Tower of London Task
The Tower of London (ToL) is a planning task used to measure executive function — the set of capacities that let us set goals, hold them in mind, and act toward them step by step. Developed as a spatial analog of the Tower of Hanoi puzzle, it has become one of the most common clinical and experimental tests of planning and problem-solving ability.
The Tower of London Procedure
The task presents two configurations of colored balls on pegs (or beads on rods): a start state and a goal state. The participant must transform the start state into the goal state by moving one ball at a time, following two rules:
- Only one ball may be moved per step.
- A peg can hold at most a limited number of balls (typically three).
The key constraint is that the participant must plan a sequence of moves before executing them, because some moves are irreversible — moving a ball to the wrong peg can block the shortest solution. Problems are graded by the minimum number of moves required (commonly 2 to 5). Harder problems require looking further ahead.
Performance is scored on several dimensions:
- Moves to solution versus the optimal (minimum) number of moves.
- Initial thinking time before the first move (a marker of planning depth).
- Number of excess moves and rule violations.
- Total solution time.
What the Tower of London Measures
The ToL isolates planning from other executive demands. Because the rules are simple and the stimuli are spatial, poor performance mainly reflects difficulty with:
- Representing the goal state and subgoals.
- Simulating move sequences in advance.
- Inhibiting the impulse to move immediately.
It is therefore a relatively pure measure of goal-directed planning, distinct from working-memory load (which is better probed by the N-Back Task) and from set-shifting (better probed by the Wisconsin Card Sorting Test).
Variations
Several variants exist. The Tower of Hanoi uses disks of different sizes that may never rest on a smaller disk, which adds a stricter constraint. The Stockings of Cambridge adapts the logic to a neuropsychological battery. Imagined (non-manual) versions ask participants to plan without moving, isolating the planning representation from motor execution.
Why It Matters
ToL performance is reliably linked to the prefrontal cortex, especially the dorsolateral region. It is sensitive to frontal-lobe injury, Parkinson's disease, schizophrenia, and developmental conditions, and is widely used to study how planning breaks down or is supported by strategy.
Methodology
Methodology: This topic is part of the experimental workflow covered in Experimentology. For the relevant methods, see Design (problem difficulty as an independent variable, and within-subject versus between-subject contrasts), Measurement (scoring planning quality and excess moves), and Models (modeling move sequences as search through a problem space).
Related Topics
- Tower of Hanoi - The size-constrained disk variant
- Wisconsin Card Sorting Test - Set-shifting and executive control
- N-Back Task - Working-memory load
- Spatial Span - Visuospatial working memory
Implementation
A jsPsych implementation of the Tower of London is part of the ExpFactory experiments collection, which provides browser-ready code for executive-function paradigms.
Try It Yourself
References
- Shallice, T. (1982). Specific impairments of planning. Philosophical Transactions of the Royal Society B, 298(1089), 199-209.