Variable Interval
Overview
The variable interval (VI) schedule reinforces the first response after a variable time period that averages around a specified mean. For example, VI30s makes reinforcement available on average every 30 seconds, but individual intervals might range from 5 to 60 seconds, drawn from a programmed distribution. VI schedules produce moderate, steady response rates without the scalloping seen in FI schedules.
VI schedules are prized in behavioral pharmacology because they generate exceptionally stable baselines against which drug effects can be measured. The steady-state responding makes it easy to detect rate-increasing or rate-decreasing drug effects. VI schedules are also used in concurrent schedule studies (e.g., VI-VI) to assess choice behavior and the matching law.
ConductMaze implements VI schedules using Fleshler-Hoffman or arithmetic progressions. The software generates non-repeating interval sequences, provides real-time response rate monitoring, and supports concurrent VI-VI configurations for two-lever choice experiments.
Trial Flow
Session Start
House light ON, levers extended, first interval begins
Interval Timer
Variable time period elapses; responses recorded but not reinforced
Interval Elapsed
Has the current variable interval passed?
First Response
First active press after interval ends triggers reinforcement
Reinforcer Delivery
Pellet dispensed, cue light activated
New Interval Drawn
Next interval sampled from distribution around mean
Session End
Levers retracted, data saved
Parameters
| Parameter | Type | Default | Description |
|---|---|---|---|
| Mean Interval | seconds | 30 | Average time between reinforcement availability periods |
| Distribution Type | enum | Fleshler-Hoffman | Distribution algorithm for generating interval values |
| Max Reinforcers | integer | 40 | Maximum reinforcers before session ends |
| Max Session Time | seconds | 3600 | Absolute session time limit |
| Active Lever Side | enum | Right | Which lever is reinforced |
| Cue Light Duration | seconds | 3 | Duration of cue light paired with reinforcer |
| Reinforcer Type | enum | Sucrose Pellet | Type of reinforcement delivered |
| Limited Hold | seconds | 0 | Time window after interval for valid response (0 = unlimited) |
Metrics
| Metric | Unit | Description |
|---|---|---|
| Total Active Presses | count | Total presses on the active lever |
| Total Inactive Presses | count | Total presses on the inactive lever |
| Reinforcers Earned | count | Total reinforcers delivered |
| Response Rate | presses/min | Mean active lever press rate |
| Inter-Response Time | seconds | Mean time between successive active presses |
| Actual Mean Interval | seconds | Observed mean interval across the session |
| Response Rate Stability | CV | Coefficient of variation of response rate across bins |
Sample Data
| Trial | Interval_s | Active_Presses | Inactive_Presses | IRT_s | Rate_ppm |
|---|
Representative data for illustration purposes. Actual values will vary by species, strain, and experimental conditions.
Applications
- 1Behavioral pharmacology — stable VI baselines for detecting drug-induced rate changes
- 2Matching law research — concurrent VI-VI schedules for quantifying choice allocation
- 3Behavioral economics — measuring response output under time-based reinforcement constraints
- 4Attention studies — VI schedules maintain vigilance-like sustained responding
- 5Schedule comparison — contrasting VI and VR to dissociate time-based vs. ratio-based control
Related Protocols
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