Navon Task

Version: v1 (current)

A hierarchical stimuli paradigm measuring global versus local processing biases in visual attention.

Overview

The Navon task uses hierarchical letter stimuli, where large "global" letters are composed of smaller "local" letters, to investigate how attention is allocated between different levels of visual structure. For example, a large letter "H" might be made up of small letter "S"s. Participants are instructed to identify either the global letter (H) or the local letters (S) by pressing the corresponding key.

This elegant paradigm reveals fundamental principles of visual perception: typically, global features are processed faster than local features ("global precedence"), and incongruent local features interfere with global identification more than vice versa. These effects vary with cognitive style, clinical conditions, and cultural background, making the Navon task a valuable tool for investigating individual and group differences in attentional scope.

The task is widely used in cognitive psychology, neuroscience, and clinical research on autism, schizophrenia, and aging.

Scientific Background

Classic Findings:

• Global Precedence: Global letters are identified faster than local letters
• Asymmetric Interference: Incongruent local letters slow global identification, but incongruent global letters have less effect on local identification
• Congruency Effects: Performance is better when global and local levels are congruent (e.g., large H made of small H's) versus incongruent (large H made of small S's)
• Individual Differences: Cognitive style (holistic vs. analytic) modulates global-local balance

Key Mechanisms:

• Low Spatial Frequency Advantage: Global structure conveyed by low spatial frequencies, processed first
• Attentional Scope: Broad attention facilitates global processing; narrow focus facilitates local processing

Seminal Paper:

• Navon, D. (1977). Forest before trees: The precedence of global features in visual perception. Cognitive Psychology, 9(3), 353-383.

Why Researchers Use This Task

1. Attention Research: Study the scope and flexibility of visual attention
2. Clinical Assessment: Evaluate global-local processing in autism (local bias) and schizophrenia
3. Cognitive Neuroscience: Investigate visual processing pathways and hierarchical perception
4. Individual Differences: Measure analytic vs. holistic cognitive styles
5. Cross-Cultural Studies: Explore cultural differences in attention to global vs. local features

Configuration Options

Response Mode

Parameter	Type	Default	Description
Time-based trials	boolean	True	If enabled, trials auto-advance after stimulus duration; if disabled, participant responds via button

Visual Settings

Parameter	Type	Default	Description
Global font size	number	120	Font size for the large letter shape (8-400 pixels)
Local font size	number	16	Font size for the small letters forming the shape (8-400 pixels)
Letter spacing	text	0.5em	CSS letter-spacing value for local letters
Line height	text	1.2	CSS line-height value for local letter rows

Practice Trials

Parameter	Type	Default	Description
Enable practice	boolean	False	Show practice trials with visual feedback before main trials
Practice trials	array	[]	Array of practice trial configurations

Keyboard Shortcuts

Researchers can customize the keyboard bindings used during the task:

Parameter	Type	Default	Description
Show keyboard hint	boolean	True	Display an on-screen hint showing the configured key
Response key	key	Any letter (A-Z)	Key accepted for letter identification response
Response action label	text	"to match the target letter"	Label shown in the keyboard hint

The default "Any letter" setting accepts any letter key A-Z, which is appropriate since the task requires identifying specific letters.

Trial Configuration

Each trial is defined in the Trials spreadsheet with the following columns:

Column	Description	Example Values
global_letter	The large letter shape formed by the pattern	H, S, E, L, T
local_letter	The small letter used to form the global shape	H, S, E, L, T
target_level	Which level the participant should respond to	Global, Local
condition	Whether global and local letters match	Congruent, Incongruent
expected_response	The correct keyboard response	H, S, E, etc.
block	Optional grouping label	Global Block, Local Block
fixation_ms	Fixation cross duration before stimulus	500
stimulus_ms	Stimulus display duration	2000

Example Trials

Global Block (respond to large letter):

| global_letter | local_letter | target_level | condition    | expected_response | fixation_ms | stimulus_ms |
|---------------|--------------|--------------|--------------|-------------------|-------------|-------------|
| H             | H            | global       | congruent    | H                 | 500         | 2000        |
| H             | S            | global       | incongruent  | H                 | 500         | 2000        |
| S             | S            | global       | congruent    | S                 | 500         | 2000        |
| S             | H            | global       | incongruent  | S                 | 500         | 2000        |

