fMRI Task Design Guide

fMRI Task Design Guide

Purpose

The experimental design is the single most important determinant of an fMRI study's statistical power, interpretability, and scientific value. Choosing between block, event-related, and mixed designs involves trade-offs between detection power and estimation efficiency that depend on the research question. Similarly, the choice of inter-stimulus interval (ISI), jittering strategy, condition ordering, and trial count directly determines whether the BOLD signal of interest can be reliably detected.

A competent programmer without neuroimaging training would not know that block designs provide higher detection power but cannot estimate HRF shape, that exponentially distributed jitter is more efficient than uniform jitter, or that the BOLD response takes 12-16 seconds to return to baseline. This skill encodes those domain-specific design decisions.

When to Use This Skill

• Planning a new task-based fMRI experiment
• Choosing between block, event-related, or mixed designs
• Optimizing inter-stimulus interval and jittering strategy
• Calculating design efficiency for contrast detection
• Determining minimum trial counts per condition
• Integrating behavioral task constraints with fMRI timing requirements
• Reviewing or troubleshooting an existing fMRI task design

Research Planning Protocol

Before executing the domain-specific steps below, you MUST:

1. State the research question — What specific question is this analysis/paradigm addressing?
2. Justify the method choice — Why is this approach appropriate? What alternatives were considered?
3. Declare expected outcomes — What results would support vs. refute the hypothesis?
4. Note assumptions and limitations — What does this method assume? Where could it mislead?
5. Present the plan to the user and WAIT for confirmation before proceeding.

For detailed methodology guidance, see the research-literacy skill.

⚠️ Verification Notice

This skill was generated by AI from academic literature. All parameters, thresholds, and citations require independent verification before use in research. If you find errors, please open an issue.

Design Type Selection

Comparison of Design Types

Design Type	Detection Power	Estimation Efficiency	Trial-Level Analysis	Best For	Source
Block	High	Low	No	Detecting whether a region is active	Friston et al., 1999; Petersen & Dubis, 2012
Event-related (slow)	Moderate	High	Yes	Estimating HRF shape	Dale, 1999
Rapid event-related	Moderate-High	Moderate-High	Yes	Flexible trial-by-trial analysis with good power	Dale, 1999; Friston et al., 1999
Mixed (hybrid)	High (sustained) + Moderate (transient)	Moderate	Yes (transient component)	Separating sustained and transient effects	Petersen & Dubis, 2012

Decision Tree

What is the primary goal?
 |
 +-- Detect presence/absence of activation (localization)
 | |
 | +-- Is HRF shape estimation needed?
 | |
 | +-- NO --> Block design (maximum detection power)
 | |
 | +-- YES --> Mixed design (blocks + events within blocks)
 |
 +-- Estimate trial-by-trial neural responses
 | |
 | +-- Are there enough trials (>40 per condition)?
 | |
 | +-- YES --> Rapid event-related design (jittered ISI)
 | |
 | +-- NO --> Slow event-related design (ISI > 12 s)
 |
 +-- Separate sustained state vs. transient item effects
 --> Mixed design (Petersen & Dubis, 2012)

Block Design Parameters

• Optimal block duration: 15-20 seconds for maximum detection power (Maus et al., 2010; Bandettini et al., 1993). Shorter blocks (< 12 s) reduce sensitivity because the BOLD response does not reach steady state. Longer blocks (> 30 s) increase habituation and strategy effects (Poldrack et al., 2011, Ch. 3)
• Minimum block duration: 12 seconds to allow the BOLD signal to reach near-plateau (Bandettini et al., 1993)
• Number of blocks per condition: At least 4-6 blocks per condition per run for stable estimates (Poldrack et al., 2011, Ch. 3)
• Condition alternation: Alternate conditions (ABAB or ABCABC) rather than grouping (AAABBB), which confounds condition with time (Poldrack et al., 2011, Ch. 3)
• Rest blocks: Include rest/fixation blocks of at least 12-16 seconds between active blocks to allow BOLD signal return to baseline (Glover, 1999)

Event-Related Design Parameters

Inter-Stimulus Interval (ISI) and Jittering

The ISI between events is critical for statistical efficiency and BOLD signal separability.

Parameter	Recommendation	Source
Minimum ISI	2-4 seconds (for partial BOLD recovery)	Dale, 1999; Glover, 1999
Mean ISI for rapid designs	4-6 seconds	Dale, 1999
ISI range for jittered designs	2-8 seconds	Dale, 1999; Wager & Nichols, 2003
Null/fixation trials	20-33% of total events	Friston et al., 1999

Jittering strategies (from most to least recommended):

1. Optimized sequences: Use design optimization tools (optseq2, NeuroDesign) to maximize efficiency for specific contrasts (Dale, 1999; Durnez et al., 2017)
2. Truncated exponential distribution: More short ISIs, fewer long ISIs; near-optimal efficiency (Hagberg et al., 2001)
3. Uniform random: Equal probability across ISI range; acceptable but suboptimal
4. Fixed ISI: Avoid for rapid event-related designs; severely reduces design efficiency

Domain warning: Jittered designs can be over 10x more efficient than fixed-ISI designs with the same mean interval (Dale, 1999). Always jitter for event-related fMRI.

