What Is Stimulus Control Transfer?
In applied behavior analysis, stimulus control transfer refers to the process of shifting behavioral control from one stimulus to another. Most commonly, this involves transferring control from a prompt (an added stimulus that increases the likelihood of a correct response) to the natural discriminative stimulus (the stimulus that should eventually occasion the behavior independently).
Effective teaching in ABA almost always involves some form of prompting. However, if prompts are not systematically faded, the learner becomes prompt-dependent, meaning they only respond correctly when prompted. Stimulus control transfer procedures ensure that the learner moves from prompted responding to independent responding under the control of naturally occurring stimuli.
Types of Prompts
Before discussing transfer procedures, it is important to understand the types of prompts used in ABA.
Response Prompts
Response prompts operate directly on the learner's behavior. They include:
- Physical prompts: Hand-over-hand guidance or physical assistance to complete a response
- Model prompts: Demonstrating the correct response for the learner to imitate
- Verbal prompts: Providing verbal instructions, hints, or cues
- Gestural prompts: Pointing, nodding, or using other gestures to indicate the correct response
Stimulus Prompts
Stimulus prompts operate on the antecedent stimulus itself, making the correct response more obvious. Examples include highlighting the correct answer, making the correct stimulus larger or a different color, or positioning the correct item closer to the learner.
Stimulus Control Transfer Procedures
Most-to-Least Prompting (Decreasing Assistance)
In most-to-least prompting, instruction begins with the most intrusive prompt level needed to ensure a correct response. Across successive trials or sessions, the prompt level is systematically reduced. A typical hierarchy moves from full physical, to partial physical, to model, to gestural, to independent.
The advantage of most-to-least prompting is that it minimizes errors during the learning process, which is important for learners who become frustrated by mistakes or whose errors are likely to be reinforced. The disadvantage is that it may take longer to reach independence because the learner starts with maximum assistance.
Least-to-Most Prompting (Increasing Assistance)
Least-to-most prompting begins with the least intrusive prompt and increases intrusiveness only as needed. On each trial, the learner is first given the opportunity to respond independently. If no correct response occurs within a set time interval, the least intrusive prompt is delivered. If the learner still does not respond correctly, the next level of prompt is provided, and so on until a correct response occurs.
This procedure is less restrictive because it gives the learner the opportunity to respond independently on every trial. It is well-suited for learners who already have some portion of the skill in their repertoire. The disadvantage is that it allows more errors than most-to-least prompting.
Graduated Guidance
Graduated guidance is a variation of most-to-least prompting that involves moment-to-moment adjustments in physical prompting intensity based on the learner's performance. The instructor physically guides the learner but reduces the intensity of the prompt (for example, moving from hand-over-hand to a light touch at the wrist, then the elbow, then the shoulder) as the learner begins to respond more independently. If the learner hesitates or begins to err, prompt intensity is immediately increased.
This fluid approach is particularly useful for teaching chained tasks like self-care routines, where different steps may require different levels of assistance.
Time Delay
Time delay procedures transfer stimulus control by inserting a delay between the presentation of the natural discriminative stimulus and the delivery of the prompt. There are two main types:
Constant time delay: After an initial round of zero-second delay trials (where the prompt is delivered simultaneously with the discriminative stimulus), the delay is set at a fixed interval (commonly 3 to 5 seconds) for all subsequent trials. The learner is reinforced for responding correctly before the prompt (an unprompted correct response) or for responding correctly after the prompt (a prompted correct response).
Progressive time delay: The delay between the discriminative stimulus and the prompt gradually increases over trials (for example, from 0 seconds to 1 second to 2 seconds to 3 seconds). This gradual increase provides a slower, more systematic transfer.
Time delay is one of the most researched and effective transfer procedures, with a strong evidence base for teaching academic skills, daily living skills, and communication to individuals with developmental disabilities.
Stimulus Fading
Stimulus fading gradually changes the physical characteristics of a stimulus prompt until it matches the natural stimulus. For example, when teaching letter discrimination, the correct letter might initially be presented in bold, bright red while incorrect options are faded gray. Across trials, the highlighting is gradually removed until all letters appear in the same format and the learner is discriminating based on the letter shapes alone.
Stimulus Shaping
Stimulus shaping involves gradually changing the form of a stimulus from one the learner can already discriminate to the target stimulus. For example, when teaching a child to read the word "cat," you might begin with a picture of a cat superimposed on the word. Across successive presentations, the picture is gradually faded until only the printed word remains.
Selecting the Right Procedure
The choice of stimulus control transfer procedure depends on several factors: the learner's history and learning profile, the type of skill being taught, the importance of minimizing errors, the setting and available resources, and data from previous teaching attempts.
For learners who are error-sensitive or whose errors are likely to strengthen, most-to-least prompting or time delay with zero-second initial trials is preferred. For learners who are relatively independent and need minimal support, least-to-most prompting may be more efficient.
Preventing Prompt Dependency
Prompt dependency occurs when a learner waits for or requires a prompt before responding, even though they are capable of independent responding. To prevent prompt dependency, always have a plan for fading prompts, collect data on prompted versus independent responses, celebrate and differentially reinforce independent correct responses, and avoid delivering prompts when the learner is capable of responding on their own.