Modelling temporal structure (in noise and signal)

Transcription

1 Modelling temporal structure (in noise and signal) Mark Woolrich, Christian Beckmann*, Salima Makni & Steve Smith FMRIB, Oxford *Imperial/FMRIB temporal noise: modelling temporal autocorrelation temporal signal: FLOBS HRF optimal basis functions temporal signal: HRF deconvolution spatiotemporally structured signal / noise: ICA functional grand-plan : integrating ICA+GLM

2 Non-independent/Autocorrelation/ Coloured FMRI noise power Coloured / autocorrelated White / independent frequency Uncorrected, this causes: Even after high-pass filtering, FMRI noise has extra power at low frequencies (positive autocorrelation or temporal smoothness) - biased stats ( increased false positives) - decreased sensitivity

3 FMRIB s Improved Linear Modelling (FILM) FILM is used to fit the GLM voxel-wise in FEAT Deals with the autocorrelation locally and uses prewhitening FILM estimates autocorrelation by looking at the residuals of the GLM fit: Y = Xβ + ɛ residuals = Y X β

4 FMRIB s Improved Linear Modelling (FILM) Power vs. freq in the residuals 1) Fit the GLM and estimate the autocorrelation on the residuals power frequency

5 FMRIB s Improved Linear Modelling (FILM) Power vs. freq in the residuals 1) Fit the GLM and estimate the autocorrelation on the residuals power frequency power 2) Spatially and spectrally smooth the data frequency

6 FMRIB s Improved Linear Modelling (FILM) Power vs. freq in the residuals 1) Fit the GLM and estimate the autocorrelation on the residuals power frequency power 2) Spatially and spectrally smooth the data power frequency 3) Construct prewhitening filter to undo autocorrelation frequency

7 FMRIB s Improved Linear Modelling (FILM) 1) Fit the GLM and estimate the autocorrelation on the residuals power frequency power 2) Spatially and spectrally smooth the data power frequency 3) Construct prewhitening filter to undo autocorrelation frequency 4) Apply filter to data and design matrix and refit power frequency

8 FMRIB s Improved Linear Modelling (FILM)

9 Dealing with Variations in Haemodynamics The haemodynamic responses vary between subjects and areas of the brain How do we allow haemodynamics to be flexible but remain plausible? Reminder: the haemodynamic response function (HRF) describes the BOLD response to a short burst of neural activity

10 Temporal Derivatives A very simple approach to providing HRF variability is to include (alongside each EV) the EV temporal derivative Including the temporal derivative of an EV allows for a small shift in time of that EV This is based upon a first order Taylor series expansion EV Temporal derivative data model fit without derivative model fit with derivative

11 Using Parameterised HRFs We need to allow flexibility in the shape of the fitted HRF Parameterise HRF shape and fit shape parameters to the data Needs nonlinear fitting - HARD

12 Using Basis Sets We need to allow flexibility in the shape of the fitted HRF Parameterise HRF shape and fit shape parameters to the data We can use linear basis sets to span the space of expected HRF shapes Needs nonlinear fitting - HARD Linear fitting (use GLM) - EASY

13 How do HRF Basis Sets Work? Different linear combinations of the basis functions can be used to create different HRF shapes basis fn 1 basis fn 2 basis fn 3 HRF 1.0* + 0.3* * =

14 How do HRF Basis Sets Work? Different linear combinations of the basis functions can be used to create different HRF shapes basis fn 1 basis fn 2 basis fn 3 HRF 1.0* + 0.3* * = 0.7* * + 0.5* =

15 FMRIB s Linear Optimal Basis Set (FLOBS) Using FLOBS we can: Specify a priori expectations of parameterised HRF shapes Generate an appropriate basis set

16 Generating FLOBs (1) Take samples of the HRF

17 Generating FLOBs (2) Perform SVD (3) Select the top eigenvectors as the optimal basis set Canonical HRF

18 Generating FLOBs (2) Perform SVD (3) Select the top eigenvectors as the optimal basis set Canonical HRF temporal derivative

