Transfer of Stimulus Control Through Fading
Fading is the gradual removal or alteration of a supplementary stimulus (prompt) or stimulus feature that initially helps the individual respond correctly. The goal of fading is to transfer stimulus control from the supplementary stimulus to the natural discriminative stimulus (SD) so the individual can respond independently. Both stimulus fading and response fading serve this purpose, but they accomplish it through different mechanisms.
Stimulus Fading Defined
Stimulus fading involves the systematic, gradual change of a physical dimension of a stimulus to transfer control from a salient but irrelevant stimulus feature to the relevant feature. In stimulus fading, the response remains the same across all trials, but the stimuli presented change progressively.
How It Works
The practitioner begins by exaggerating a stimulus dimension that makes the correct response obvious. Across successive trials, this exaggerated dimension is gradually reduced until the stimuli appear in their natural form and the individual responds to the relevant features alone.
Classic Example: Terrace's Errorless Discrimination
Terrace (1963) demonstrated stimulus fading in a landmark study with pigeons. He taught pigeons to discriminate between a red key (S+) and a green key (S-) by initially presenting the S- as very dim and small, then gradually increasing its brightness and size until it matched the S+ in all dimensions except color. Because the S- was introduced gradually, the pigeons made very few errors during discrimination training.
Clinical Example
Teaching a child to discriminate the letter "b" from the letter "d." Initially, the correct letter ("b") is presented in a much larger font and bold color, while the incorrect letter ("d") appears small and faded. Across trials, the size and color differences are gradually reduced until both letters appear identical in all dimensions except their orientation, and the child reliably selects "b" based on its relevant features.
Response Fading Defined
Response fading, more commonly referred to as prompt fading, involves the systematic, gradual reduction of a prompt that assists the individual in making the correct response. In response fading, the stimuli remain constant, but the level of assistance provided changes across successive trials.
How It Works
The practitioner begins by providing a level of prompting that ensures the individual responds correctly (typically a full physical or model prompt). Across successive trials, the prompt is gradually reduced in intrusiveness or intensity until the individual responds to the SD alone.
Prompt Fading Hierarchies
The most common approach to response fading follows a most-to-least prompt hierarchy:
- Full physical prompt: Hand-over-hand guidance to complete the response.
- Partial physical prompt: Light touch or nudge to initiate the response.
- Model prompt: Demonstrating the correct response for the individual to imitate.
- Gestural prompt: Pointing to or looking at the correct response.
- Verbal prompt: Providing a verbal cue or instruction.
- Independent: The individual responds to the SD alone with no supplementary prompts.
Clinical Example
Teaching a client to wash their hands independently. Initially, the practitioner provides full hand-over-hand guidance through each step (turn on water, apply soap, rub hands, rinse, turn off water, dry hands). Across sessions, the physical guidance is reduced to a light touch, then a gesture, then the verbal instruction "wash your hands," until the client completes the entire sequence independently in response to the natural context (dirty hands, mealtime).
Key Differences at a Glance
- What changes: In stimulus fading, the stimulus dimensions change. In response fading (prompt fading), the prompt changes.
- What stays the same: In stimulus fading, the response stays the same. In response fading, the stimulus stays the same.
- Error patterns: Stimulus fading tends to produce fewer errors overall because the correct response is obvious at each step. Response fading may produce more errors during transitions between prompt levels if the individual has become dependent on a specific prompt level.
- Best applications: Stimulus fading is well suited for discrimination training and academic tasks. Response fading is well suited for chained behaviors, self-care skills, and motor tasks.
Choosing Between the Two
Consider stimulus fading when the teaching goal involves discrimination between stimuli, when you want to minimize errors during acquisition, and when the relevant and irrelevant stimulus dimensions can be clearly manipulated. Consider response fading when the teaching goal involves a motor response or chain, when the natural SD is already present and a prompt is being used to occasion the response, and when a prompt hierarchy can be clearly defined.
Common Mistakes
- Fading too quickly: Both procedures require gradual changes. Jumping steps often leads to errors that can be difficult to correct.
- Not planning the fading sequence in advance: Each step in the fading process should be predetermined. Ad hoc fading decisions during sessions lead to inconsistency.
- Prompt dependency: Failing to fade prompts systematically can result in the individual becoming dependent on the supplementary stimulus rather than transferring control to the natural SD.
BCBA Exam Relevance
The BCBA exam frequently tests the distinction between stimulus fading and response fading. Be prepared to identify each procedure from clinical descriptions, explain which component is being changed (stimulus vs. prompt), and recommend which procedure is more appropriate for a given teaching objective. Also review how both procedures relate to errorless learning and stimulus control transfer.