ACT-R Model Builder

Purpose

This skill encodes expert knowledge for constructing computational cognitive models within the ACT-R (Adaptive Control of Thought -- Rational) architecture. It provides guidance on chunk type definition, production rule authoring, subsymbolic parameter selection with empirically validated defaults, model fitting workflows, and validation procedures. A general-purpose programmer would not know the architecture constraints, parameter defaults, or model validation standards without specialized cognitive modeling training.

When to Use This Skill

• Designing a new ACT-R model for a cognitive task (memory retrieval, decision-making, skill acquisition)
• Setting subsymbolic parameters and understanding their theoretical justification
• Structuring chunk types and production rules for a specific experimental paradigm
• Fitting an ACT-R model to behavioral data (RT, accuracy)
• Validating a model via parameter recovery, cross-validation, or qualitative predictions
• Choosing between ACT-R 7.x (Lisp) and pyactr (Python) for implementation

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.

ACT-R Architecture Overview

ACT-R is a hybrid cognitive architecture with symbolic and subsymbolic components (Anderson, 2007; Anderson & Lebiere, 1998).

Core Modules and Buffers

Module	Buffer	Function	Source
Declarative memory	retrieval	Stores and retrieves chunks (facts)	Anderson, 2007, Ch. 2
Procedural memory	(none; fires productions)	Stores production rules (skills)	Anderson, 2007, Ch. 3
Goal	goal	Tracks current task state	Anderson, 2007, Ch. 4
Imaginal	imaginal	Holds intermediate problem representations	Anderson, 2007, Ch. 4
Visual	visual, visual-location	Attends to and encodes visual objects	Anderson, 2007, Ch. 6
Motor	manual	Executes motor responses (keypresses)	Anderson, 2007, Ch. 6
Temporal	temporal	Tracks time intervals	Taatgen et al., 2007

Processing Cycle

1. Buffers hold one chunk each (the "bottleneck" assumption; Anderson, 2007, Ch. 1)
2. Productions match against buffer contents (pattern matching)
3. Conflict resolution selects one production per cycle (~50 ms per production firing; Anderson, 2007)
4. Selected production modifies buffers or makes requests to modules
5. Modules process requests asynchronously

Building the Symbolic Model

Step 1: Define Chunk Types

Chunk types define the structure of declarative knowledge:

;; ACT-R 7.x Lisp syntax
(chunk-type addition-problem arg1 arg2 answer)
(chunk-type counting-fact number next)

Decision rules for chunk type design:

1. Each chunk type represents one category of knowledge (Anderson, 2007, Ch. 2)
2. Slots should correspond to meaningful features of the domain
3. Use inheritance when chunk types share structure (e.g., a "problem" parent type)
4. Keep chunks small -- typically 3-6 slots per chunk type (Anderson & Lebiere, 1998)

Step 2: Write Production Rules

Productions follow an IF-THEN structure:

(p retrieve-answer
 =goal>
 isa addition-problem
 arg1 =num1
 arg2 =num2
 answer nil
 ?retrieval>
 state free
==>
 +retrieval>
 isa addition-fact
 addend1 =num1
 addend2 =num2
 =goal>
)

Production rule guidelines:

Guideline	Rationale	Source
One request per production	Module bottleneck constraint	Anderson, 2007, Ch. 3
Test buffer state before requesting	Prevents jamming the module	Bothell, 2023, ACT-R reference manual
Use =goal> to maintain goal buffer	Prevents goal harvesting	Bothell, 2023
Minimize productions per task step	Simpler models are preferred (parsimony)	Anderson, 2007, Ch. 1

Step 3: Structure the Goal Stack

Is the task sequential with clear phases?
 |
 +-- YES --> Use a single goal chunk with a "step" slot
 | that tracks the current phase
 |
 +-- NO --> Does the task require subgoaling?
 |
 +-- YES --> Use goal push/pop (stack)
 |
 +-- NO --> Use the imaginal buffer for
 intermediate representations

Subsymbolic Parameters

These parameters govern memory activation, retrieval, and production selection. See references/parameter-table.yaml for the complete table.