Local Block (respond to small letters):

| global_letter | local_letter | target_level | condition    | expected_response | fixation_ms | stimulus_ms |
|---------------|--------------|--------------|--------------|-------------------|-------------|-------------|
| H             | H            | local        | congruent    | H                 | 500         | 2000        |
| H             | S            | local        | incongruent  | S                 | 500         | 2000        |
| S             | S            | local        | congruent    | S                 | 500         | 2000        |
| S             | H            | local        | incongruent  | H                 | 500         | 2000        |

Practice Trials

The task supports three practice modes:

• None: Task begins directly with main trials
• Optional: Practice trials available; participant can skip after any trial
• Mandatory: Practice trials must be completed before main trials

During practice, participants receive visual feedback after each response (green checkmark for correct, red X for incorrect).

Participant Experience

Trial Sequence

1. Main Instructions: Overview of the task structure
2. (Optional) Practice Instructions: If practice enabled
3. (Optional) Practice Trials: With visual feedback
4. (Optional) Trials Instructions: Shown before main trials
5. Main Trials: Each trial follows this sequence:
   • Fixation cross (+) appears (if fixation_ms > 0)
   • Hierarchical stimulus appears (large letter made of small letters)
   • Participant presses the appropriate letter key
   • Trial advances automatically (time-based) or after response (button-based)

Response Methods

Keyboard (recommended):

• Press the letter key corresponding to the target level (default accepts any A-Z key -- configurable by researcher)
• For global trials: respond to the large letter shape
• For local trials: respond to the small letters

Buttons (if time-based mode disabled):

• Click the appropriate response button

All keyboard bindings are configurable by the researcher in the study configuration. The keys listed above are the defaults.

Data Output

Markers and Responses

Markers (stimulus_shown):

{
  "type": "stimulus_shown",
  "ts": "2024-01-01T00:00:01.000Z",
  "hr": 1234.56,
  "data": {
    "trial_index": 1,
    "stimulus_id": "navon_0_1",
    "global_letter": "H",
    "local_letter": "S",
    "target_level": "global",
    "condition": "incongruent",
    "expected_response": "H",
    "block": "global_block"
  }
}

Response Data:

{
  "trial_index": 1,
  "stimulus_id": "navon_0_1",
  "source": "keyboard",
  "raw_key": "h",
  "global_letter": "H",
  "local_letter": "S",
  "target_level": "global",
  "condition": "incongruent",
  "expected_response": "H",
  "response_value": "H",
  "correct": true,
  "latency_ms": 623,
  "ts": "2024-01-01T00:00:01.623Z",
  "hr": 1857.56
}

Summary Artifact

A JSON file (navon_summary_<taskIndex>.json) with comprehensive statistics:

{
  "task_kind": "navon",
  "total_trials": 60,
  "overall": {
    "accuracy": 0.92,
    "mean_rt_ms": 512,
    "mean_correct_rt_ms": 498
  },
  "by_target_level": {
    "global": {
      "accuracy": 0.94,
      "mean_correct_rt_ms": 478
    },
    "local": {
      "accuracy": 0.90,
      "mean_correct_rt_ms": 542
    }
  },
  "by_condition": {
    "congruent": {
      "accuracy": 0.96,
      "mean_correct_rt_ms": 465
    },
    "incongruent": {
      "accuracy": 0.88,
      "mean_correct_rt_ms": 545
    }
  },
  "effects": {
    "global_precedence_ms": -64,
    "global_interference_ms": 54,
    "local_interference_ms": 66
  },
  "trials": [ /* per-trial data */ ]
}

Key metrics:

• global_precedence_ms: Difference between local and global mean RTs (negative = global faster)
• global_interference_ms: Incongruent - congruent RT for global trials
• local_interference_ms: Incongruent - congruent RT for local trials

Instructions

The task uses a four-tier instruction system:

1. Main Instructions: Shown on a dedicated page before the task begins
2. Practice Instructions: Shown before practice trials (if practice enabled)
3. Trials Instructions: Shown before main trials after practice (if practice enabled)
4. Hint Instructions: Quick-reference help available via "?" button during task

All instruction text can be customized in rich-text format during study configuration.

