Statistical Parametric Mapping Lecture 4 - Chapter 7 Spatial and temporal resolution of fMRI Textbook: Functional MRI an introduction to methods, Peter Jezzard, Paul Matthews, and Stephen Smith Many thanks to those that share their MRI slides online

Spatial and Temporal Resolution Issues Spatial ResolutionSpatial Resolution –Spatial sampling and aliasing –Partial volume averaging alters strength of response based on voxel size and size of responding region Temporal ResolutionTemporal Resolution –Temporal sampling and filtering –Would like to sample electrical activity which happens earlier than BOLD –Order and timing of events would improve modeling capabilities

Spatial Resolution Issues Excitatory and Inhibitory neural activity are both energy consuming, but inhibited neurons produce less neuronal activity. Need to include all regions of brain involved in the designed brain tasks (whole brain preferred). –Activity could be weaker due to partial volume effects in smaller components of a system level activated brain network. –Need to improve task induced change and reduce partial volume averaging. Position errors due to motion, veins, macroscopic susceptibility, etc.

Impact of Spatial Resolution Extent of BOLD response (rb) is related to the extent of neuro- vascular response (rv) and the imaging spatial resolution extent (rs). General relationship rb 2 = rv 2 + rs 2 BOLD signal is variable due to partial volume averaging When rv < rs (voxel larger than signal region) rb ~ rs Bold signal is reduced by partial volume averaging When rv > rs (voxel smaller than signal region) rb ~ rv BOLD signal minimally affected by rs Based on classical linear system where output(x,y,z) = input(x,y,z)  PSF(x,y,z) But?

Two Main Focus Points Responding well to changing hemodynamicsResponding well to changing hemodynamics –Initial dip in BOLD response more spatially specific to activated brain area than later hypoxic rise in response, but later phase response is larger and needed for fMRI. –Late hyperoxic response more broadly distributed spatially. Techniques to eliminate unwanted contributions to signalTechniques to eliminate unwanted contributions to signal (increase contrast to noise ratio - CNR). –Short duration stimuli seem to be more narrowly distributed spatially than long duration stimuli in BOLD studies. –Higher B 0 appears to improve microvascular signals more than interfering signals –Better RF coils improve SNR –Improved motion correction improves CNR –Multi-shot EPI to reduce T2* blurring supports smaller voxels

Neuro-Vascular Signalling If signalling is mediated by diffusion then densely packed vasculature such as in V1 would show faster BOLD response. If signalling is mediated through membrane potentials on glia then different areas within brain would have similar BOLD response timing. Neural activitySignallingVascular response Vascular tone (reactivity) Autoregulation Metabolic signalling BOLD signal glia arteriole venule B 0 field Synaptic signalling Blood flow, oxygenation and volume dendrite end bouton

Temporal Hemodynamics Arterial inflow effects Venous outflow effects

fMRI response ratio drops off with stimulus duration Dilution of signal into larger extent seems to be dominant effect Stimulus duration (s) fMRI response ratio Figure 7.3 from textbook. time BOLD response, % initial dip positive BOLD response post stimulus undershoot overshoot stimulus Initial dip – localized response (low signal) Overshoot next in extent (high signal) Plateau has greatest extent (high signal) Response extent Figure 8.1. from textbook.

Brodmann’s Functional Map

Visual Field Mapping Visual information from right visual field sent to left V1 area in occipital lobe Vice-versa for left visual field Right and left eye views used to form 3D images through stereo effect (overlapping central visual area) V1 – Primary visual cortex

Anatomy of the Visual System Both eyes project to each visual cortex, but at the primary visual area (BA17), they remain largely segregated into ocular dominance columns. RIGHT VISUAL FIELD MAPPING

Occular Dominance Column Imaging Figure 7.1 –Blue is right eye response when corresponding (right eye right visual field) stimulated. Red is left eye (right visual field) stimulation. –Note similarity in columnar (connected) organization for each eye’s response. Figure 7.2 shows timing of visual stimulus and BOLD response. TR = 1sec. Note the spatial detail in figure 7.1. Short duration stimuli used. Figure 7.1 from textbook Time (s) Percent signal change Corresponding eye stimulation Other eye stimulation Figure 7.2 from textbook.

Typical Paradigm Instruction Presentation –stimulation –timing Processing –sensing –decision Response –plan –motor fMRI responses time (s) Trial #1 Trial #2 Presentation Response Behaviour time (s) Figure 7.4 from textbook. BOLD signal time course presentation (black) processing (light grey) response (dark grey) Task Behavior Onset and Width of BOLD response as temporal measures Not time to peak ----

Estimating Neural Processing Time From BOLD Response Onset V1 SMA M1 time fMRI response ampitude   (a) Figure 7.5 from textbook. Task – use joystick to move cursor from start box to target box as rapidly and accurately as possible (10 trials in multiple subjects). BOLD response – V1 (primary visual cortex), SMA (supplementary motor area), M1 (primary motor area) Analysis –  but not  increases with increasing reaction time (RT). Conclusion – Delay in reaction time from planning rather than execution of movement.

Estimating Neural Processing Time From from BOLD Response Width fMRI signal change from SPL Time after presentation (s) fMRI (b) Trial A Trial B (more angular disparity) RT(A) RT(B) Task (a) (c) Normalized width of BOLD response (s) Reaction Time (s) Figure 7.6 from textbook. Task – determine if one object could be rotated to match a second. Rotation angle varied by design. Press button yes or no. BOLD response – Superior Parietal Lobule (SPL) Analysis – Normalized width of BOLD response correlated with reaction time (RT). Conclusion – SPL intimately involved in mental rotation of object.

Forward Connections Parietal Temporal Visual overview.pdf

Parietal Lobe

Mango and Anatomy Talairach Daemon (TD) –Anatomical/functional labels –5 hierarchical levels Hemispheres Lobes Gyri Tissue Cellular Spatial Normalization –Supports x-y-z coordinate lookup of anatomical/functional labels using the TD

Cell Level Tissue Level Gyrus Level Hemisphere Level Lobe Level Talairach Daemon Atlas Sections at Z = +1