Core Declarative Memory Parameters

Parameter	Symbol	Default	Typical Range	Source
Base-level learning decay	d	0.5	0.1 -- 1.0	Anderson & Schooler, 1991; Anderson, 2007
Activation noise	s	0.4	0.1 -- 0.8	Anderson, 2007
Latency factor	F	1.0	0.2 -- 5.0	Anderson, 2007
Latency exponent	f	1.0	Fixed in most models	Anderson, 2007
Retrieval threshold	tau	-infinity (default)	Set empirically; often 0.0 to -2.0	Anderson, 2007
Maximum associative strength	S (mas)	context-dependent	1.0 -- 5.0	Anderson & Reder, 1999
Mismatch penalty	P	application-dependent	0.5 -- 2.0	Anderson, 2007

Production Utility Parameters

Parameter	Symbol	Default	Typical Range	Source
Utility noise	sigma	0.0 (deterministic)	0.1 -- 2.0 when enabled	Anderson, 2007
Utility learning rate	alpha	0.2	0.01 -- 1.0	Anderson, 2007
Initial utility	U0	0.0	Set per production	Anderson, 2007
Production compilation	enabled/disabled	Disabled by default	--	Taatgen & Anderson, 2002

Timing Parameters

Parameter	Value	Source
Production cycle time	50 ms	Anderson, 2007
Visual encoding time	85 ms	Anderson, 2007, Ch. 6
Motor initiation time	50 ms	Anderson, 2007, Ch. 6
Motor execution time	100 ms (Fitts' law applies)	Anderson, 2007, Ch. 6
Imaginal delay	200 ms	Anderson, 2007, Ch. 4

Activation Equation

Total activation of chunk i:

A_i = B_i + sum_j(W_j * S_ji) + PM_i + noise

Where:

• B_i = base-level activation (log of weighted recency; decay d; Anderson & Schooler, 1991)
• W_j * S_ji = spreading activation from source j (Anderson, 2007, Ch. 5)
• PM_i = partial matching component (Anderson, 2007)
• noise = logistic noise with scale s (Anderson, 2007)

Retrieval time: RT = F * e^(-f * A_i) (Anderson, 2007)

Model Fitting Workflow

Step 1: Identify Free Parameters

How many free parameters?
 |
 +-- <= 3 --> Standard practice; proceed
 |
 +-- 4-6 --> Acceptable if justified by model complexity
 |
 +-- > 6 --> Warning: overfitting risk. Consider fixing some
 to default values (Anderson, 2007)

Rule of thumb: The number of free parameters should be substantially less than the number of independent data points being fit (Roberts & Pashler, 2000).

Step 2: Choose Fitting Method

Method	When to Use	Source
Grid search	Few parameters (1-3), bounded space	Standard practice
Simplex (Nelder-Mead)	Moderate parameters, smooth landscape	Anderson, 2007
Differential evolution	Many parameters, multimodal landscape	Storn & Price, 1997
Bayesian optimization	Expensive evaluations, informed priors	Palestro et al., 2018

Step 3: Fit to Multiple Dependent Variables

ACT-R models should simultaneously account for:

• Response times (correct trials, mean or quantiles)
• Accuracy (proportion correct by condition)
• Qualitative patterns (error types, learning curves)

Use weighted sum of squared deviations or log-likelihood across measures (Anderson, 2007, Ch. 4).

Step 4: Parameter Recovery

Before trusting fitted parameter values, conduct a parameter recovery study. See the parameter-recovery-checker skill.

Common Model Patterns

See references/model-patterns.md for detailed implementations of:

1. Memory retrieval -- Paired associates, fan effect (Anderson, 2007, Ch. 5)
2. Skill acquisition -- Production compilation, power law of practice (Taatgen & Anderson, 2002)
3. Decision-making -- Instance-based learning, utility-based selection (Gonzalez et al., 2003)
4. Problem solving -- Means-ends analysis, goal stacking (Anderson, 2007, Ch. 8)

Model Validation Checklist

Validation Step	Method	Minimum Standard
Parameter recovery	Simulate and refit	r > 0.9 between true and recovered (Heathcote et al., 2015)
Cross-validation	Fit half, predict half	Prediction RMSE within 2x of fitting RMSE
Qualitative predictions	Novel conditions	Model predicts ordinal pattern correctly
Model comparison	AIC/BIC or Bayes factor	Compare against plausible alternatives (Burnham & Anderson, 2002)
Sensitivity analysis	Vary fixed parameters	Conclusions robust to +/-20% variation

Software and Implementation

Platform	Language	URL	Notes
ACT-R 7.x	Common Lisp	act-r.psy.cmu.edu	Reference implementation (Bothell, 2023)
pyactr	Python	github.com/jakdot/pyactr	Python interface, good for batch simulations (Dotlacil, 2018)
jACT-R	Java	jactr.org	Java implementation

Recommendation: Use ACT-R 7.x for model development and validation. Use pyactr when integrating with Python data analysis pipelines or running large parameter sweeps (Dotlacil, 2018).

