia. LCBF changes were always prompt and localized over the appropriate barrel. Stimulus- related changes in parenchymal flow monitored continuously with H2 electrodes recorded short latency flow changes initiated in middle cortical layers. Activation that increased flow to particular barrels often led to reduced flow to adjacent cortex. The matching between a capillary plexus (a vascular module) and a barrel (a functional neuronal unit) is a spatial organization of neurons and blood vessels that optimizes local interactions between the two. The paths of communication probably include: neurons to neurons, neurons to glia, neurons to vessels, glia to vessels, vessels to vessels and vessels to brain. Matching a functional grouping of neurons with a vascular module is an elegant means of reducing the risk of embarrassment for energy-expensive neuronal activity (ion pumping) while minimizing energy spent for delivery of the energy (cardiac output). For imaging studies this organization sets biological limits to spatial, temporal and magnitude resolution. Reduced flow to nearby inactive cortex enhances local differences Woolsey et al. Cerebral Cortex, 95: , 1996 Whisker Barrel Model

Yang, Hyder, Shulman PNAS, 93: , 1996 Rat Single Whisker Barrel fMRI Activation 7 Tesla 200  m x 200  m x 1000  m

Berwick et al, JCBFM, 2002 Optical imaging of rat barrel cortex Hb02= oxyhemoglobin, Hbr = deoxyhemoglobin, Hbt = total blood flow

Berwick et al, JCBFM, 2002 (a) Outside activated region, (b) ipsilateral whisker

Relationship between field potentials and functional MRI

LMY1

LTO10 DWT1 LSOP5 LPT6 LPT7 LTO4

SOP5 PT6 PT7 TO4 TO10 LG FG Pole V1-V2 MT

Timing of activations compared to neuronal activation

Subdural Electrode Strips

Face-Specific N200

Face-House Attention Task

Attend House Attend Face NBH1 CDOB1

Negative activations

Harel et al. JCBFM, 2002

9 sec a b

180° phase-reversed responses to faces among objects

41088

Is there evidence for inhibition?

RTP2-5LTTP2-2

- + Excitatory Inhibitory + - Face-specific cellWord-specific cell N200P200

Rat Olfactory Bulb Structural MRI Yang, Renken, Hyder, Siddeek, Greer, Shepherd, Shulman PNAS, 95: , Tesla 100  m x 100  m x 1000  m

Yang, Renken, Hyder, Siddeek, Greer, Shepherd, Shulman PNAS, 95: , 1998 Rat Olfactory Bulb fMRI Activation 7 Tesla 200  m x 200  m x 1000  m