19 Generating FLOBs (2) Perform SVD (3) Select the top eigenvectors as the optimal basis set dispersion derivative Canonical HRF temporal derivative The resulting basis set can then be used in FEAT

20 Bayesian Inference Priors on HRF parameters HRF Joint posterior distribution p(a, c, m 1, m 2... Y ) BOLD FMRI data Neural Activity *A + Gaussian noise Woolrich et al., TMI, 2004

21 Bayesian Inference Priors on HRF parameters HRF Marginal posterior distribution p(a Y ) = p(a, c, m 1, m 2... Y )dcdm 1 dm 2... BOLD FMRI data Neural Activity *A + Gaussian noise Infer using MCMC Woolrich et al., TMI, 2004

22 Temporal deconvolution of FMRI timecourses Inputs are raw paradigm (stimulation and modulation ) timecourses Model based on Bilinear Dynamical Systems (Penny 2005), where modulatory input changes neural response to stimulation What s new: estimate HRF from data full Bayesian inference on model, using VB Makni, NeuroImage 2008

23 CORRECTED PROOF

24 To do the Bayes, either: MCMC (computer takes ages) Variational Bayes (maths takes ages) CORRECTED PROOF

25 To do the Bayes, either: MCMC (computer takes ages) Variational Bayes (maths takes ages) CORRECTED PROOF

26 Model-free Functional Data Analysis MELODIC Multivariate Exploratory Linear Optimised Decomposition into Independent Components decomposes data into a set of statistically independent spatial component maps and associated time courses can perform multi-subject/ multi-session analysis fully automated (incl. estimation of the number of components) inference on IC maps using alternative hypothesis testing

27 Model-free Functional Data Analysis MELODIC Multivariate Exploratory Linear Optimised Decomposition into Independent Components decomposes data into a set of statistically independent spatial component maps and associated time courses can perform multi-subject/ multi-session analysis fully automated (incl. estimation of the number of components) inference on IC maps using alternative hypothesis testing

28 EDA techniques for FMRI are mostly multivariate often provide a multivariate linear decomposition: space # maps space time Scan #k FMRI data = time # maps spatial maps Data is represented as a 2D matrix and decomposed into factor matrices (or modes)

29 Model Order Selection can estimate the model order from the Eigenspectrum of the data covariance matrix (corrected using Wishart random matrix theory) approximate the Bayesian evidence for the model order for a probabilistic PCA model (PPCA) AR 16 noise + signal Figure Laplace approximation 3: Bayesian BIC AIC Minka, TR 514 MIT Media Lab 2000

30 Probabilistic ICA GLM analysis standard ICA (unconstrained)

31 Probabilistic ICA GLM analysis probabilistic ICA

32 Probabilistic ICA GLM analysis probabilistic ICA designed to address the overfitting problem : tries to avoid generation of spurious results high spatial sensitivity and specificity

33 Applications EDA techniques can be useful to investigate the BOLD response estimate artefacts in the data find areas of activation which respond in a nonstandard way analyse data for which no model of the BOLD response is available

34 Investigate BOLD response estimated signal time course standard hrf model

35 Applications EDA techniques can be useful to investigate the BOLD response estimate artefacts in the data find areas of activation which respond in a nonstandard way analyse data for which no model of the BOLD response is available

36 slice drop-outs

37 gradient instability

38 EPI ghost

39 high-frequency noise

40 head motion

41 field inhomogeneity

42 eye-related artefacts

43 eye-related artefacts

44 eye-related artefacts Wrap around

45 Structured Noise and the GLM structured noise appears: in the GLM residuals and inflate variance estimates (more false negatives) in the parameter estimates (more false positives and/or false negatives) In either case lead to suboptimal estimates and wrong inference!

46 Structured noise and GLM Z-stats bias Correlations of the noise time courses with typical FMRI regressors can cause a shift in the histogram of the Z-statistics Thresholded maps will have wrong false-positive rate

47 Denoising FMRI before denoising 500 PE raw 300 Z raw Example: left vs right hand finger tapping !1000! PE clean RIGHT 0!10! Z clean Johansen-Berg et al. PNAS !1000! after denoising 0!10!