Common Pitfalls

1. Too many free parameters: Fitting more than 5-6 free parameters without strong justification risks overfitting (Roberts & Pashler, 2000). Fix well-established parameters (d = 0.5, production cycle = 50 ms) to defaults.
2. Ignoring parameter correlations: Parameters like s (noise) and tau (threshold) trade off. Run parameter recovery to verify identifiability.
3. Fitting only means: ACT-R makes distributional predictions. Fitting only mean RT discards information. Use quantile-based fitting where possible (Heathcote et al., 2015).
4. Incorrect timing alignment: ACT-R's predicted RT includes perceptual and motor times. Account for these when comparing to behavioral RT.
5. Overly complex models: Prefer models with fewer productions and chunk types that still capture the qualitative pattern. Complexity should be motivated by the data (Anderson, 2007, Ch. 1).
6. Neglecting model comparison: Always compare your ACT-R model against at least one alternative (simpler ACT-R variant or a different architecture) using formal model comparison (Burnham & Anderson, 2002).

Minimum Reporting Checklist

Based on best practices from Anderson (2007) and Heathcote et al. (2015):

• Architecture version (e.g., ACT-R 7.27)
• All chunk types and their slots
• Number of production rules
• All parameter values: fixed (with default source) and free (with fitted values and confidence intervals)
• Fitting method and objective function
• Data fitted: number of conditions, number of data points, dependent variables
• Goodness of fit: R-squared, RMSE, or log-likelihood per dependent variable
• Parameter recovery results (r, bias, RMSE for each free parameter)
• Model comparison results (AIC/BIC/Bayes factor vs. alternatives)
• Qualitative predictions and whether they matched data

References

• Anderson, J. R. (2007). How Can the Human Mind Occur in the Physical Universe? Oxford University Press.
• Anderson, J. R., & Lebiere, C. (1998). The Atomic Components of Thought. Lawrence Erlbaum Associates.
• Anderson, J. R., & Reder, L. M. (1999). The fan effect: New results and new theories. Journal of Experimental Psychology: General, 128(2), 186-197.
• Anderson, J. R., & Schooler, L. J. (1991). Reflections of the environment in memory. Psychological Science, 2(6), 396-408.
• Bothell, D. (2023). ACT-R 7.27 Reference Manual. Carnegie Mellon University.
• Burnham, K. P., & Anderson, D. R. (2002). Model Selection and Multimodel Inference (2nd ed.). Springer.
• Dotlacil, J. (2018). Building an ACT-R reader for eye-tracking corpus data. Topics in Cognitive Science, 10(1), 144-160.
• Gonzalez, C., Lerch, J. F., & Lebiere, C. (2003). Instance-based learning in dynamic decision making. Cognitive Science, 27(4), 591-635.
• Heathcote, A., Brown, S. D., & Wagenmakers, E.-J. (2015). An introduction to good practices in cognitive modeling. In B. U. Forstmann & E.-J. Wagenmakers (Eds.), An Introduction to Model-Based Cognitive Neuroscience. Springer.
• Palestro, J. J., Sederberg, P. B., Osth, A. F., Van Zandt, T., & Turner, B. M. (2018). Likelihood-free methods for cognitive science. Springer.
• Roberts, S., & Pashler, H. (2000). How persuasive is a good fit? A comment on theory testing. Psychological Review, 107(2), 358-367.
• Storn, R., & Price, K. (1997). Differential evolution. Journal of Global Optimization, 11(4), 341-359.
• Taatgen, N. A., & Anderson, J. R. (2002). Why do children learn to say "Broke"? A model of learning the past tense without feedback. Cognition, 86(2), 123-155.
• Taatgen, N. A., van Rijn, H., & Anderson, J. R. (2007). An integrated theory of prospective time interval estimation. Psychological Review, 114(3), 577-598.

See references/ for detailed parameter tables and common model patterns.