HRF Timing Constraints

The BOLD hemodynamic response imposes hard constraints on fMRI design timing:

• HRF peak: 4-6 seconds after neural event onset (Glover, 1999)
• Return to baseline: 12-16 seconds after a brief event (Glover, 1999)
• BOLD nonlinearity: Responses to stimuli separated by < 2 seconds sum nonlinearly (reduced amplitude), making them harder to separate (Glover, 1999; Wager & Nichols, 2003)

Trial Count Requirements

Design Type	Minimum Trials per Condition	Recommended Trials	Source
Event-related (detection)	20	30-50	Desmond & Glover, 2002
Event-related (HRF estimation)	30	50+	Murphy & Garavan, 2005
Rapid event-related	30	40-60	Desmond & Glover, 2002
FIR/deconvolution	40+	60+	Glover, 1999

Domain insight: These are per-condition minimums. If comparing conditions (A vs. B), each condition needs this many trials. More conditions require longer scan sessions or fewer trials per condition, creating a power trade-off.

Design Efficiency

Efficiency Calculation

Design efficiency quantifies how well a given design matrix allows detection of specific contrasts:

Detection efficiency = 1 / trace(c' * inv(X'X) * c)

where c is the contrast vector and X is the design matrix (Dale, 1999; Liu et al., 2001).

Detection vs. Estimation Trade-off

• Detection power: Ability to detect whether an effect exists. Maximized by block designs and rapid event-related designs with high event density (Liu et al., 2001)
• Estimation efficiency: Ability to accurately characterize the HRF shape. Maximized by jittered designs with sufficient ISI variability (Liu et al., 2001)
• These are inherently in tension: block designs maximize detection but cannot estimate HRF shape

Design Optimization Tools

• optseq2 (FreeSurfer): Optimizes event ordering and null events for maximum efficiency (Dale, 1999)
• NeuroDesign (Python): Genetic algorithm-based optimization (Durnez et al., 2017)
• fMRIpower: Power calculations accounting for design and temporal autocorrelation (Mumford & Nichols, 2008)

Mixed Design Specification

Mixed designs combine sustained (block-level) and transient (event-level) components (Petersen & Dubis, 2012):

1. Sustained regressor: Models the tonic task state (e.g., "task block on" vs. "rest"), boxcar convolved with HRF
2. Transient regressors: Model individual trial onsets within each block, convolved with HRF
3. These regressors are separable because they operate at different temporal frequencies

Key parameters:

• Block duration: 20-40 seconds to provide enough events within each block (Petersen & Dubis, 2012)
• Events per block: At least 4-6 for stable transient estimates
• Inter-block rest: 16-20 seconds minimum for BOLD signal recovery (Glover, 1999)

Scanner-Related Timing Constraints

Constraint	Guideline	Rationale
TR synchronization	Stimulus onsets need not align with TR boundaries for event-related designs	Jittered onsets relative to TR improve temporal sampling of HRF
Trigger pulses	Start experiment on scanner trigger pulse (TTL signal)	Ensures precise alignment between stimulus and acquisition timing
Total scan duration	5-15 minutes per run	Longer runs increase motion and fatigue; shorter runs waste setup time (Poldrack et al., 2011, Ch. 3)
Number of runs	2-4 runs typical; split conditions across runs if needed	Allows rest between runs; run-level effects can be modeled
Dummy scans	First 3-5 TRs (5-10 s) are T1 equilibration artifacts	Discard or model as confounds (Poldrack et al., 2011, Ch. 5)

Rest/Fixation Baseline

• Allocate 25-30% of total scan time to rest/fixation baseline (Friston et al., 1999)
• Rest periods serve dual purpose: allow BOLD signal recovery and provide baseline estimate
• For block designs, rest blocks of 12-16 seconds between active blocks (Glover, 1999)
• For event-related designs, null trials (fixation) distributed throughout the sequence (Friston et al., 1999)

Condition Ordering

• m-sequences: Pseudo-random sequences with optimal counterbalancing properties (Buracas & Boynton, 2002)
• Optimized pseudo-random: Generated by optimization algorithms (optseq2, NeuroDesign) to maximize design efficiency while controlling trial-order effects (Dale, 1999)
• Avoid: Simple alternation (ABABAB) which confounds condition with time, or purely random sequences which may produce long runs of the same condition

Behavioral Task Constraints

Constraint	Guideline	Source
Response mapping	Counterbalance button assignments across subjects	Prevents lateralized motor confound
Practice effects	Include out-of-scanner practice until performance plateaus	Reduces learning-related activation changes during scanning
Task difficulty	Aim for 70-85% accuracy	Floor/ceiling effects eliminate behavioral variance (Poldrack et al., 2011, Ch. 3)
Response window	Allow 1.5-3 seconds for speeded responses	Accommodate scanner environment slowing (~200 ms; Haatveit et al., 2010)
Stimulus duration	0.5-4 seconds typical for visual stimuli	Long enough for perceptual processing, short enough for event separation