Design Recommendations

Trial Design

Balanced design (recommended):

• Equal numbers of global and local trials
• Equal numbers of congruent and incongruent trials within each target level
• Randomize order to prevent anticipation
• Counterbalance target level order (global-first vs. local-first) across participants

Minimum for reliable effects:

• 20 trials per target level (40 total)
• 10 congruent + 10 incongruent per target level

Timing Guidelines

Parameter	Standard	Brief	Extended
Fixation (ms)	500ms	300ms	800ms
Stimulus (ms)	1500-2000ms	150-250ms	3000ms+

Note: Brief presentations (150-250ms) enhance global precedence effects. Longer presentations allow more time but may reduce processing biases.

Practice Trials

Include 8-16 practice trials:

• Mix global and local target levels
• Mix congruent and incongruent conditions
• Provide feedback so participants learn the task
• Consider starting with congruent trials, then introducing incongruent

Letter Selection

Recommended letter set:

• Use letters with distinct shapes: H, S, E, L, T
• Avoid confusable letters (e.g., O and Q)
• Letters should form clear global shapes when repeated

Enhancing Global Precedence

• Brief stimulus duration (150-250ms)
• Use monospace font with adequate letter spacing
• Larger size ratio (global letter 10x larger than local)

Enhancing Local Processing

• Long stimulus duration (500ms+)
• Reduce global-local size ratio
• Use clear, high-contrast local letters

Common Issues and Solutions

No Global Precedence Observed

Problem: Local and global RTs are similar

Possible causes:

• Stimulus duration too long
• Size ratio not large enough
• Participants treating task as unspeeded

Solutions:

• Reduce stimulus_ms to 200-300ms
• Increase global font size or decrease local font size
• Emphasize speed in instructions ("Respond as quickly as possible")

High Error Rate (>15%)

Problem: Accuracy below 85%

Possible causes:

• Stimulus duration too short
• Participants confusing target level instructions
• Local letters not clearly visible

Solutions:

• Increase stimulus_ms to 2500-3000ms
• Provide clearer instructions with visual examples
• Increase local font size
• Extend practice with feedback

Keyboard Responses Not Working

Problem: Letter keys don't register

Solutions:

• Ensure task window has focus (click on task area)
• Check browser compatibility (works best in Chrome/Edge/Firefox)
• Use button-based mode as alternative

Population-Specific Adaptations

Children (8+ years)

• Larger stimuli (global: 150px, local: 20px)
• Longer exposure (2500-3000ms)
• Simpler letter set (3-4 letters total)
• Extended practice until child demonstrates understanding
• Button-based mode may be easier than keyboard

Older Adults (65+)

• Larger stimuli and longer exposure
• High contrast (black on white)
• Generous response window (3000ms+)
• Clear block instructions with examples
• Self-paced mode (button-based)

Clinical Populations

• Autism: May show reduced or reversed global advantage (local bias)
• Schizophrenia: May show disrupted global processing
• Adapt timing and size to individual capabilities
• Focus on accuracy over speed for some populations

References

• Navon, D. (1977). Forest before trees: The precedence of global features in visual perception. Cognitive Psychology, 9(3), 353-383.
• Lamb, M. R., Robertson, L. C., & Knight, R. T. (1989). Attention and interference in the processing of global and local information: Effects of unilateral temporal-parietal junction lesions. Neuropsychologia, 27(4), 471-483.
• Dale, G., & Arnell, K. M. (2013). Investigating the stability of and relationships among global/local processing measures. Attention, Perception, & Psychophysics, 75(3), 394-406.
• Happé, F., & Frith, U. (2006). The weak coherence account: Detail-focused cognitive style in autism spectrum disorders. Journal of Autism and Developmental Disorders, 36(1), 5-25.

See Also

• Flanker Task - Related selective attention paradigm
• Stroop Task - Another interference/attention task
• Visual Search - Feature vs. conjunction search