Laboratory 2: Introduction to fMRI Data and AnalysisWhat is functional MRI?How do we measure neuronal activity with MRI?BOLD ImagingBOLD Imaging: TimingSlide 6Example Hemodynamic Response (impulse response)Slide 8Goals of laboratory 2:Slide 10Slide 11So where should we expect activation?Slide 13Laboratory 2: Introduction Laboratory 2: Introduction to fMRI Data and Analysisto fMRI Data and AnalysisSeptember 18, 2006September 18, 2006HST.583HST.583Divya BolarDivya BolarWhat is functional MRI?What is functional MRI?Broad sense: fMRI refers to any MR technique Broad sense: fMRI refers to any MR technique that goes beyond anatomy to measure that goes beyond anatomy to measure aspects of local physiology.aspects of local physiology.Specific sense: fMRI refers to MR techniques Specific sense: fMRI refers to MR techniques that investigate changes in brain function that investigate changes in brain function over time.over time.Brain function/ information processing results Brain function/ information processing results from the activity of ensembles of neurons.from the activity of ensembles of neurons.Primary goal of fMRI is to detect signal Primary goal of fMRI is to detect signal changes corresponding to neuronal activity. changes corresponding to neuronal activity. Buxton RB. Introduction to Functional Magnetic Resonance Imaging, 2002.Huettel S, Song AW, McCarthy G. Funcitonal Magnetic Resonance Imaging, 2004.How do we measure neuronal How do we measure neuronal activity with MRI?activity with MRI?Currently not possible to directly measure Currently not possible to directly measure neural activity (i.e. firing of action neural activity (i.e. firing of action potentials) with MRIpotentials) with MRICan visualize downstream correlates of Can visualize downstream correlates of neural activityneural activityNeural ActivityCellular MetabolismBlood FlowVenous [deoxyHb]Blood oxygen level dependence (BOLD)Simplified FlowchartBOLD ImagingBOLD ImagingDeoxyhemoglobin is paramagnetic; Deoxyhemoglobin is paramagnetic; changes susceptibility of bloodchanges susceptibility of bloodAs blood becomes more deoxygenated As blood becomes more deoxygenated (i.e. (i.e. ↑ ↑ [dHb]), MRI signal decreases[dHb]), MRI signal decreasesIncreased neural activity results in Increased neural activity results in decreaseddecreased [dHb] in venous blood [dHb] in venous bloodMRI signal thus increases with MRI signal thus increases with increased brain activity*increased brain activity*BOLD Imaging: Timing BOLD Imaging: Timing BOLD effect is dependent on so-called BOLD effect is dependent on so-called “hemodynamic response”“hemodynamic response”Hemodynamic response describes Hemodynamic response describes how blood flow changes over time, in how blood flow changes over time, in response to neural activationresponse to neural activationHemodynamic response does not Hemodynamic response does not instantaneously follow neural activity; instantaneously follow neural activity; it occurs with delay and dispersion it occurs with delay and dispersionExample Example Hemodynamic Hemodynamic Response (impulse response)Response (impulse response)0 2 4 6 8 10 12 14 16 18 20-0.200.20.40.60.811.2Time (s)AmplitudeEnsemble Neural FiringHemodynamic ResponseDelayDispersionImpulseTotal task/ scan time: 6 minutesTotal task/ scan time: 6 minutesSample fMRI paradigm:Sample fMRI paradigm:Simple visual task; flashing circle checkerboard (8 Hz)Simple visual task; flashing circle checkerboard (8 Hz)OFFOFFONONOFFOFFONONOFFOFFONON•Time series fMRI data Time series fMRI data of visual cortex voxel of visual cortex voxel matches paradigmmatches paradigm•Indicates a positive Indicates a positive BOLD effect; i.e. BOLD effect; i.e. decreased [dHb] as a decreased [dHb] as a result of neural result of neural activationactivation•This is fMRI!This is fMRI!1 min1 minGoals of laboratory 2:Goals of laboratory 2:Goal 1: Get familiar with sample fMRI data Goal 1: Get familiar with sample fMRI data set from visual hemifield stimulation task set from visual hemifield stimulation task ((NeurolensNeurolens11 tutorial, exercises 1 and 2) tutorial, exercises 1 and 2)Goal 2: Successfully analyze data set and Goal 2: Successfully analyze data set and show appropriate activations show appropriate activations (Neurolens (Neurolens tutorial, exercises 3 and 4)tutorial, exercises 3 and 4)Goal 3: Answer laboratory questions and do Goal 3: Answer laboratory questions and do exercises on handout (exercises on handout (graded)graded)Bonus: Neurolens tutorial, exercises 5 and 6Bonus: Neurolens tutorial, exercises 5 and 61. Hoge RD, www.neurolens.org, 2006.Left hemifieldRight hemifieldVisual stimulus seen by subject32 secVisual Task:Interleaved left and right visual field stimulation (8 Hz flashing checkerboard)So where should we expect activation??So where should we expect activation??So where should we expect So where should we expect activation?activation?Primary activation should be in Primary activation should be in contralateral contralateral visual cortexvisual cortexWill be Will be slightslight activation in activation in ipsilateral ipsilateral visual cortex, due to some crosstalk. visual cortex, due to some crosstalk. In other words, the flashing In other words, the flashing checkerboard stiumulus does not checkerboard stiumulus does not completely isolate L/R visual fieldscompletely isolate L/R visual