Common Pitfalls

1. Fixed ISI in event-related designs: Dramatically reduces design efficiency compared to jittered designs. Always jitter ISI for event-related fMRI (Dale, 1999)
2. Too few trials per condition: Fewer than 20 events per condition yields unreliable single-subject estimates (Desmond & Glover, 2002). Plan for at least 30 per condition
3. Ignoring HRF recovery time: Events separated by < 2 seconds produce nonlinear BOLD summation, making responses difficult to separate (Glover, 1999)
4. No baseline/rest periods: Without rest periods, the model cannot estimate absolute activation levels and efficiency drops substantially (Friston et al., 1999)
5. Confounding condition with time: Presenting all trials of one condition before another confounds the effect with scanner drift and fatigue
6. Not counterbalancing response mappings: Lateralized motor responses (left vs. right hand) produce motor cortex activation that confounds task effects
7. Ceiling/floor performance: If accuracy is near 100% or chance, there is no behavioral variance to correlate with brain activity
8. Not optimizing the design matrix: Using arbitrary event timing instead of optimized sequences wastes statistical power that could be gained at no additional cost

Minimum Reporting Checklist

Based on COBIDAS guidelines (Nichols et al., 2017) and Poldrack et al. (2008):

• Design type (block, event-related, mixed, rapid event-related)
• Block duration (for block designs) or ISI distribution parameters (for event-related)
• Number of conditions and number of trials per condition
• Stimulus duration and response window
• Jittering strategy and ISI range (min, max, mean, distribution)
• Null trial proportion and distribution
• Condition ordering method (optimized, m-sequence, pseudo-random)
• Total scan duration per run and number of runs
• Design optimization tool used (if any) and efficiency metric
• Response mapping and counterbalancing scheme
• Practice procedure (in-scanner or out-of-scanner, duration)
• TR and its relationship to stimulus timing

References

• Bandettini, P. A., Jesmanowicz, A., Wong, E. C., & Hyde, J. S. (1993). Processing strategies for time-course data sets in functional MRI of the human brain. Magnetic Resonance in Medicine, 30(2), 161-173.
• Buracas, G. T., & Boynton, G. M. (2002). Efficient design of event-related fMRI experiments using m-sequences. NeuroImage, 16(3), 801-813.
• Dale, A. M. (1999). Optimal experimental design for event-related fMRI. Human Brain Mapping, 8(2-3), 109-114.
• Desmond, J. E., & Glover, G. H. (2002). Estimating sample size in functional MRI (fMRI) neuroimaging studies: Statistical power analyses. Journal of Neuroscience Methods, 118(2), 115-128.
• Durnez, J., Blair, R., & Poldrack, R. A. (2017). NeuroDesign: Optimal experimental designs for task fMRI. bioRxiv, 119594.
• Friston, K. J., Zarahn, E., Josephs, O., Henson, R. N. A., & Dale, A. M. (1999). Stochastic designs in event-related fMRI. NeuroImage, 10(5), 607-619.
• Glover, G. H. (1999). Deconvolution of impulse response in event-related BOLD fMRI. NeuroImage, 9(4), 416-429.
• Haatveit, B. C., Sundet, K., Hugdahl, K., et al. (2010). The validity of d prime as a working memory index. Neuropsychology, 24(5), 629-640.
• Hagberg, G. E., Zito, G., Patria, F., & Sanes, J. N. (2001). Improved detection of event-related functional MRI signals using probability functions. NeuroImage, 14(5), 1193-1205.
• Liu, T. T., Frank, L. R., Wong, E. C., & Buxton, R. B. (2001). Detection power, estimation efficiency, and predictability in event-related fMRI. NeuroImage, 13(4), 759-773.
• Maus, B., van Breukelen, G. J. P., Goebel, R., & Berger, M. P. F. (2010). Optimal design of multi-subject blocked fMRI experiments. NeuroImage, 51(3), 1338-1348.
• Mumford, J. A., & Nichols, T. E. (2008). Power calculation for group fMRI studies accounting for arbitrary design and temporal autocorrelation. NeuroImage, 39(1), 261-268.
• Murphy, K., & Garavan, H. (2005). Deriving the optimal number of events for an event-related fMRI study based on the spatial extent of activation. NeuroImage, 27(4), 771-777.
• Nichols, T. E., Das, S., Eickhoff, S. B., et al. (2017). Best practices in data analysis and sharing in neuroimaging using MRI (COBIDAS). Nature Neuroscience, 20(3), 299-303.
• Petersen, S. E., & Dubis, J. W. (2012). The mixed block/event-related design. NeuroImage, 62(2), 1177-1184.
• Poldrack, R. A., Fletcher, P. C., Henson, R. N., et al. (2008). Guidelines for reporting an fMRI study. NeuroImage, 40(2), 409-414.
• Poldrack, R. A., Mumford, J. A., & Nichols, T. E. (2011). Handbook of Functional MRI Data Analysis. Cambridge University Press.
• Wager, T. D., & Nichols, T. E. (2003). Optimization of experimental design in fMRI: A general framework using a genetic algorithm. NeuroImage, 18(2), 293-309.

See references/ for detailed design optimization examples and parameter lookup tables.