48 Denoising FMRI before denoising 600 PE raw 350 Z raw Example: left vs right hand finger tapping !1000! PE clean LEFT 0!10! Z clean Johansen-Berg et al. PNAS !1000! after denoising 0!10!

49 Denoising FMRI before denoising Example: left vs right hand finger tapping LEFT - RIGHT contrast Johansen-Berg et al. PNAS 2002 after denoising

50 Apparent variability McGonigle et al.: 33 Sessions under motor paradigm de-noising data by regressing out noise: reduced apparent session variability

51 Applications EDA techniques can be useful to investigate the BOLD response estimate artefacts in the data find areas of activation which respond in a nonstandard way analyse data for which no model of the BOLD response is available

52 PICA on resting data perform ICA on null data and compare spatial maps between subjects/ scans ICA maps depict spatially localised and temporally coherent signal changes Example: ICA maps - 1 subject at 3 different sessions

53 Spatial characteristics (a) x=17 y=-73 z=-12 (b) x=-13 y=-61 z=6 (a) x=17 y=-73 z=-12 (b) x=-13 y=-61 z=6 R (c) L Medial visual cortex y=-17 x=3 x=-4 y=-29 x=45 R (d) z=33 (f) y=-29 x=1 x=5 (f) L (h) z=27 L Sensori-motor system y=6 x=-45 z=27 L R z=47 z=51 y=6 x=5 L y=-42 y=-21 R z=33 z=51 L R Auditory system x=-4 Lateral Visual Cortex L R (g) z=1.5 L y=-21 x=1 L R (e) (d) L R (e) z=1.5 y=-17 x=3 R (c) R y=-42 z=47

54 Spatial characteristics (e) x=-4 y=-29 z=33 (f) x=5 y=6 z=27 R L R L Visuospatial system Executive control (g) x=45 y=-42 z=47 (h) x=-45 y=-42 z=47 R L Visual Stream R L

55 Temporal deconvolution of ICA timecourses What are the task-related components? Use explicit time series model on the ICgenerated temporal modes (using BDS)

56 Example: BDS/ICA integration (b) (c) Estimate the mean of the posterior distribution over the neuronal response Estimate the mean of the Gaussian posterior distribution of the HRF 1 (d) 1.5 (e) 0.5 0! posterior probability of neuronal response > 0 for the IC Time (s) Assess the total model fit

57 Integrating GLM and ICA for FMRI analysis GLM β i = + Noise Data Predicted response Estimated effect

58 Integrating GLM and ICA for FMRI analysis GLM β i = + Noise Data Predicted response Estimated effect + clear conclusions on a particular question - results depend on the model

59 Integrating GLM and ICA for FMRI analysis PICA space # maps space time Scan #k FMRI data = time # maps spatial maps + Noise Data Estimated artefact time courses Estimated spatially independent maps

60 Integrating GLM and ICA for FMRI analysis PICA space # maps space time Scan #k FMRI data = time # maps spatial maps + Noise Data Estimated artefact time courses Estimated spatially independent maps + data driven and multivariate approach - no knowledge about the fmri paradigm is used - can be hard to interpret activation results

61 Integrating GLM and ICA for FMRI analysis GLM + ICA space # maps space time Scan #k FMRI data = time # maps spatial maps + Noise Data Predicted response Estimated artefact time courses Estimated spatially independent maps

62 Integrating GLM and ICA for FMRI analysis GLM + ICA space # maps space time Scan #k FMRI data = time # maps spatial maps + Noise Data Predicted response Estimated artefact time courses Estimated spatially independent maps Stimulus model-based hypothesis testing Adaptive model-free artefact modelling - e.g. stimulus correlated motion, physiological noise, networks of spontaneous neuronal activity

63 Modelled ICs Estimated HRFs 10 auditory stimulus visual stimulus Time (s) Spatial map Time course Example model free IC # maps space time # maps spatial maps Spatial map Time course